William Paley Institute
Intelligent Design


Electronic Books


Teaching About Evolution and the Nature of Science

National Academy Press
Washington, DC



Donald Kennedy (Chairman)
Bing Professor of Environmental Studies
Stanford University
Stanford, California John Moore
Professor Emeritus of Biology
University of California
Riverside, California
Bruce Alberts
National Academy of Sciences
Washington, DC Eugenie Scott
Executive Director
National Center for Science Education
El Cerrito, California
Danine Ezell
Science Department
Bell Junior High School
San Diego, California Maxine Singer
Carnegie Institution of Washington
Washington, DC
Tim Goldsmith
Department of Biology
Yale University
New Haven, Connecticut Mike Smith
Associate Professor of Medical Education
Mercer University School of Medicine
Macon, Georgia
Robert Hazen
Staff Scientist, Geophysical Laboratory
Carnegie Institution of Washington
Washington, DC Marilyn Suiter
Director, Education and Human Resources
American Geological Institute
Alexandria, Virginia
Norman Lederman
Professor, College of Science
Science and Mathematics Education
Oregon State University
Corvallis, Oregon Rachel Wood
Science Specialist
Delaware State Department of Public Instruction
Dover, Delaware
Joseph McInerney
Biological Sciences Curriculum Study
Colorado Springs, Colorado

Rodger Bybee, Executive Director
Peggy Gill, Research Assistant
Jay Hackett, Visiting Fellow
Patrice Legro, Division Director
Steve Olson, Consultant Writer

Any opinions, findings, conclusions, or recommendations expressed in this
publication are those of the authors and do not necessarily reflect the view of
the organizations that provided financial support for this project.



The National Academy of Sciences gratefully acknowledges contributions from:

Howard Hughes Medical Institute
The Esther A. and Joseph Klingenstein Fund, Inc.
The Council of the National Academy of Sciences

The 1997 Annual Fund of the National Academy of Sciences,
whose donors include:

NAS members and other science-interested individuals.
We also extend special thanks to members of the
Council of State Science Supervisors
and teachers who participated in focus groups and provided guidance
on the development of this document.


In a 1786 letter to a friend, Thomas Jefferson called for "the diffusion of
knowledge among the people. No other sure foundation can be devised for the
preservation of freedom and happiness."1 Jefferson saw clearly what has become
increasingly evident since then: the fortunes of a nation rest on the ability of
its citizens to understand and use information about the world around them.
We are about to enter a century in which the United States will be even more
dependent on science and technology than it has been in the past. Such a future
demands a citizenry able to use many of the same skills that scientists use in
their work—close observation, careful reasoning, and creative thinking based on
what is known about the world.

The ability to use scientific knowledge and ways of thinking depends to a
considerable extent on the education that people receive from kindergarten
through high school. Yet the teaching of science in the nation's public schools
often is marred by a serious omission. Many students receive little or no
exposure to the most important concept in modern biology, a concept essential to
understanding key aspects of living things—biological evolution. People and
groups opposed to the teaching of evolution in the public schools have pressed
teachers and administrators to present ideas that conflict with evolution or to
teach evolution as a "theory, not a fact." They have persuaded some textbook
publishers to downplay or eliminate treatments of evolution and have championed
legislation and policies at the state and local levels meant to discourage the
teaching of evolution.

These pressures have contributed to widespread misconceptions about the state of
biological understanding and about what is and is not science. Fewer than
one-half of American adults believe that humans evolved from earlier species.2
More than one half of Americans say that they would like to have creationism
taught in public school classrooms3—even though the Supreme Court has ruled that
"creation science" is a religious idea and that its teaching cannot be mandated
in the public schools.4

The widespread misunderstandings about evolution and the conviction that
creationism should be taught in science classes are of great concern to the
National Academy of Sciences, a private nonpartisan group of 1,800 scientists
dedicated to the use of science and technology for the general welfare. The
Academy and its affiliated institutions—the National Academy of Engineering, the
Institute of Medicine, and the National Research Council—have all sought to
counter misinformation about evolution because of the enormous body of data
supporting evolution and because of the importance of evolution as a central
concept in understanding our planet.

The document that you are about to read is addressed to several groups at the
center of the ongoing debate over evolution: the teachers, other educators, and
policy makers who design, deliver, and oversee classroom instruction in biology.
It summarizes the overwhelming observational evidence for evolution and suggests
effective ways of teaching the subject. It explains the nature of science and
describes how science differs from other human endeavors. It provides answers to
frequently asked questions about evolution and the nature of science and offers
guidance on how to analyze and select teaching materials.

This publication does not attempt specifically to refute the ideas proffered by
those who oppose the teaching of evolution in public schools. A related
document, Science and Creationism: A View from the National Academy of Sciences,
discusses evolution and "creation science" in detail.5 This publication instead
provides information and resources that teachers and administrators can use to
inform themselves, their students, parents, and others about evolution and the
role of science in human affairs.

One source of resistance to the teaching of evolution is the belief that
evolution conflicts with religious principles. But accepting evolution as an
accurate description of the history of life on earth does not mean rejecting
religion. On the contrary, most religious communities do not hold that the
concept of evolution is at odds with their descriptions of creation and human

Nevertheless, religious faith and scientific knowledge, which are both useful
and important, are different. This publication is designed to help ensure that
students receive an education in the sciences that reflects this distinction.
The book is divided into seven chapters and five appendices, plus three
interspersed "dialogues" in which several fictional teachers discuss the
implications of the ideas discussed in the book.

Chapter 1, "Why Teach Evolution," introduces the basic concepts of
evolutionary theory and provides scientific definitions of several common
terms, such as "theory" and "fact," used throughout the book.

The first dialogue, "The Challenge to Teachers," follows the conversation of
three teachers as they discuss some of the problems that can arise in teaching
evolution and the nature of science.

Chapter 2, "Major Themes in Evolution," provides a general overview of
evolutionary processes, describes the evidence supporting evolution, and shows
how evolutionary theory is related to other areas of biology.

The second dialogue, "Teaching About the Nature of Science," follows the three
teachers as they engage in a teaching exercise designed to demonstrate several
prominent features of science.

Chapter 3, "Evolution and the Nature of Science," uses several scientific
theories, including evolution, to highlight important characteristics of
scientific endeavors.

The third dialogue, "Teaching Evolution Through Inquiry," presents a teacher
using an exercise designed to interest and educate her students in fossils and
the mechanisms of evolution.

Chapter 4, "Evolution and the National Science Education Standards," begins by
describing the recent efforts to specify what students should know and be able
to do as a result of their education in the sciences. It then reproduces
sections from the 1996 National Science Education Standards released by the
National Research Council that relate to biological evolution and the nature
and history of science.

Chapter 5, "Frequently Asked Questions About Evolution and the Nature of
Science," gives short answers to some of the questions asked most frequently
by students, parents, educators, and others.

Chapter 6, "Activities for Teaching About Evolution and the Nature of
Science," provides eight sample activities that teachers can use to develop
students' understanding of evolution and scientific inquiry.

Chapter 7, "Selecting Instructional Materials," lays out criteria that can be
used to evaluate school science programs and the content and design of
instructional materials.

The appendices summarize significant court decisions regarding evolution and
creationism issues, reproduce statements from a number of organizations
regarding the teaching of evolution, provide references for further reading
and other resources, and conclude with a list of reviewers.

Teaching About Evolution and the Nature of Science was produced by the Working
Group on Teaching Evolution under the Council of the National Academy of
Sciences. The Working Group consists of 13 scientists and educators who have
been extensively involved in research and education on evolution and related
scientific subjects. The group worked closely with teachers, school
administrators, state officials, and others in preparing this publication,
soliciting suggestions for what would be most useful, and responding to comments
on draft materials. We welcome additional input and guidance from readers that
we can incorporate into future versions of this publication. Please visit our
World Wide Web site at www4.nas.edu/opus/evolve.nsf for additional information.


1. Thomas Jefferson, To George Wythe, "Crusade Against Ignorance" in Thomas
Jefferson on Education, ed. Gordon C. Lee. 1961. New York: Teachers College
Press, pp. 99-100.
2. National Science Board. 1996. Science and Engineering Indicators—1996.
Washington, DC: U.S. Government Printing Office.
3. Gallup Poll, News Release, May 24, 1996.
4. In the 1987 case Edwards v. Aguillard, the U.S. Supreme Court reaffirmed
the 1982 decision of a federal district court that the teaching of "creation
science" in public schools violates the First Amendment of the U.S.
5. National Academy of Sciences. (in press). Science and Creationism: A View
from the National Academy of Sciences. Washington, DC: National Academy Press.

Chapter 1

Why Teach Evolution?

Why is it so important to teach evolution? After all, many questions in biology
can be answered without mentioning evolution: How do birds fly? How can certain
plants grow in the desert? Why do children resemble their parents? Each of these
questions has an immediate answer involving aerodynamics, the storage and use of
water by plants, or the mechanisms of heredity. Students ask about such things
all the time.

The answers to these questions often raise deeper questions that are sometimes
asked by students: How did things come to be that way? What is the advantage to
birds of flying? How did desert plants come to differ from others? How did an
individual organism come to have its particular genetic endowment? Answering
questions like these requires a historical context—a framework of understanding
that recognizes change through time.

People who study nature closely have always asked these kinds of questions. Over
time, two observations have proved to be especially perplexing. The older of
these has to do with the diversity of life: Why are there so many different
kinds of plants and animals? The more we explore the world, the more impressed
we are with the multiplicity of kinds of organisms. In the mid-nineteenth
century, when Charles Darwin was writing On the Origin of Species, naturalists
recognized several tens of thousands of different plant and animal species. By
the middle of the twentieth century, biologists had paid more attention to less
conspicuous forms of life, from insects to microorganisms, and the estimate was
up to 1 or 2 million. Since then, investigations in tropical rain forests—the
center of much of the world's biological diversity—have multiplied those
estimates at least tenfold. What process has created this extraordinary variety
of life?

The second question involves the inverse of life's diversity. How can the
similarities among organisms be explained? Humans have always noticed the
similarities among closely related species, but it gradually became apparent
that even distantly related species share many anatomical and functional
characteristics. The bones in a whale's front flippers are arranged in much the
same way as the bones in our own arms. As organisms grow from fertilized egg
cells into embryos, they pass through many similar developmental stages.
Furthermore, as paleontologists studied the fossil record, they discovered
countless extinct species that are clearly related in various ways to organisms
living today.

This question has emerged with even greater force as modern experimental biology
has focused on processes at the cellular and molecular level. From bacteria to
yeast to mice to humans, all living things use the same biochemical machinery to
carry out the basic processes of life. Many of the proteins that make up cells
and catalyze chemical reactions in the body are virtually identical across
species. Certain human genes that code for proteins differ little from the
corresponding genes in fruit flies, mice, and primates. All living things use
the same biochemical system to pass genetic information from one generation to

From a scientific standpoint, there is one compelling answer to questions about
life's commonalities. Different kinds of organisms share so many characteristics
of structure and function because they are related to one another. But how?
Solving the Puzzle

Investigations of forest
ecosystems have helped
reveal the incredible
diversity of earth's
living things.

The concept of biological evolution addresses both of these fundamental
questions. It accounts for the relatedness among organisms by explaining that
the millions of different species of plants, animals, and microorganisms that
live on earth today are related by descent from common ancestors—like distant
cousins. Organisms in nature typically produce more offspring than can survive
and reproduce given the constraints of food, space, and other resources in the
environment. These offspring often differ from one another in ways that are
heritable—that is, they can pass on the differences genetically to their own
offspring. If competing offspring have traits that are advantageous in a given
environment, they will survive and pass on those traits. As differences continue
to accumulate over generations, populations of organisms diverge from their

This straightforward process, which is a natural consequence of biologically
reproducing organisms competing for limited resources, is responsible for one of
the most magnificent chronicles known to science. Over billions of years, it has
led the earliest organisms on earth to diversify into all of the plants,
animals, and microorganisms that exist today. Though humans, fish, and bacteria
would seem to be so different as to defy comparison, they all share some of the
characteristics of their common ancestors.

Evolution also explains the great diversity of modern species. Populations of
organisms with characteristics enabling them to occupy ecological niches not
occupied by similar organisms have a greater chance of surviving. Over time—as
the next chapter discusses in more detail—species have diversified and have
occupied more and more ecological niches to take advantage of new resources.

Living fish and fossil fish share many similarities, but
the fossil fish clearly belongs to a different species that
no longer exists. The progression of species found in the
fossil record provides powerful evidence for evolution.

Evolution explains something else as well. During the billions of years that
life has been on earth, it has played an increasingly important role in altering
the planet's physical environment. For example, the composition of our
atmosphere is partly a consequence of living systems. During photosynthesis,
which is a product of evolution, green plants absorb carbon dioxide and water,
produce organic compounds, and release oxygen. This process has created and
continues to maintain an atmosphere rich in oxygen. Living communities also
profoundly affect weather and the movement of water among the oceans,
atmosphere, and land. Much of the rainfall in the forests of the western Amazon
basin consists of water that has already made one or more recent trips through a
living plant. In addition, plants and soil microorganisms exert important
controls over global temperature by absorbing or emitting "greenhouse gases"
(such as carbon dioxide and methane) that increase the earth's capacity to
retain heat.

Living things have altered the earth's oceans, land surfaces, and
atmosphere. For example,
photosynthetic organisms are responsible for the oxygen that makes up
about a fifth of the
earth's atmosphere. The rapid accumulation of atmospheric oxygen about 2
billion years ago
led to the evolution of more structured eucaryotic cells, which in turn
gave rise to multicellular
plants and animals.

In short, biological evolution accounts for three of the most fundamental
features of the world around us: the similarities among living things, the
diversity of life, and many features of the physical world we inhabit.
Explanations of these phenomena in terms of evolution draw on results from
physics, chemistry, geology, many areas of biology, and other sciences. Thus,
evolution is the central organizing principle that biologists use to understand
the world. To teach biology without explaining evolution deprives students of a
powerful concept that brings great order and coherence to our understanding of

The teaching of evolution also has great practical value for students. Directly
or indirectly, evolutionary biology has made many contributions to society.
Evolution explains why many human pathogens have been developing resistance to
formerly effective drugs and suggests ways of confronting this increasingly
serious problem (this issue is discussed in greater detail in Chapter 2).
Evolutionary biology has also contributed to many important agricultural
advances by explaining the relationships among wild and domesticated plants and
animals and their natural enemies. An understanding of evolution has been
essential in finding and using natural resources, such as fossil fuels, and it
will be indispensable as human societies strive to establish sustainable
relationships with the natural environment.

Such examples can be multiplied many times. Evolutionary research is one of the
most active fields of biology today, and discoveries with important practical
applications occur on a regular basis.

Those who oppose the teaching of evolution in public schools sometimes ask that
teachers present "the evidence against evolution." However, there is no debate
within the scientific community over whether evolution occurred, and there is no
evidence that evolution has not occurred. Some of the details of how evolution
occurs are still being investigated. But scientists continue to debate only the
particular mechanisms that result in evolution, not the overall accuracy of
evolution as the explanation of life's history.

Evolution and the Nature of Science

Teaching about evolution has another important function. Because some people see
evolution as conflicting with widely held beliefs, the teaching of evolution
offers educators a superb opportunity to illuminate the nature of science and to
differentiate science from other forms of human endeavor and understanding.
Chapter 3 describes the nature of science in detail. However, it is important
from the outset to understand how the meanings of certain key words in science
differ from the way that those words are used in everyday life.

Glossary of Terms Used in Teaching About the Nature of Science
Fact: In science, an observation that has been repeatedly confirmed.
Law: A descriptive generalization about how some aspect of the natural
world behaves under stated circumstances.
Hypothesis: A testable statement about the natural world that can be
used to build more complex inferences and explanations.
Theory: In science, a well-substantiated explanation of some aspect of
the natural world that can incorporate facts, laws, inferences, and
tested hypotheses.

Think, for example, of how people usually use the word "theory." Someone might
refer to an idea and then add, "But that's only a theory." Or someone might
preface a remark by saying, "My theory is . . . ." In common usage, theory often
means "guess" or "hunch."

In science, the word "theory" means something quite different. It refers to an
overarching explanation that has been well substantiated. Science has many other
powerful theories besides evolution. Cell theory says that all living things are
composed of cells. The heliocentric theory says that the earth revolves around
the sun rather than vice versa. Such concepts are supported by such abundant
observational and experimental evidence that they are no longer questioned in

Sometimes scientists themselves use the word "theory" loosely and apply it to
tentative explanations that lack well-established evidence. But it is important
to distinguish these casual uses of the word "theory" with its use to describe
concepts such as evolution that are supported by overwhelming evidence.
Scientists might wish that they had a word other than "theory" to apply to such
enduring explanations of the natural world, but the term is too deeply engrained
in science to be discarded.

As with all scientific knowledge, a theory can be refined or even replaced by an
alternative theory in light of new and compelling evidence. For example, Chapter
3 describes how the geocentric theory that the sun revolves around the earth was
replaced by the heliocentric theory of the earth's rotation on its axis and
revolution around the sun. However, ideas are not referred to as "theories" in
science unless they are supported by bodies of evidence that make their
subsequent abandonment very unlikely. When a theory is supported by as much
evidence as evolution, it is held with a very high degree of confidence.
In science, the word "hypothesis" conveys the tentativeness inherent in the
common use of the word "theory." A hypothesis is a testable statement about the
natural world. Through experiment and observation, hypotheses can be supported
or rejected. As the earliest level of understanding, hypotheses can be used to
construct more complex inferences and explanations.

Like "theory," the word "fact" has a different meaning in science than it does
in common usage. A scientific fact is an observation that has been confirmed
over and over. However, observations are gathered by our senses, which can never
be trusted entirely. Observations also can change with better technologies or
with better ways of looking at data. For example, it was held as a scientific
fact for many years that human cells have 24 pairs of chromosomes, until
improved techniques of microscopy revealed that they actually have 23.
Ironically, facts in science often are more susceptible to change than
theories—which is one reason why the word "fact" is not much used in science.
Finally, "laws" in science are typically descriptions of how the physical world
behaves under certain circumstances. For example, the laws of motion describe
how objects move when subjected to certain forces. These laws can be very useful
in supporting hypotheses and theories, but like all elements of science they can
be altered with new information and observations.

Scientists examining the
head of Chasmosaurus
mariscalensis hone their
understanding of nature
by comparing it against
observations of the world.
Clockwise from upper left:
Prof. Paul Sereno, Univ.
of Chicago; assistant
Cathy Forster, Univ. of
Chicago; students Hilary
Tindle and Tom Evans, who
discovered the skull in the
field in March 1991 in Big
Bend National Park, Texas.

Those who oppose the teaching of evolution often say that evolution should be
taught as a "theory, not as a fact." This statement confuses the common use of
these words with the scientific use. In science, theories do not turn into facts
through the accumulation of evidence. Rather, theories are the end points of
science. They are understandings that develop from extensive observation,
experimentation, and creative reflection. They incorporate a large body of
scientific facts, laws, tested hypotheses, and logical inferences. In this
sense, evolution is one of the strongest and most useful scientific theories we

Evolution and Everyday Life

The concept of evolution has an importance in education that goes beyond its
power as a scientific explanation. All of us live in a world where the pace of
change is accelerating. Today's children will face more new experiences and
different conditions than their parents or teachers have had to face in their

The story of evolution is one chapter—perhaps the most important one—in a
scientific revolution that has occupied much of the past four centuries. The
central feature of this revolution has been the abandonment of one notion about
stability after another: that the earth was the center of the universe, that the
world's living things are unchangeable, that the continents of the earth are
held rigidly in place, and so on. Fluidity and change have become central to our
understanding of the world around us. To accept the probability of change—and to
see change as an agent of opportunity rather than as a threat—is a silent
message and challenge in the lesson of evolution.

The following dialogue dramatizes some of the problems educators encounter in
teaching evolution and demonstrates ways of overcoming these obstacles. Chapter
2 returns to the basic themes that characterize evolutionary theory, and Chapter
3 takes a closer look at the nature of science.


The Challenge to Teachers

Teaching evolution presents special challenges to science teachers.
Sources of support upon which teachers can draw include high-quality
curricula, adequate preparation, exposure to information useful in
documenting the evidence for evolution, and resources and contacts
provided by professional associations.
One important source of support for teachers is to share problems and
explore solutions with other teachers. The following vignette
illustrates how a group of teachers—in this case, three biology
teachers at a large public high school—can work together to solve
problems and learn from each other.

It is the first week of classes at Central High School. As the bell rings
for third period, Karen, the newest teacher on the faculty, walks into the
teachers' lounge. She greets her colleagues, Barbara and Doug.
"How are your first few days going?" asks Doug.

"Fine," Karen replies. "The second-period Biology I class is full, but
it'll be okay. By the way, Barbara, thanks for letting me see your
syllabus for Bio I. But I wanted to ask you about teaching evolution—I
didn't see it there."

"You didn't see it on my syllabus because it's not a separate topic,"
Barbara says. "I use evolution as a theme to tie the course together, so
it comes into just about every unit. You'll see a section called 'History
of Life' on the second page, and there's a section called 'Natural
Selection.' But I don't treat evolution separately because it is related
to almost every other topic in biology."1

"Wait a minute, Barbara," Doug says. "Is that good advice for a new
teacher? I mean, evolution is a controversial subject, and a lot of us
just don't get around to teaching it. I don't. You do, but you're braver
than most of us."

"It's not a matter of bravery, Doug," Barbara replies. "It's a matter of
what needs to be taught if we want students to understand biology.
Teaching biology without evolution would be like teaching civics and never
mentioning the United States Constitution."

"But how can you be sure that evolution is all that important. Aren't
there a lot of scientists who don't believe in evolution? Say it's too

"The debate in science is over some of the details of how evolution
occurred, not whether evolution happened or not. A lot of science and
science education organizations have made statements about why it is
important to teach evolution...."2

"I saw a news report when I was a student," Karen interjects, "about a
school district or state that put a disclaimer against evolution in all
their biology textbooks. It said that students didn't need to believe in
evolution because it wasn't a fact, only a theory. The argument was that
no one really knows how life began or how it evolved because no one was
there to see it happen."3

"If I taught evolution, I'd sure teach it as a theory—not a fact," says

"Just like gravity," Barbara says.

"Now, Barbara, gravity is a fact, not a theory."

"Not in scientific terms. The fact is that things fall. The explanation
for why things fall is the theory of gravitation. Our problem is
definitions. You're using 'fact' and 'theory' the way we use them in
everyday life, but we need to use them as scientists use them. In science,
a 'fact' is an observation that has been made so many times that it's
assumed to be okay. How facts are explained is where theories come in:
theories are explanations of what we observe. One place where students get
confused about evolution is that they think of 'theory' as meaning 'guess'
or 'hunch.' But evolution isn't a hunch. It's a scientific explanation,
and a very good one."

"But how good a theory is it?" asks Doug. "We don't know everything about

"That's true," says Karen. "A student in one of my classes at the
university told me that there are big gaps in the fossil record. Do you
know anything about that?"

A fossil of Archaeopteryx,
a bird that lived about 150
million years ago and had
many reptilian characteristics,
was discovered in 1861 and
helped support the hypothesis
of evolution proposed by
Charles Darwin in The Origin
of Species two years earlier.

"Well, there's Archaeopteryx," says Doug. "It's a fossil that has feathers
like a bird but the skeleton of a small dinosaur. It's one of those
missing links that's not missing any more."

"In fact, there are good transitional fossils between primitive fish and
amphibians and between reptiles and mammals," Barbara says. "Our knowledge
of fossil intermediates is actually pretty good.4 And, Doug, it sounds
like you know more about evolution than you're letting on. Why don't you
teach it?"

"I don't want any trouble. Every time I teach evolution, I have a student
announce that 'evolution is against his religion.'"

"But most of the major religious denominations have taken official
positions that accept evolution," says Barbara. "One semester a friend of
mine in the middle school started out her Life Science unit by having her
students interview their ministers or priests or rabbis about their
religion's views on evolution. She said that most of her students came
back really surprised. 'Hey,' they said, 'evolution is okay.' It defused
the controversy in her class."

"She didn't have Stanley in her class," says Doug.

"Who's Stanley?" asks Karen.

"The son of a school board member. Given his family's religious views, I'm
sure he would not come back saying evolution was okay."

"That can be a hard situation," says Barbara. "But even if Stanley came
back to class saying that his religion does not accept evolution, it could
help a teacher show that there are many different religious views about
evolution. That's the point: religious people can still accept evolution."
"Stanley will never believe in evolution."

"We talk about 'believing' in evolution, but that's not necessarily the
right word. We accept evolution as the best scientific explanation for a
lot of observations—about fossils and biochemistry and evolutionary
changes we can actually see, like how bacteria become resistant to certain
medicines. That's why people accepted the idea that the earth goes around
the sun—because it accounted for many different observations that we make.
In science, when a better explanation comes around, it replaces earlier

"Does that mean that evolution will be replaced by a better theory some
day?" asks Karen.

"It's not likely. Not all old theories are replaced, and evolution has
been tested and has a lot of evidence to support it. The point is that
doing science requires being willing to refine our theories to be
consistent with new information."

"But there's still Stanley," says Doug. "He doesn't even want to hear
about evolution."

"I had Stanley's sister in AP biology one year," Barbara replies. "She
raised a fuss about evolution, and I told her that I wasn't going to grade
her on her opinion of evolution but on her knowledge of the facts and
concepts. She seemed satisfied with that and actually got an A in the

"I still think that if you teach evolution, it's only fair to teach both."
"What do you mean by both?" asks Barbara. "If you mean both evolution and
creationism, what kind of creationism do you want to teach? Will you teach
evolution and the Bible? What about other religions like Buddhism or the
views of Native Americans? It's hard to argue for 'both' evolution and
creationism when people have many different ideas about creation."
"I can't teach a whole bunch of creation stories in my Bio class," says

"That's the point. We can't add subjects to the science curriculum to be
fair to groups that hold certain beliefs. Teaching ecology isn't fair to
the polluter, either. Biology is a science class, and what should be
taught is science."

"But isn't there something called 'creation science'?" asks Karen. "Can
creationism be made scientific?"

"That's an interesting story. 'Creation science' is the idea that
scientific evidence can support a literal interpretation of Genesis—that
the whole universe was created all at once about 10,000 years ago."
"It doesn't sound very likely."

"It's not. Scientists have looked at the arguments and have found they are
not supported by verifiable data. Still, back in the early 1980s, some
states passed laws requiring that 'creation science' be taught whenever
evolution was taught. But the Supreme Court threw out 'equal time' laws,
saying that because creationism was inherently a religious and not a
scientific idea, it couldn't be presented as 'truth' in science classes in
the public schools."5

"Well, I'm willing to teach evolution," says Karen, "and I'd like to try
it your way, Barbara, as a theme that ties biology together. But I really
don't know enough about evolution to do it. Do you have any suggestions
about where I can get information?"

"Sure, I'd be glad to share what I have. But an important part of teaching
evolution has to do with explaining the nature of science. I'm trying out
a demonstration after school today that I'm going to use with my Bio I
class tomorrow. Why don't you both come by and we can try it out?"
"Okay," say Karen and Doug. "We'll see you then."

Barbara, Doug, and Karen's discussion of evolution and the nature of
science resumes following Chapter 2.


The National Science Education Standards cite "evolution and
equilibrium" as one of five central concepts that unify all of the
sciences. (See www.nap.edu/readingroom/books/nses)
Appendix C contains statements from science and science education
organizations that support the need to teach evolution.
In 1995, the Alabama board of education ordered that all biology
textbooks in public schools carry inserts that read, in part, as
follows: "This textbook discusses evolution, a controversial theory some
scientists present as a scientific explanation for the origin of living
things, such as plants, animals, and humans. No one was present when
life first appeared on earth. Therefore, any statement about life's
origins should be considered theory, not fact." Other districts have
required similar disclaimers.

The book From So Simple a Beginning: The Book of Evolution by Philip
Whitfield (New York: Macmillan, 1993) presents a well-illustrated
overview of evolutionary history. Evolution by Monroe W. Strickberger
(Boston: Jones and Bartlett, 2nd edition, 1995) is a thorough text
written at the undergraduate level.

In the 1987 case Edwards v. Aguillard, the U.S. Supreme Court reaffirmed
the 1982 decision of a federal district court that the teaching of
"creation science" in public schools violates the First Amendment of the
U.S. Constitution.

Chapter 2

Major Themes in Evolution

The world around us changes. This simple fact is obvious everywhere we look.
Streams wash dirt and stones from higher places to lower places. Untended
gardens fill with weeds.

Other changes are more gradual but much more dramatic when viewed over long time
scales. Powerful telescopes reveal new stars coalescing from galactic dust, just
as our sun did more than 4.5 billion years ago. The earth itself formed shortly
thereafter, when rock, dust, and gas circling the sun condensed into the planets
of our solar system. Fossils of primitive microorganisms show that life had
emerged on earth by about 3.8 billion years ago.

Similarly, the fossil record reveals profound changes in the kinds of living
things that have inhabited our planet over its long history. Trilobites that
populated the seas hundreds of millions of years ago no longer crawl about.
Mammals now live in a world that was once dominated by reptilian giants such as
Tyrannosaurus rex. More than 99 percent of the species that have ever lived on
the earth are now extinct, either because all of the members of the species
died, the species evolved into a new species, or it split into two or more new

The Hubble Space Telescope
has revealed many astronomical
phenomena that ground-based
telescopes cannot see. The
images at right show disks of
matter around young stars
that could give rise to planets.
In the image below, stars are
forming in the tendrils of gas
and dust extending from a
gigantic nebula.

Many kinds of cumulative change through time have been described by the term
"evolution," and the term is used in astronomy, geology, biology, anthropology,
and other sciences. This document focuses on the changes in living things during
the long history of life on earth—on what is called biological evolution. The
ancient Greeks were already speculating about the origins of life and changes in
species over time. More than 2,500 years ago, the Greek philosopher Anaximander
thought that a gradual evolution had created the world's organic coherence from
a formless condition, and he had a fairly modern view of the transformation of
aquatic species into terrestrial ones. Following the rise of Christianity,
Westerners generally accepted the explanation provided in Genesis, the first
book of the Judeo-Christian-Muslim Bible, that God created everything in its
present form over the course of six days. However, other explanations existed
even then. Among Christian theologians, for example, Saint Thomas Aquinas (1225
to 1274) stated that the earth had received the power to produce organisms and
criticized the idea that species had originated in accordance with the
timetables in Genesis.1

Charles Darwin (1809-1882)

Charles Darwin, Alfred Russel Wallace, and Gregor Mendel laid the
foundations of modern evolutionary theory.

During the early 1800s, many naturalists speculated about changes in organisms,
especially as geological investigations revealed the rich story laid out in the
fossilized remains of extinct creatures. But although ideas about evolution were
proposed, they never gained wide acceptance because no one was able to propose a
plausible mechanism for how the form of an organism might change from one
generation to another. Then, in 1858, two English naturalists—Charles Darwin and
Alfred Russel Wallace—simultaneously issued papers proposing such a mechanism.
Both men observed that the individual members of a particular species are not
identical but can differ in many ways. For example, some will be able to run a
little faster, have a different color, or respond to the same circumstance in
different ways. (Humans— including any class of high school students—have many
such differences.) Both men further observed that many of these differences are
inherited and can be passed on to offspring. This conclusion was evident from
the experiences of plant and animal breeders.

Alfred Russel Wallace (1823-1913)

Darwin and Wallace were both deeply influenced by the realization that, even
though most species produce an abundance of offspring, the size of the overall
population usually remains about the same. Thus, an oak tree might produce many
thousands of acorns each year, but few, if any, will survive to become
full-grown trees.

Darwin—who conceived of his ideas in the 1830s but did not publish them until
Wallace came to similar conclusions—presented the case for evolution in detail
in his 1859 book On the Origin of Species by Natural Selection. Darwin proposed
that there will be differences between offspring that survive and reproduce and
those that do not. In particular, individuals that have heritable
characteristics making them more likely to survive and reproduce in their
particular environment will, on average, have a better chance of passing those
characteristics on to their own offspring. In this way, as many generations
pass, nature would select those individuals best suited to particular
environments, a process Darwin called natural selection. Over very long times,
Darwin argued, natural selection acting on varying individuals within a
population of organisms could account for all of the great variety of organisms
we see today, as well as for the species found as fossils.

Gregor Mendel (1822-1884)

If the central requirement of natural selection is variation within populations,
what is the ultimate source of this variation? This problem plagued Darwin, and
he never found the answer, although he proposed some hypotheses. Darwin did not
know that a contemporary, Gregor Mendel, had provided an important part of the
solution. In his classic 1865 paper describing crossbreeding of varieties of
peas, Mendel demonstrated that organisms acquire traits through discrete units
of heredity which later came to be known as genes. The variation produced
through these inherited traits is the raw material on which natural selection

Mendel's paper was all but forgotten until 1890, when it was rediscovered and
contributed to a growing wave of interest and research in genetics. But it was
not immediately clear how to reconcile new findings about the mechanisms of
inheritance with evolution through natural selection. Then, in the 1930s, a
group of biologists demonstrated how the results of genetics research could both
buttress and extend evolutionary theory. They showed that all variations, both
slight and dramatic, arose through changes, or mutations, in genes. If a
mutation enabled an organism to survive or reproduce more effectively, that
mutation would tend to be preserved and spread in a population through natural
selection. Evolution was thus seen to depend both on genetic mutations and on
natural selection. Mutations provided abundant genetic variation, and natural
selection sorted out the useful changes from the deleterious ones.

Selection by natural processes of favored variants explained many observations
on the geography of species differences—why, for example, members of the same
bird species might be larger and darker in the northern part of their range, and
smaller and paler in the southern part. In this case, differences might be
explained by the advantages of large size and dark coloration in forested, cold
regions. And, if the species occupied the entire range continuously, genes
favoring light color and small size would be able to flow into the northern
population, and vice versa—prohibiting their separation into distinct species
that are reproductively isolated from one another.

Glossary of Terms Used in Teaching About Evolution
Evolution: Change in the hereditary characteristics of groups of
organisms over the course of generations. (Darwin referred to this
process as "descent with modification.")
Species: In general, a group of organisms that can potentially breed
with each other to produce fertile offspring and cannot breed with the
members of other such groups.
Variation: Genetically determined differences in the characteristics of
members of the same species.
Natural selection: Greater reproductive success among particular members
of a species arising from genetically determined characteristics that
confer an advantage in a particular environment.

How new species are formed was a mystery that eluded biologists until
information about genetics and the geographical distribution of animals and
plants could be put together. As a result, it became clear that the most
important source of new species is the process of geographical isolation—through
which barriers to gene flow can be created. In the earlier example, the
interposition of a major mountain barrier, or the origin of an intermediate
desert, might create the needed isolation.

Other situations also encourage the formation of new species. Consider fish in a
river that, over time, changes course so as to isolate a tributary. Or think of
a set of oceanic islands, distant from the mainland and just far enough from one
another that interchange among their populations is rare. These are ideal
circumstances for creating reproductive barriers and allowing populations of the
same species to diverge from one another under the influence of natural
selection. After a time, the species become sufficiently different that even
when reunited they remain reproductively isolated. They have become so different
that they are unable to interbreed.

In the 1950s, the study of evolution entered a new phase. Biologists began to be
able to determine the exact molecular structure of the proteins in living
things—that is, the actual sequences of the amino acids that make up each
protein. Almost immediately, it became clear that certain proteins that serve
the same function in different species have very similar amino acid sequences.
The protein evidence was completely consistent with the idea of a common
evolutionary history for the planet's living things. Even more important, this
knowledge provided important clues about the history of evolution that could not
be obtained through the fossil record.

Discovery of a Missing Link

As a zoologist I have discovered many phenomena that can be rationally
explained only as products of evolution, but none so striking as the
ancestor of the ants. Prior to 1967 the fossil record had yielded no
specimens of wasps or other Hymenopterous insects that might be
interpreted as the ancestors of the ants. This hypothetical form was a
missing link of major importance in the study of evolution. We did have
many fossils of ants dating back 50 million years. These were different
species from those existing today, but their bodies still possessed the
basic body form of modern ants. The missing link of ant evolution was
often cited by creationists as evidence against evolution. Other ant
specialists and I were convinced that the linking fossils would be
found, and that most likely they would be associated with the late
Mesozoic era, a time when many dinosaur and other vertebrate bones were
fossilized but few insects. And that is exactly what happened. In 1967 I
had the pleasure of studying two specimens collected in amber
(fossilized resin) from New Jersey, and dating to the late Mesozoic
about 90 million years ago. They were nearly exact intermediates between
solitary wasps and the highly social modern ants, and so I gave them the
scientific name Sphecomyrma, meaning "wasp ant." Since that time many
more Sphecomyrma specimens of similar age have been found in the United
States, Canada, and Siberia, but none belonging to the modern type. With
each passing year, such fossils and other kinds of evidence tighten our
conception of the evolutionary origin of this important group of insects.

—Edward O. Wilson

The discovery of the structure of DNA by Francis Crick and James Watson in 1953
extended the study of evolution to the most fundamental level. The sequence of
the chemical bases in DNA both specifies the order of amino acids in proteins
and determines which proteins are synthesized in which cells. In this way, DNA
is the ultimate source of both change and continuity in evolution. The
modification of DNA through occasional changes or rearrangements in the base
sequences underlies the emergence of new traits, and thus of new species, in
evolution. At the same time, all organisms use the same molecular codes to
translate DNA base sequences into protein amino acid sequences. This uniformity
in the genetic code is powerful evidence for the interrelatedness of living
things, suggesting that all organisms presently alive share a common ancestor
that can be traced back to the origins of life on earth.

One common misconception among students is that individual organisms change
their characteristics in response to the environment. In other words, students
often think that the environment acts on individual organisms to generate
physical characteristics that can then be passed on genetically to offspring.
But selection can work only on the genetic variation that already is present in
any new generation, and genetic variation occurs randomly, not in response to
the needs of a population or organism. In this sense, as Francois Jacob has
written, evolution is a "tinkerer, not an engineer."2 Evolution does not design
new organisms; rather, new organisms emerge from the inherent genetic variation
that occurs in organisms.

Genetic variation is random, but natural selection is not. Natural selection
tests the combinations of genes represented in the members of a species and
allows to proliferate those that confer the greatest ability to survive and
reproduce. In this sense, evolution is not the simple product of random chance.
The booklet Science and Creationism: A View from the National Academy of
Sciences3 summarizes several compelling lines of evidence that demonstrate
beyond any reasonable doubt that evolution occurred as a historical process and
continues today. In brief:

Fossils found in rocks of increasing age attest to the interrelated lineage of
living things, from the single-celled organisms that lived billions of years
ago to Homo sapiens. The most recent fossils closely resemble the organisms
alive today, whereas increasingly older fossils are progressively different,
providing compelling evidence of change through time.

Even a casual look at different kinds of organisms reveals striking
similarities among species, and anatomists have discovered that these
similarities are more than skin deep. All vertebrates, for example, from fish
to humans, have a common body plan characterized by a segmented body and a
hollow main nerve cord along the back. The best available scientific
explanation for these common structures is that all vertebrates are descended
from a common ancestor species and that they have diverged through evolution.
In the past, evolutionary relationships could be studied only by examining the
consequences of genetic information, such as the anatomy, physiology, and
embryology of living organisms. But the advent of molecular biology has made
it possible to read the history of evolution that is written in every
organism's DNA. This information has allowed organisms to be placed into a
common evolutionary family tree in a much more detailed way than possible from
previous evidence. For example, as described in Chapter 3, comparisons of the
differences in DNA sequences among organisms provides evidence for many
evolutionary events that cannot be found in the fossil record.

Evolution is the only plausible scientific explanation that accounts for the
extensive array of observations summarized above. The concept of evolution
through random genetic variation and natural selection makes sense of what would
otherwise be a huge body of unconnected observations. It is no longer possible
to sustain scientifically the view that the living things we see today did not
evolve from earlier forms or that the human species was not produced by the same
evolutionary mechanisms that apply to the rest of the living world.
The following two sections of this chapter examine two important themes in
evolutionary theory. The first concerns the occurrence of evolution in "real
time"—how changes come about and result in new kinds of species. The second is
the ecological framework that underlies evolution, which is needed to understand
the expansion of biological diversity.

Evolution as a Contemporary Process

Evolution by natural selection is not only a historical process—it still
operates today. For example, the continual evolution of human pathogens has come
to pose one of the most serious public health problems now facing human
societies. Many strains of bacteria have become increasingly resistant to
once-effective antibiotics as natural selection has amplified resistant strains
that arose through naturally occurring genetic variation. The microorganisms
that cause malaria, gonorrhea, tuberculosis, and many other diseases have
demonstrated greatly increased resistance to the antibiotics and other drugs
used to treat them in the past. The continued use and overuse of antibiotics has
had the effect of selecting for resistant populations because the antibiotics
give these strains an advantage over nonresistant strains.4

Similar episodes of rapid evolution are occurring in many different organisms.
Rats have developed resistance to the poison warfarin. Many hundreds of insect
species and other agricultural pests have evolved resistance to the pesticides
used to combat them—and even to chemical defenses genetically engineered into
plants. Species of plants have evolved tolerance to toxic metals and have
reduced their interbreeding with nearby nontolerant plants—an initial step in
the formation of separate species. New species of plants have arisen through the
crossbreeding of native plants with plants introduced from elsewhere in the

The North American species Chrysoperla carnea and Chrysoperla downesi
separated from a common ancestor species recently in evolutionary time and
are very similar. But they are different in color, reflecting their
different habitats,
and they breed at different times of the year.

The creation of a new species from a pre-existing species generally requires
thousands of years, so over a lifetime a single human usually can witness only a
tiny part of the speciation process. Yet even that glimpse of evolution at work
powerfully confirms our ideas about the history and mechanisms of evolution. For
example, many closely related species have been identified that split from a
common ancestor very recently in evolutionary terms. An example is provided by
the North American lacewings Chrysoperla carnea and Chrysoperla downesi. The
former lives in deciduous woodlands and is pale green in summer and brown in
winter. The latter lives among evergreen conifers and is dark green all year
round. The two species are genetically and morphologically very similar. Yet
they occupy different habitats and breed at different times of the year and so
are reproductively isolated from each other.

The fossil record also sheds light on speciation. A particularly dramatic
example comes from recently discovered fossil evidence documenting the evolution
of whales and dolphins. The fossil record shows that these cetaceans evolved
from a primitive group of hoofed mammals called Mesonychids. Some of these
mammals crushed and ate turtles, as evidenced by the shape of their teeth. This
mammal gave rise to a species with front forelimbs and powerful hind legs with
large feet that were adapted for paddling. This animal, known as Ambulocetus,
could have moved between sea and land. Its fossilized vertebrae also show that
this animal could move its back in a strong up and down motion, which is the
method modern cetaceans use to swim and dive. A later fossil in the series from
Pakistan shows an animal with smaller functional hind limbs and even greater
back flexibility. This species, Rodhocetus, probably did not venture onto land
very often, if at all. Finally, Basilosaurus fossils from Egypt and the United
States present a recognizable whale, with front flippers for steering and a
completely flexible backbone. But this animal still has hind limbs (thought to
have been nonfunctional), which have become further reduced in modern whales.5

Modern whales evolved from a primitive group of hoofed mammals into
species that were progressively more adapted to life in the water.

Another focus of research has been the evolution of ancient apelike creatures
through many intermediate forms into modern humans. Homo sapiens, one of 185
known living species in the primate order, is a member of the hominoids, a
category that includes orangutans, gorillas, and chimpanzees. The succession of
species that would give rise to humans seems to have separated from the
succession that would lead to the apes about 5 to 8 million years ago. The first
members of our genus, Homo, had evolved by about 1.5 million years ago.
According to recent evidence—based on the sequencing of DNA found in a part of
human cells known as mitochondria—it has been proposed that a small group of
modern humans evolved in Africa about 150,000 years ago and spread throughout
the world, replacing archaic populations of Homo sapiens.

A. afarensis
A. africanus
early Homo
H. erectus
H. sapiens
Early hominids had smaller brains and larger faces than species belonging
to the genus Homo, including our own species, Homo sapiens. White parts of
the skulls are reconstructions, and the skulls are not all on the same

Evolution and Ecology

Animals and plants do not live in isolation, nor do they evolve in isolation.
Indeed, much of the pressure toward diversification comes not only from physical
factors in the environment but from the presence of other species. Any animal is
a potential host for parasites or prey for a carnivore. A plant has other plants
as competitors for space and light, can be a host for parasites, and provides
food for herbivores. The interactions within the complex communities, or
ecosystems, in which organisms live can generate powerful evolutionary forces.

Ongoing Evolution Among Darwin's Finches

A particularly interesting example of contemporary evolution involves
the 13 species of finches studied by Darwin on the Galapagos Islands,
now known as Darwin's finches. A research group led by Peter and
Rosemary Grant of Princeton University has shown that a single year of
drought on the islands can drive evolutionary changes in the finches.6
Drought diminishes supplies of easily cracked nuts but permits the
survival of plants that produce larger, tougher nuts. Drought thus
favors birds with strong, wide beaks that can break these tougher seeds,
producing populations of birds with these traits. The Grants have
estimated that if droughts occur about once every 10 years on the
islands, a new species of finch might arise in only about 200 years.7

Evolution in natural communities arises from both constraints and opportunities.
The constraints come from competitors, primarily among the same species. There
are only so many nest holes for bluebirds and so much food for mice. Genetically
different individuals that are able to move to a different resource—a new food
supply, for example, or a hitherto uninhabited area—are able to exploit that
resource free of competition. As a result, the trait that opened up the new
opportunity will be favored by natural selection because the individuals
possessing it are able to survive and reproduce better than other members of
their species in the new environment.

An ecologist would say that the variant had occupied a new niche—a term that
defines the "job description" of an organism. (For example, a bluebird would
have the niche of insect- and fruit-eater, inhabitant of forest edges and
meadows, tree-hole nester, and so on.) One often finds closely related species
in the same place and occupying what look like identical niches. However, if the
niches were truly identical, one of the species should have a competitive
advantage over the other and eventually drive the less fit species to extinction
or to a different niche. That leads to a tentative hypothesis: where we find
such a situation, careful observation should reveal subtle niche specialization
of the apparently competing species.

This hypothesis has been tested by many biologists. For example, in the 1960s
Robert MacArthur carefully studied three North American warblers of the same
genus that were regularly seen feeding on insects in coniferous trees in the
same areas—indeed, often in the same trees. MacArthur's painstaking observations
revealed that the three were actually specialists: one fed on insects on the
major branches near the trunk; another occupied the mid-regions of branches and
ate from different parts of the foliage; and the third fed on insects occupying
the finest needles near the periphery of the tree. Although the three warblers
occurred together, they were in fact not competitors for the same food

A Chemical Distress Signal

J. H. Tumlinson and colleagues at the U.S. Department of Agriculture's
Research Service Laboratories in Gainesville, Florida, have explored a
fascinating case that illustrates the intricacy of many ecological
relationships. Cotton plants, like many other crops, are attacked by
caterpillars. One destructive cotton pest, the army worm, produces a
complex series of reactions when it feeds on the plant—a reaction that
involves the caterpillar itself, the tissues of the plant, and a third
participant, a wasp that preys on the caterpillar. When the caterpillar
chews on the cotton plant leaf, a reaction occurs that causes the plant
to synthesize and release a class of volatile chemicals that escape into
the air and travel rapidly downwind. The chemicals are detected by
wasps, who follow the scent and are able to find the caterpillars and
deposit eggs within them. The eggs hatch, and the wasp larvae destroy
the caterpillar.8

This complex case of "chemical ecology" required a series of linked
coevolutionary events: the response of the plant to a special signal
from its predator, and the response of the wasp to a special signal from
the host of its prey.

Often, species that are evolving together in the same ecosystem do so through a
highly interactive process. For example, natural selection will favor organisms
with defenses against predation; in turn, predators experience selection for
traits that overcome those defenses. Such coevolutionary competitions are common
in nature. Many plants manufacture and store chemicals that deter herbivorous
insects; but usually one or more insect species will have evolved biochemical
mechanisms for inactivating the deterrent, providing them with a plant they can
eat relatively free of competitors.

Another classic example of coevolution involves the introduction of rabbits and
the myxomatosis virus into Australia. After rabbits were brought to Australia,
they multiplied rapidly and threatened the wool industry because they grazed on
the same plants as sheep. To control the rabbit population, a virulent pathogen
of rabbits, the myxomatosis virus, also was introduced into Australia. Within a
decade, rabbits had become more resistant to the virus, and the virus had
evolved into a less virulent form, allowing both the host and pathogen to


As the examples in this chapter demonstrate, evolutionary biology provides an
extremely active and rich source of new insights into the world. By exploring
the history of life on earth and shedding light on how evolution works,
evolutionary biology is linking fundamental scientific research to knowledge
needed to meet important societal needs, including the preservation of our
environment. Few other ideas in science have had such a far-reaching impact on
our thinking about ourselves and how we relate to the world.


Biological Sciences Curriculum Study. 1978. Biology Teachers' Handbook. 3rd
ed. William V. Mayer, ed. New York: John Wiley and Sons.
Francois Jacob. June 10, 1977. Evolution and tinkering. Science 196:1161-1166.
National Academy of Sciences. (in press). Science and Creationism: A View from
the National Academy of Sciences. Washington, DC: National Academy Press. (See
P. Ewald. 1994. The Evolution of Infectious Disease. New York: Oxford
University Press.
"Evolution, Science, and Society: A White Paper on Behalf of the Field of
Evolutionary Biology," Draft, June 4, 1997.
Jonathan Weiner. 1994. The Beak of the Finch. New York: Alfred A. Knopf.
Peter R. Grant. 1991. Natural selection and Darwin's finches. Scientific
American, October, pp. 82-87.
James H. Tumlinson, W. Joe Lewis, and Louise E. M. Vet. 1993. How parasitic
wasps find their hosts. Scientific American, March, pp. 100-106.
F. Fenner and F.N. Ratcliffe. 1965. Myxomatosis. Cambridge: Cambridge
University Press.


Teaching about the Nature of Science

In the following vignette, Barbara, Doug, and Karen use a model to
continue their discussion of the nature of science and its
implications for the teaching of evolution.

"Thanks for meeting with me this afternoon," Barbara says. "To begin
this demonstration I first need to ask you what you think science is."
"Oh, I had that in college," says Karen. "The scientific method is to
identify a question, gather information about it, develop a hypothesis
that answers the question, and then do an experiment that either proves
or disproves the hypothesis."

"But that was one of my points about evolution," Doug says. "No one was
there when evolution happened and we can't do any experiments about what
happened in the past. So by your definition, Karen, evolution isn't

"Science is a lot more than just supporting or rejecting hypotheses,"
Barbara replies. "It also involves observation, creativity, and
judgment. Here's an activity I use to teach the nature of science."
Barbara takes a cardboard mailing tube about one foot long that has the
ends of four ropes extending from it.

As Barbara tugs on the various ropes one at a time, she has Doug and
Karen make observations of what happens. After three or four pulls, she
asks Karen and Doug to predict what will happen when she pulls on one of
the ropes. Both are able to predict that if Barbara pulls rope A, rope B
will move. Barbara then asks if there are additional manipulations they
would like to see, and she follows their requests.

Barbara then asks Doug and Karen to sketch a model of what is inside the
tube that could explain their observations.

When Karen and Doug show their sketches to each other, they realize that
they have come up with different models. Barbara asks them if they want
to make any changes to their sketches based on the comparison, and both
of them make modifications, although their final models are still

Barbara then gives them their own cardboard tubes and some string and
asks them to build the model they proposed. When their models are built,
Barbara holds up her tube and asks Doug and Karen to follow her actions
with their own models, to see if the two models behave in the same way
as Barbara's tube. But when Barbara pulls string A in her tube, Karen's
model does not work the same way. Karen asks if she can make some
changes in her model, and once she does her new model seems to work the
same way as Barbara's. Doug's model consistently behaves the same way as
Barbara's tube.

"Now wait a minute," Karen says. "What do ropes and tubes have to do
with science and evolution?"

"You might not know it, but what we just did is much of what science is
about. You observed what happened when I pulled these ropes. Then, based
on your initial observations, you made a prediction about what would
happen if we manipulated the system in a specific way. How accurate was
your prediction?"

"We were right," Doug responds.

"And why were you able to predict what would happen before I pulled the

"I used what I observed in the first few pulls to help me predict what
would happen later."

"Basically what each of you did was to speculate about how my tube was
working on the basis of some limited observations. Scientists do that
type of thing all the time. They make observations and try to explain
what's going on, or sometimes they recognize that more than one
explanation fits their data. Then they try out their proposed
explanations by making predictions that they test. At first I had you
draw a picture of how you thought my tube worked and had you each
explain your picture. You got to hear each other's view on how the
system worked. Doug, did you change your ideas at all based on what you
heard from Karen?"

"Well, yes. I first thought that ropes A and C were the two ends of the
same rope and B and D were two ends of another rope. Karen had A and B
as ends of the same rope and C and D as ends of another rope, and her
explanation seemed to fit better than mine."

"Right. Communication about observations and interpretations is very
important among scientists because different scientists may interpret
data in different ways. Hearing someone else's views can help a
scientist revise his or her interpretation. In essence that was what you
were doing when you shared your diagrams. Karen, when your model didn't
work, what did you do?"

"All I did was adjust the length of one rope, and then it worked fine."
"So as a result of your formal testing of the predictions from your
model, you revised your explanation of the system. Your understanding
improved. In scientific terms, you revised your model to make it more
consistent with your further observations. In science, the validity of
any explanation is determined by its coherence with observations in the
natural world and by its ability to predict further observations."

"But we still have different models," Karen observes. "How do we know
which one is right?"

Doug says: "You told us that, didn't you, Barbara. There can be two
possible explanations for the same observation."

"So it's possible for scientists to disagree sometimes," says Karen.
"But does that mean that we don't understand evolution because
scientists disagree about how evolution takes place?"

"Not at all," Barbara answers, "you both created different models of my
tube, but both of your models are fairly accurate. And don't forget
there were constraints on the possible models you could create that
would be consistent with the data. Just any explanation would not be
acceptable. In evolution, there are some things we know could not have
happened, just as we are confident that some things have happened."
"And if different scientists can have different explanations, like Karen
and I did, then I guess science also has to involve judgment to some
extent," Doug says.

"But I thought scientists were supposed to be totally objective," says

"Good science always attempts to be objective, but it also relies on the
individual insights of scientists. And the questions they choose to ask
as well as the methods they choose to use, not to mention the
interpretations they may have, can be colored by their individual
interests and backgrounds. But scientific explanations are reviewed by
other scientists and must be consistent with the natural world and
future experiments, so there are checks on subjectivity. What we read in
science books is a combination of observations and inferred explanations
of those observations that can change with new research."

"Still, I'm wondering," says Karen, "how can we find out which model is

"Let's just open up Barbara's tube," says Doug.

"We could do that," Barbara says. "But let's assume in this analogy that
opening the tube is not possible. Sometimes scientists figure out how to
open up the natural world and look inside, but sometimes they can't. And
not opening up the tube is a good metaphor for how science often works.
Science involves coming up with explanations that are based on evidence.
With time, additional evidence might require changing the explanations,
so that at any time what we have is the best explanation possible for
how things work. In the future, with additional data, we may change our
original explanation—just like you did, Karen.

"Remember when we were talking this morning about evolution being fact
or theory? That conversation is very relevant to what we have been doing
with the tubes. As scientists started to notice patterns in nature, they
began to speculate about some explanations for these patterns. These
explanations are analogous to your initial ideas about how my tube
worked. In the terms of science, these initial ideas are called
hypotheses. You noticed some patterns in how the ropes were related to
each other, and you used these patterns to develop a model to explain
the patterns. The model you created is analogous to the beginning of a
scientific theory. Except in science, theories are only formalized after
many years of testing the predictions that come from the model.
"Because of our human limitations in collecting complete data, theories
necessarily contain some judgments about what is important. Judgments
aren't a weakness of scientific theory. They are a basic part of how
science works."

"I always thought of science as a bunch of absolute facts," says Doug.
"I never thought about how knowledge is developed by scientists."
"Creativity and insight are what help make science such a powerful way
of understanding the natural world.

"There's another important thing that I try to teach my students with
this activity," Barbara continues. "It's important for them to be able
to distinguish questions that can be answered by science from those that
cannot be answered by science. Here's a list of questions that I use to
get them talking. I ask them if a question can be answered by science,
cannot be answered by science, or has some parts that belong to science
and others that do not. Then I ask the group to select a couple of
questions and discuss how they would go about answering them."

Barbara hands Doug and Karen the following list of questions:

Do ghosts haunt old houses at night?

How old is the earth?

Should I follow the advice of my daily horoscope?

Do species change over long periods of time?

Should I exercise regularly?

"Of course, you can make up other questions if something is happening in
the news or if it's related to an earlier lesson. And sometimes I
include moral or religious questions to make it clear that they lie
outside science."

"I can see that these would get students thinking," says Karen. "I guess
understanding the nature of science really is relevant to real life."
"That's what this exercise is about."

Chapter 3

Evolution and the Nature of Science

Science is a particular way of knowing about the world. In science, explanations
are restricted to those that can be inferred from confirmable data—the results
obtained through observations and experiments that can be substantiated by other
scientists. Anything that can be observed or measured is amenable to scientific
investigation. Explanations that cannot be based on empirical evidence are not a
part of science.

The history of life on earth is a fascinating subject that can be studied
through observations made today, and these observations have led to compelling
accounts of how organisms have changed over time. The best available evidence
suggests that life on earth began more than three and a half billion years ago.
For more than two billion years after that, life was housed in the bodies of
many kinds of tiny, single-celled organisms, some of which produced the oxygen
that now makes up more than a fifth of the earth's atmosphere. Less than a
billion years ago, much more complex organisms appeared. By about half a billion
years ago, evolution had resulted in a wide variety of multicellular animals and
plants living in the sea that are the clear ancestors of many of the major types
of organisms that continue to live to this day. Somewhat more than 400 million
years ago, some marine plants and animals began one of the greatest of all
innovations in evolution—they invaded dry land. For our own phylum, the
Chordata, this move away from the nurturing sea led to the appearance of
amphibians, reptiles, birds, and mammals—the latter including, of course, our
own species, Homo sapiens.

This chapter looks at how science works in the context of our overall
understanding of how biological evolution occurred. It begins, however, by
discussing another scientific development that challenged long-held
understandings and beliefs: the discovery of heliocentricism.

and the Nature of Science

Surely one of the first major natural phenomena to be understood was the cause
of night and day. Some of the earliest surviving human records left on clay
tablets relate to the movements of the sun and other celestial bodies. The
obvious cause of day and night is the rising and setting of the sun. This is an
observation that can be made today by anyone and, seemingly, requires no further

Archaeological evidence and early records make it clear that our ancestors
realized that not only does the sun appear to rise and set, but so do the moon
and stars. The movements of the moon and stars, however, are not precisely
synchronized with those of the sun. The moon is slower by about one hour per
day. The stars remain almost the same on successive nights, but slowly it
becomes obvious that they, too, are slowed in their movements compared to the
sun. Thus, the stars of summer are different from those visible in the winter.
In fact, it takes a full year for the stars to return to their previous
position, an interval of time that defines our year.

The ancient observers realized that not all stars move in unison. Although most
move in majestic unity, a few others are "wanderers"—appearing now with one
group of stars and a week later somewhere else. The majority were called "fixed
stars," the wanderers were called "planets."

Nicolaus Copernicus (1473-1543)

Nicolaus Copernicus, Johannes Kepler, Galileo Galilei, and Isaac Newton
led the way to a new understanding of the relationship between the earth
and the sun and initiated an age of scientific progress that continues

During the late Middle Ages, and especially in the Renaissance, beautiful brass
models known as orreries were made to show the relative positions and movements
of the sun, planets, and moon as they circled the earth. As the center of the
universe, the earth was a sphere in the center of the orrery. The other
celestial bodies were positioned on rings of metal, each moving by clockwork at
its own rate. The fixed stars required a simple solution—they could be
considered stuck in an outermost shell, also moved by clockwork.

Johannes Kepler (1571-1630)

The problem with orreries—and with the theories of the cosmos then
prevailing—was that they had to become successively more complex as more became known. Careful observations of the movements of the stars and planets greatly complicated the hypotheses used to account for those movements. This growing complexity stimulated some of the leading astronomers of the 16th and 17th centuries, including Copernicus, Kepler, and Galileo, to make even more precise observations of the movements of the heavenly bodies. Astronomers used these measurements to demonstrate that the age-old human explanations of the heavens were incomplete. In the process, they replaced a complex and confusing
explanation with a simple one: the sun, rather than the earth, is at the center
of a "solar system," and the earth revolves around it. That simple step—a bold
departure from past thinking due mainly to the insights of Copernicus (1473 to
1543)—dramatically changed the picture of the then known universe.

Galileo Galilei (1564-1642)

This concept of heliocentricism initially ran counter to the positions of
religious authorities. The view of Christianity over most of its history, based
on a literal interpretation of the Bible, was that the earth is the center of
the universe around which the celestial bodies revolve. Copernicus dedicated his
book describing the theory of heliocentricism, De revolutionibus orbium
coelestium, to Pope Paul III and promptly died. That saved him the troubles that
were to beset Galileo (1564 to 1642), whose astronomical observations confirmed
the views of Copernicus. Galileo was told to abandon his beliefs, and he later
was tried by the Inquisition and sentenced to the equivalent of house arrest.
The Church held that his views were dangerous to faith.

Isaac Newton (1642-1727)

Continued study and ever more careful measurements of the movements of the
planets and sun continued to support the heliocentric hypothesis. Then, in the
latter half of the 17th century, Isaac Newton (1642 to 1727) showed that the
force of gravity—as measured on the earth—could account for the movements of the
planets given the laws of motion that Newton derived. As a result of the steady
accumulation of evidence, the theological interpretation of celestial movements
gave way to the naturalistic explanation, and it is now accepted that night and
day are the consequences of the rotation of the earth on its axis. Today, we can
see for ourselves the rotation of the earth from satellites orbiting the planet.

Illustration from the 18th
century depicts the
Ptolemaic system in the
upper left corner and the
Copernican system in
other corners and center.

Like biological evolution, the theory of heliocentricism brought order and new
understanding to an otherwise chaotic and confusing aspect of nature. It also
had great practical applications, in that the exploration of the world by
European seafarers used the more accurate understanding of celestial mechanics
to assist in navigation.

Looking at the night sky remains a powerful experience. But that experience is
now informed not only by the beauty and majesty of the heavens, but by a deeper
understanding of nature and by an appreciation of the power of the human

This triumph of the human mind says a great deal about the nature of science.
First, science is not the same as common sense. Common sense indicates that the
sun does rise and set. Nevertheless, there can be other explanations of that
phenomenon, and one of them, the rotation of the earth on its axis, is
responsible for day and night. A concept based on observation proved to need
extensive modification as new observations accumulated.

Second, the statements of science should never be accepted as "final truth."
Instead, over time they generally form a sequence of increasingly more accurate
statements. Nevertheless, in the case of heliocentricism as in evolution, the
data are so convincing that the accuracy of the theory is no longer questioned
in science.

Third, scientific progress depends on individuals, but the contributions of one
individual could be made by others. If Copernicus had kept his ideas to himself,
the discovery of heliocentricism would have been postponed, but it would not
have been blocked, since other astronomers eventually would have come to the
same conclusion. Similarly, had Darwin and Wallace not published their
hypotheses, the concept of biological evolution would nevertheless have emerged
as the accepted explanation for the history of life on earth. The same cannot be
said in other areas of human endeavor; for example, had Shakespeare never
published, we would most assuredly never have had his plays. The publications of
scientists, unlike those of playwrights, are a means to an end—they are not the
end itself.

Science Requires Careful Description

What are the scientific methods that have led to our current understanding of
the history of life over vast eons of time? They begin with careful descriptions
of the material being studied.

The material for the study of biological evolution is life itself. One basic
aspect of life is that individuals can be grouped as similar kinds, or species.
Another important observation is that many species seem to be closely related to
each other. The scientific classification of species and their arrangement into
groups began with the publication in 1758 of Systema Naturae, or system of
nature, by the Swedish naturalist Carolus Linnaeus (1707 to 1778). For example,
Linnaeus knew seven dog-like species, and he gave each a double name.
Subsequently many more species were discovered and some of the names were
changed—and continue to be changed as more information is obtained. The domestic
dog is Canis familiaris; the coyote of North America is Canis latrans; the
Australian dingo is Canis dingo; and the wolf of the northern hemisphere is
Canis lupus. Thus Canis is the name of the genus of dog-like animals, and the
distinctive second name is the species name.

Biologists have used
construction cranes to
study the many newly
discovered species that
live in the canopies of
tropical forests, as in
this research project
in Panama.

Generations of scientists have discovered new species, described them, and
arranged them into the system first suggested by Linnaeus. Whereas Linnaeus
recognized about 9,000 species, systematists now have recognized about 1.5
million. The task of categorizing and describing species is still far from
complete. Most species of smaller invertebrates, and many bacteria and other
microscopic organisms, remain to be discovered. The plant kingdom is also
incompletely known. Though the flowering plants of many areas, such as Europe
and North America, are fairly well described, many other regions have not been
nearly as well explored by botanists.

Recent investigations in the exceptionally diverse rainforests of South America
have caused biologists to raise their estimates of the number of undescribed
species. For example, a very high proportion of insects collected from the
forest canopy are "new" species to science. It is now believed that the number
of different species of plants and animals in the world may be ten million, or
even more.

The scientific methods used in classifying organisms have been greatly improved
over time. The process begins with the intensive field work in which the
animals, plants, and microorganisms are collected and carefully examined. Most
will be known to a specialist, but there might be some unusual examples.
However, none is likely to be a complete stranger, since the specialist will
probably recognize that any puzzling specimen is similar to some familiar
species. Next the specialist must check all that has been published on the group
of organisms that contains the similar species. If, after an exhaustive search,
there is no record of a described species that corresponds to the one being
examined, the specimen is probably a new species. The specialist will then
prepare a careful description of the new species and publish it in a scientific
journal. There is a permanent reward for being the describer of a new species:
thereafter monographs that deal with the classification of the group to which
the new species belongs will add the describer's name at the end of the
scientific name. Thus, for example, "Homo sapiens Linnaeus" is our own proper
identification, because Linnaeus was the first to give us our scientific name.

Despite their
similarities with
birds, bats are
mammals that
evolved from
flightless ancestors.

This example makes it clear that not all scientific data are derived as the
result of experimentation. The conventional classification of species into
seemingly natural groups involved the careful observation of a variety of
different species, followed by the use of selected characteristics in an attempt
to define groups of species thought to be related. But the groupings are not
always obvious. For example, it might have seemed reasonable to classify bats
with birds, since the most conspicuous characteristic of each is the ability to
fly. But bats are mammals. Like all mammals, their bodies are covered with hair
and their young are born alive (instead of hatching from eggs) and are nourished
by milk from the mother's mammary glands.

Although most of the species we know today were described after the time of
Linnaeus, we continue to use his basic system of hierarchical classification.
For example, similar genera are united in families, similar families in orders,
similar orders in classes, and similar classes in phyla. The dog-like species
listed above (the genus Canis), plus a number of similar but more distant
dog-like animals, are grouped as the family Canidae. This family plus the
families of cats, bears, seals, and weasels form the order Carnivora. The
carnivores and all other animals with hair are combined as the class Mammalia.
Mammals are combined with the birds, reptiles, amphibians, fishes, plus some
small marine animals in the phylum Chordata. Today, many systematists group
organisms according to a system known as cladistics. By determining which traits
of a species evolved earlier and which evolved later, this system seeks to
classify organisms according to their evolutionary history.

Science as Explanation

In the quest for understanding, science involves a great deal of careful
observation that eventually produces an elaborate written description of the
natural world. This description is communicated to scientists in scientific
journals or at scientific meetings, so that others can build on pre-existing
work. In this way, the accuracy and sophistication of the description tends to
increase with time, as subsequent generations of scientists correct and extend
the observations of their predecessors. Because the total sum of scientific
knowledge increases relentlessly, scientific progress is something that all
scientists take for granted.

Evolutionary relationships
often are depicted in
diagrams that resemble
the branches of trees.
Closely related species
(denoted S1, S2, etc.)
are grouped into genera,
genera into families, and
so on. The result is a
hierarchical diagram
showing how different
species evolved from
common ancestor
species (represented in
this diagram by the
letters A through E).

But science is not just description. Even as observations are being made, the
human mind attempts to sort, or organize, the observations in a way that reveals
some underlying order in the objects or phenomena being observed. This sorting
process, which involves a great deal of trial and error, seems to be driven by a
fundamental human urge to make sense of our world.

The sorting process also suggests new observations that might otherwise not be
made. For example, the suggestion that bats should be grouped with mammals led
to an intensified examination of the similarities between bats and rodents—first
at the anatomical level, and later with respect to the genes and protein
molecules that form their cells. In this case, new evidence was obtained that
confirmed the suggested relationship. In other cases, the further observations
inspired by a tentative grouping have caused the rejection of a new idea.
The realization that species can be arranged in a hierarchy of groups of
seemingly similar forms raised an obvious question: What accounts for the
relatedness of different groups of organisms? The mechanism that was proposed by
Darwin directly addressed this question. It suggested that all animals
classified as belonging to the same group had a common ancestor species. That
is, dogs, wolves, coyotes and all members of the genus Canis are descended from
a common ancestor species that lived in the remote past. In a similar manner all
species in a family, an order, a class, or a phylum share a common inheritance.

Sedimentary rocks
are formed when
solid materials
carried by wind or
water accumulate
in layers and then
are compressed by
overlying deposits.
Sedimentary rocks
sometimes contain
fossils formed from the
parts of organisms
deposited along with
other solid materials.

How could one possibly test such a hypothesis? In the decades before Darwin
proposed his hypothesis, geologists realized that the sedimentary rocks of the
earth's crust contain a running diary of earth's history. This record of past
events comes about because the earth's crust is in a constant state of change.
This observation might not be obvious in the lifetime of an individual, but it
is dramatic over thousands of years. Relatively flat surfaces are uplifted to
form mountains, and then the mountains slowly erode to form flatlands. Storms
produce powerful waves that erode cliffs at the seashore. These phenomena have
the common feature of moving solid materials, and the subsequent settling out of
these materials makes possible the formation of a special form of rock that
contains a record of the earth's past.

Consider the case of a river with a source in the mountains. As the water moves
downstream, it erodes the slopes of the mountains. Tiny grains produced by the
erosion, called silt, are relatively easy to move. When the river reaches the
flatlands, a lake, or the ocean, the solid material being carried by the water
is deposited—often reaching great thicknesses over long periods of time. Then
the pressure of the sediments on top can cause the sediments beneath to harden
into "sedimentary rocks."

The river may carry things other than silt, sand, and rocks. Hard structures of
organisms such as the bones and teeth of animals may be carried along as well.
These, too, will be deposited with the silt, sand, and rocks. Under certain
circumstances, these remains of organisms undergo a chemical change in which the
original material is replaced by molecules that form stone. In this way, the
organic remains of living things are fossilized (changed into stone), creating
the evidence of ancient life studied by scientists.

Because of the order in which the sediments are deposited, the most recent layer
of rocks normally will be on top and the oldest layer will be on the bottom
(though sometimes sediments are flipped upside down by the geologic folding of
rock layers). Also, the fossils in each layer usually will be of those organisms
that lived at the time the layer was formed. Thus, the fossils in the lower
layers will represent species that lived earlier than those found in the upper

The relative position of fossils tells only which are older and which younger.
One can estimate the difference in the ages of the two fossils by noting the
thickness of the rock that separates them. If the difference is only one foot,
one might guess the interval of time is less than if two fossils are separated
by 50 feet of rock layers. Today, however, far more accurate methods of dating
fossils are available, as described on the next page. Because these methods are
based on the known rates of radioactive decay, they provide valuable measures of
absolute time.

A fossil just predating
the Cambrian shows
the outlines of a marine
invertebrate that might
have resembled a

The scientific study of fossils is called paleontology, and the methods used for
their identification and classification are similar to those used for living
species. But in some respects the task of the paleontologist is far more
difficult. Many species lack hard parts such as bones and shells, and such
organisms almost always decay without becoming fossilized. This is the case for
many groups of soft-bodied invertebrates—such as worms of many kinds, jellyfish,
and protozoans. Even for such species as mammals, birds, reptiles, and
amphibians, death is usually followed by the skeleton being dismembered and the
bones scattered. For this reason, whereas isolated bones are often fossilized,
it is exceptionally rare for an intact skeleton to be found.

Before the start of
the Cambrian period
about 550 million
years ago, multicellular
organisms lacked
hard parts like shells
and bones and rarely
left fossils. However, a
few pre-Cambrian
organisms left traces
of their existence.
Some ancient rocks
contain stromatolites—
the remnants of bacteria
that grew in columns
like stacked pancakes.

Tiny fossils first reveal the existence of bacteria 3.5 to 3.8 billion years
ago, and animals composed of more than a single cell are known from about 670
million years ago. But the organisms that lived between these two dates lacked
hard parts and, hence, were rarely preserved as fossils. Then, about 570 million
years ago, a dramatic change took place. At the beginning of the Cambrian
period, animals evolved that had calcified shells and other types of body
coverings that had a far better chance of becoming fossilized. These fossils
demonstrate that Cambrian seas were populated with a variety of invertebrates.
The earliest vertebrate fossils date from about 500 million years ago.
Thereafter early amphibians and reptiles appeared. Birds and mammals appear in
the fossil record only about 200 million years ago, while dinosaurs first appear
about 225 million years ago and disappear suddenly about 160 million years

In the fossil record, most species are characterized
by a specific appearance, a duration over time, and
extinction. The evolutionary origins of species are
inferred from the morphological relations among fossils.

In the 1830s, when Darwin began his studies, the essential features of the
fossil record were known (although absolute dates had not yet been determined).
Many thousands of living species had been described, and it was clearly
recognized that they could be organized into various groups—suggesting that they
are somehow relatives. In addition, analysis of the fossil record revealed that
the organisms on the ancient earth had undergone major changes over time—with
whole groups of animals appearing, persisting for long periods of time, and then

Darwin was an unusually keen observer. But he was not content to catalogue facts
and observations. Instead, the natural world to him was a gigantic, very
challenging puzzle that demanded an explanation for its otherwise bewildering
complexity. Why are different organisms so similar? Why has there been a
succession of different kinds of species throughout geologic time?
Certain observations seemed particularly important. For example:

In South America,
Darwin found fossil
species that were
clearly related to
modern armadillos,
yet neither the fossils
nor the living animals
were found anywhere
else in the world. In
The Origin of Species,
he explained that "the
inhabitants of each
quarter of the world
will obviously tend
to leave in that quarter
closely allied through
modified descendants."

1) In South America, the only continent where living armadillos were found,
Darwin discovered fossil evidence for the prior existence of ancient species
that had many of the unique features of living armadillos, yet were clearly
different. Such fossils were found nowhere else in the world. Why were both
living and ancient armadillo-like species confined to the same geographical

Dating the Earth

One of the greatest scientific triumphs of the last two centuries has
been the discovery of the vast expanse of geologic time. Early methods
of calculating the age of the earth relied on measures of the rate of
sedimentation or the cooling of the earth from an initially molten
state. The relative ages of rocks also were calculated early in the
1800s by noting what kinds of fossils the rocks contained. But the
absolute age of the earth and the timing of many events in geologic
history required the discovery late in the 19th century of a previously
unknown phenomenon: radioactivity.

Some elements, such as uranium, undergo radioactive decay to produce
other elements. By measuring the quantities of radioactive elements and
the elements into which they decay in rocks, geologists can determine
how much time has elapsed since the rock cooled from an initially molten
state. For example, the oldest known rocks are found in Greenland and
date from about 3.8 billion years ago. Scientists believe the earth's
age to be about 4.6 billion years because meteorites and rocks of the
moon—both of which formed about the same time as the earth—date from
this time. Radiometric dating also shows that the period of earth's
history during which large fossils can be readily found in rocks began
only about 570 million years ago.

Radiometric dating draws on information and insights from many areas of
science. For example, it requires that the rate of radioactive decay is
constant over time and is not influenced by such factors as temperature
or pressure—conclusions supported by extensive research in physics. It
also assumes that the rocks being analyzed have not been altered over
time by the migration of atoms into or out of the rocks, which requires
detailed information from both the geologic and chemical sciences.

2) On the Galapagos Islands, 600 miles off the coast of Ecuador, Darwin observed
many distinct living species of birds and reptiles that closely resembled each
other—yet were different on each tiny island. Why, for example, should the beak
size of the mockingbirds on one island be different from that of a closely
related mockingbird on an island only 30 miles away? And why were the various
types of animals on these islands related, but distinct from, the animals in
Ecuador, whereas those on the otherwise very similar islands off the coast of
Africa were related to the animals in Africa instead?

Darwin could not see how these observations could be explained by the prevailing
view of his time: that each species had been independently created, with the
species that were best suited to each location on the earth being created at
each particular site. It looked instead as though species could evolve from one
into another over time, with each being confined to the particular geographical
region where its ancestors happened to be—particularly if isolated by major
barriers to migration, such as vast expanses of ocean.

A timeline of evolution demonstrates the tremendous expanse of geologic
time compared to the period since humans evolved. Each higher scale
part of the scale beneath it. While the estimated times of various
events continue to change as new fossils are discovered and dating methods
are refined, the overall sequence demonstrates both the scope and grandeur
of evolutionary change.

But how could one species turn into another over the course of time? In
constructing his hypothesis of how this occurred, Darwin was struck by several
other observations that he and others before him had made.

1) People who bred domesticated animals and plants for commercial or
recreational use had found and exploited a great deal of variation among the
progeny of their crosses. Pigeon breeders, for example, had observed wide
differences in colors, beaks, necks, feet, and tails of the offspring from a
single mating pair. They routinely enhanced their stocks for desired traits—for
example, selectively breeding those animals that shared a particular type of
beak. Through such artificial selection, pigeon fanciers had been able to create
many different-looking pigeons, known as breeds. A similar type of artificial
selection for mating pairs of dogs had likewise created the whole variety of
shapes and sizes of these common pets—ranging from a Great Dane to a dachshund.
2) Animals living in the wild can face a tremendous struggle for survival. For
some birds, for example, fewer than one in 100 animals born in one year will
survive over a harsh winter into a second year. Those with characteristics best
suited for a particular environment—for example, those individual birds who are
best able to find scarce food in the winter while avoiding becoming food for a
larger animal—tend to have better chances of surviving. Darwin called this
process natural selection to distinguish it from the artificial selection used
by dog and pigeon breeders to determine which animals to mate to produce

At least 20 years elapsed between the time that Darwin conceived of descent with
modification and 1859, the year that he revealed his ideas to the world in On
the Origin of Species. Throughout these 20 years, Darwin did what scientists
today do: he tested his ideas of how things work with new observations and
experiments. In part, he did this by thinking up every possible objection he
could to his own hypothesis. For each such argument, Darwin tried to find an
observation made by others, make an observation, or do an experiment of his own
that might imply that his ideas were in fact not valid. When he could
successfully counter such objections, he strengthened his theory. For example,
Darwin's ideas readily explained why distant oceanic islands were generally
devoid of terrestrial mammals, except for flying bats. But how could the land
snails, so common on such islands, have traversed the hundreds of miles of open
ocean that separate the islands from the mainland where the snails first
evolved? By floating snails on salt-water for prolonged periods, Darwin
convinced himself that, on rare occasions in the past, snails might in fact have
"floated in chunks of drifted timber across moderately wide arms of the sea."
This example shows how a hypothesis can drive a scientist to do experiments that
would otherwise not be done. Prior to Darwin, the existence of land snails and
bats, but not typical terrestrial mammals, on the oceanic islands was simply
noted and catalogued as a fact. It is unlikely that anyone would have thought to
test the snails for their ability to survive for prolonged periods in salt
water. Even if they had, such an experiment would have had little meaning or

By publishing his ideas, Darwin subjected his hypothesis to the tests of others.
This process of public scrutiny is an essential part of science. It works to
eliminate individual bias and subjectivity, because others must also be able to
determine whether a proposed explanation is consistent with the available
evidence. It also leads to further observations or to experiments designed to
test hypotheses, which has the effect of advancing science.
Many of the hypotheses advanced by scientists turn out to be incorrect when
tested by further observations or experiments. But skillful scientists like
Darwin tend to have good ideas that end up increasing the amount of knowledge in
the world. For this reason, the ideas of scientists have been—over the long
run—central to much of human progress.

Science as Cumulative Knowledge

The ability to analyze individual biological molecules has added
great detail to biologists' understanding of the tree of life. For
example, molecular analyses indicate that all living things fall
into three domains—the Bacteria, Archaea, and Eucarya—
related by descent from a common ancestor.

At the time of Darwin, there were many unsolved puzzles, including missing links
in the fossil record between major groups of animals. Guided by the central idea
of evolution, thousands of scientists have spent their lives searching for
evidence that either supports or conflicts with the idea. For example, since
Darwin's time, paleontologists have discovered many ancient organisms that
connect major groups—such as Archaeopteryx between ancient reptiles and birds,
and Ichthyostega between ancient fish and amphibians. By now, so much evidence
has been found that supports the fundamental idea of biological evolution that
its occurrence is no longer questioned in science.

Even more striking has been the information obtained during the 20th century
from studies on the molecular basis of life. The theory of evolution implies
that each organism should contain detailed molecular evidence of its relative
place in the hierarchy of living things. This evidence can be found in the DNA
sequences of living organisms. Before a cell can divide to produce two daughter
cells, it must make a new copy of its DNA.

Continental Drift and Plate Tectonics: A Scientific Revolution of the
Past 50 Years

The theory of plate tectonics demonstrates that revolutions in science
are not just a thing of the past, thus suggesting that more revolutions
can be expected in the future.

World maps have long indicated a curious "jigsaw puzzle fit" of the
continents. This is especially apparent between the facing coastlines of
South America and Africa. Alfred Wegener (1880 to 1930), a German
meteorologist who was dissatisfied with explanations that relied on
expanding and contracting crust to account for mountain building and the
formation of the ocean floor, pursued other lines of reasoning. Wegener
suggested that all of earth's continents used to be assembled in a
single ancient super-continent he called Pangea. He hypothesized that
Pangea began to break up approximately 200 million years ago, with South
America and Africa slowly drifting apart to their present positions,
leaving the southern Atlantic Ocean between them. This was an
astonishing hypothesis: could huge continents really move?
Wegener cited both geological and biological evidence in support of his
explanation. Similar plant and animal fossils are found in rock layers
more than 200 million years old in those regions where he claimed that
different continents were once aligned. Wegener attributed this to the
migration of plants and animals freely throughout these broad regions.
If 200 million years ago Africa and South America had been separated by
the Atlantic Ocean as they are today, their climates, environments, and
life forms should have been very different from each other—but they were

Despite Wegener's use of evidence and logic to develop his explanations,
other scientists found it difficult to imagine how solid, brittle
continents could plow through the equally solid and brittle rock
material of the ocean floor. Wegener did not have an explanation for how
the continents moved. Since there was no plausible mechanism for
continental drift, the idea did not take hold. The hypothesis of
continental drift was equivalent to the hypothesis of evolution in the
decades before Darwin, when evolution lacked the idea of variation
followed by natural selection as an explanatory mechanism.
The argument essentially lay dormant until improved technologies allowed
scientists to gather previously unobtainable data. From the mid 1950s
through the early 1970s, new evidence for a mechanism to explain
continental drift became available that the scientific community could
accept. Sonar mapping of the ocean floor revealed the presence of a
winding, continuous ridge system around the globe. These ridges were
places where molten material was welling up from the earth's interior
and pushing apart the plates that form the earth's surface.
In a relatively short time, these new observations, measurements, and
interpretations provoked a complete shift in the thinking of the
scientific community. Geologists now accept the idea that the surface of
the earth is broken up into about a dozen large pieces, as well as a
number of smaller ones, called tectonic plates.

On a time scale of millions of years, these plates shift about on the
planet's surface, changing the relative positions of the continents. The
plate tectonic model provides explanations that are widely accepted for
the evolution of crustal features such as folded mountain chains, zones
of active volcanoes and earthquakes, and deep ocean floor trenches.
Direct measurements using the satellite-based global positioning system
(GPS) to measure absolute longitude and latitude verify that the plates
collide, move apart, and slide past one another in different areas along
their adjacent boundaries at speeds comparable to the growth rate of a
human fingernail.

In copying its DNA nucleotides, however, cells inevitably make a small number of
mistakes. For this reason, a few nucleotides are changed through random error
each time that a cell divides. (For example, an A in the DNA sequence of a gene
in a chromosome may be replaced with a G in the new copy made as the cell
divides.) Therefore, the larger number of cell divisions that have elapsed
between the time that two organisms diverged from their common ancestor, the
more differences there will be in their DNA sequences due to chance errors.
This molecular divergence allows researchers to track evolutionary events by
sequencing the DNA of different organisms. For example, the lineage that led to
humans and to chimpanzees diverged about 5 million years ago—whereas one needs
to look back in time about 80 million years to find the last common ancestor
shared by mice and humans. As a result, there is a much smaller difference
between human and chimpanzee DNA than between human (or chimpanzee) and mouse
DNA. In fact, scientists today routinely use the differences they can measure
between the DNA sequences of organisms as "molecular clocks" to decipher the
relationships between living things.

Organisms ranging from yeast to humans use an enzyme
known as cytochrome C to produce high-energy molecules
as part of their metabolism. The gene that codes for
cytochrome C gradually has changed over the course of
evolution. The greater the differences in the DNA bases that
code for the enzyme, the longer the time since two organisms
shared a common ancestor. This DNA evidence for evolution
has confirmed evolutionary relationships derived from other

The same comparisons among organisms can be made using the proteins encoded by
DNA. For example, every living cell uses a protein called cytochrome c in its
energy metabolism. The cytochrome c proteins from humans and chimpanzees are
identical. But there is only an 86 percent overlap in the molecules between
humans and rattlesnakes, and only a 58 percent overlap between us and brewer's
yeast. This is explained by the evolutionary proposition that we shared a common
ancestor with chimps relatively recently, whereas the common ancestor that we,
as vertebrates, shared with rattlesnakes is much more ancient. Still farther in
the past, we and yeast shared a common ancestor—and the molecular data reflect
this pattern.

In the past few decades, new methods have been developed that are allowing us to
obtain the exact sequence of all of the DNA nucleotides in chromosomes. The
Human Genome Project, for example, will produce when completed the entire
sequence of the 3 billion nucleotides that make up our genetic inheritance. The
complete sequence of the yeast genome (12 million nucleotides) is already known,
as are the genomes for numerous species of bacteria (from 0.5 to 5 million
nucleotides each, depending on the species). Similar sequencing efforts will
soon yield the complete sequences for hundreds of bacteria and other organisms
with small genomes.

These molecular studies are powerful evidence for evolution. The exact order of
the genes on our chromosomes can be used to predict the order on monkey or even
mouse chromosomes, since long stretches of the chromosomes of mammalian species
are so similar. Even the parts of our DNA that do not code for proteins and at
this point have no known function are similar to the comparable parts of DNA in
related organisms.

The confirmation of Darwin's ideas about "descent with modification" by this
recent molecular evidence has been one of the most exciting developments in
biology in this century. In fact, as the chromosomes of more and more organisms
are sequenced over the next few decades, these data will be used to reconstruct
much of the missing history of life on earth—thereby compensating for many of
the gaps that still remain in the fossil record.


One goal of science is to understand nature. "Understanding" in science means
relating one natural phenomenon to another and recognizing the causes and
effects of phenomena. Thus, scientists develop explanations for the changing of
the seasons, the movements of heavenly bodies, the structure of matter, the
shaping of mountains and valleys, the changes in the positions of continents
over time, and the diversity of living things.

The statements of science must invoke only natural things and processes. The
statements of science are those that emerge from the application of human
intelligence to data obtained from observation and experiment. These fundamental
characteristics of science have demonstrated remarkable power in allowing us to
describe the natural world accurately and to identify the underlying causes of
natural phenomena. This understanding has great practical value, in part because
it allows us to better predict future events that rely on natural processes.
Progress in science consists of the development of better explanations for the
causes of natural phenomena. Scientists can never be sure that a given
explanation is complete and final. Yet many scientific explanations have been so
thoroughly tested and confirmed that they are held with great confidence.
The theory of evolution is one of these explanations. An enormous amount of
scientific investigation has converted what was initially a hypothesis into a
theory that is no longer questioned in science. At the same time, evolution
remains an extremely active field of research, with an abundance of new
discoveries that are continually increasing our understanding of exactly how the
evolution of living organisms actually occurred.

An Excerpt from the Book
This Is Biology: The Science of the Living World (1997)
By Ernst Mayr

It has been said that the scientist searches for truth, but many people
who are not scientists claim the same. The world and all that is in it
are the sphere of interest not only of scientists but also of
theologians, philosophers, poets, and politicians. How can one make a
demarcation between their concerns and those of the scientist?

How Science Differs from Theology

The demarcation between science and theology is perhaps easiest, because
scientists do not invoke the supernatural to explain how the natural
world works, and they do not rely on divine revelation to understand it.
When early humans tried to give explanations for natural phenomena,
particularly for disasters, invariably they invoked supernatural beings
and forces, and even today divine revelation is as legitimate a source
of truth for many pious Christians as is science. Virtually all
scientists known to me personally have religion in the best sense of
this word, but scientists do not invoke supernatural causation or divine

Another feature of science that distinguishes it from theology is its
openness. Religions are characterized by their relative inviolability;
in revealed religions, a difference in the interpretation of even a
single word in the revealed founding document may lead to the origin of
a new religion. This contrasts dramatically with the situation in any
active field of science, where one finds different versions of almost
any theory. New conjectures are made continuously, earlier ones are
refuted, and at all times considerable intellectual diversity exists.
Indeed, it is by a Darwinian process of variation and selection in the
formation and testing of hypotheses that science advances.
Despite the openness of science to new facts and hypotheses, it must be
said that virtually all scientists—somewhat like theologians—bring a set
of what we might call "first principles" with them to the study of the
natural world. One of these axiomatic assumptions is that there is a
real world independent of human perceptions. This might be called the
principle of objectivity (as opposed to subjectivity) or common-sense
realism. This principle does not mean that individual scientists are
always "objective" or even that objectivity among human beings is
possible in any absolute sense. What it does mean is that an objective
world exists outside of the influence of subjective human perception.
Most scientists—though not all—believe in this axiom.

Second, scientists assume that this world is not chaotic but is
structured in some way, and that most, if not all, aspects of this
structure will yield to the tools of scientific investigation. A primary
tool used in all scientific activity is testing. Every new fact and
every new explanation must be tested again and again, preferably by
different investigators using different methods. Every confirmation
strengthens the probability of the "truth" of a fact or explanation, and
every falsification or refutation strengthens the probability that an
opposing theory is correct. One of the most characteristic features of
science is this openness to challenge. The willingness to abandon a
currently accepted belief when a new, better one is proposed is an
important demarcation between science and religious dogma.
The method used to test for "truth" in science will vary depending on
whether one is testing a fact or an explanation. The existence of a
continent of Atlantis between Europe and America became doubtful when no
such continent was discovered during the first few Atlantic crossings in
the period of discoveries during the late fifteenth and early sixteenth
centuries. After complete oceanographic surveys of the Atlantic Ocean
were made and, even more convincingly, after photographs from satellites
were taken in this century, the new evidence conclusively proved that no
such continent exists. Often, in science, the absolute truth of a fact
can be established. The absolute truth of an explanation or theory is
much harder, and usually takes much longer, to gain acceptance. The
"theory" of evolution through natural selection was not fully accepted
as valid by scientists for over 100 years; and even today, in some
religious sects, there are people who do not believe it.

Third, most scientists assume that there is historical and causal
continuity among all phenomena in the material universe, and they
include within the domain of legitimate scientific study everything
known to exist or to happen in this universe. But they do not go beyond
the material world. Theologians may also be interested in the physical
world, but in addition they usually believe in a metaphysical or
supernatural realm inhabited by souls, spirits, angels, or gods, and
this heaven or nirvana is often believed to be the future resting place
of all believers after death. Such supernatural constructions are beyond
the scope of science.


Teaching Evolution Through Inquiry

The following dialogue demonstrates a way of teaching about evolution
using inquiry-based learning. High school students are often
interested in fossils and in what fossils indicate about organisms and
their habitats. In the investigation described here, the students
conduct an inquiry to answer an apparently simple question: What
influence has evolution had on two slightly different species of
fossils? The investigation begins with a straightforward
task—describing the characteristics of two species of brachiopods.

"Students, I want you to look at some fossils," says Karen. She gives the
students a set of calipers and two plastic sheets that each contain about
100 replicas of carefully selected fossil brachiopods.1 "These two sheets
contain fossils from two different species of a marine animal called a
brachiopod. Let's begin with some observations of what they look like."
"They look like butterflies," replies one student.

"They are kind of triangular with a big middle section and ribs," says
another student.

"Can you tell if there are any differences between the fossils in the two

The students quickly conclude that the fossils have different sizes but
that they cannot really tell any other difference.

"In that case, how could you tell if the fossil populations are
different?" Karen asks.

"We can count the ribs."

"We can measure them."

"Those are both good answers. Here's what I want you to do. Break into
groups of four and decide among yourselves which of those two
characteristics of the fossils you want to measure. Then graph your
measurements for each of the two different populations."
For the rest of the class period, the students investigate the fossils.
They soon realize that the number of ribs is related to the size of the
fossils, so the groups focus on measuring the lengths and widths of the
fossils. They enter the data on the two different populations into a
computer data base. Two of the graphs that they generate are shown on the
facing page.

Graphs showing characteristics of brachiopod populations.
"Now that we have these graphs of the fossils' lengths and widths," Karen
says at the beginning of the next class period, "we can begin to talk
about what these measurements mean. We see from one set of graphs that the
fossils in the second group tend to be both wider and longer than those in
the other group. What could that mean?"

"Maybe one group is older," volunteers one of the students.

"Maybe they're different kinds of fossils," says another.

"Let's think about that," says Karen. "How could their lengths and widths
have made a difference to these organisms?"

"It could have something to do with the way they moved around."

"Or how they ate."

"That's good," says Karen. "Now, if you had dug up these fossils, you
would have some additional information to work with, so let me give you
some of that background. As I mentioned last week, these fossils are from
marine animals known as brachiopods. When they die their shells are often
buried in sediments and fossilized. What I know about the fossils you have
is that they were taken from sediments that are about 400 million years
old. But the two sets of fossils were separated in time by about 10
million years.

"Taking that information, I'd like you to do some research on brachiopods
and develop some hypotheses about whether or not evolution has influenced
their size. Here are some of the questions you can consider as you're
writing up your arguments."

Karen hands out a sheet of paper containing the following questions:
What differences in structure and function might be represented in the
length and width of the brachiopods? Could efficiency in burrowing or
protection against predators have influenced their shapes?

Why might natural selection influence the lengths and widths of

What could account for changes in their dimensions?

The following week, Karen holds small conferences at which the students'
papers are presented and discussed. She focuses students on their ability
to ask skeptical questions, evaluate the use of evidence, assess the
understanding of geological and biological concepts, and review aspects of
scientific inquiries. During the discussions, students are directed to
address the following questions: What evidence would you look for that
might indicate these brachiopods were the same or different species? How
could changes in their shapes have affected their ability to reproduce
successfully? What would be the likely effects of other changes in the
environment on the species?


The materials needed to carry out this investigation are available from
Carolina Biological Supply Company, 2700 York Rd., Burlington, NC 27215.
Phone: 1-800-334-5551. www.carolina.com

Chapter 4

Evolution and the
National Science Education Standards

Over the last six years, several major documents have been released that
describe what students from kindergarten through twelfth grade should know and
be able to do as a result of their instruction in the sciences. These include
the National Science Education Standards released by the National Research
Council in 1996,1 the Benchmarks for Science Literacy released by the American
Association for the Advancement of Science in 1993,2 and The Content Core: A
Guide for Curriculum Designers released by the Scope, Sequence, Coordination
project of the National Science Teachers Association in 1992.3

These documents agree that all students should leave biology class with an
understanding of the basic concepts of biological evolution and of the limits,
possibilities, and dynamics of science as a way of knowing. Benchmarks for
Science Literacy, for example, states that "the educational goal should be for
all children to understand the concept of evolution by natural selection, the
evidence and arguments that support it, and its importance in history." For
biology educators, these documents offer significant support for the inclusion
of evolution in school science programs.
Structure and Overview of the
National Science Education Standards
This chapter focuses on the treatment of evolution in the National Science
Education Standards. The Standards are divided into six broad sections. The
first set of standards, the science teaching standards, describes what teachers
of science at all grade levels should know and be able to do. The professional
development standards describe the experiences necessary for teachers to gain
the knowledge, understanding, and ability to implement the Standards. The
assessment standards provide criteria against which to judge whether assessments
are contributing fully to the goals outlined in the Standards. The science
content standards outline what students should know, understand, and be able to
do in the natural sciences. The science education program standards discuss the
planning and actions needed to translate the Standards into programs that
reflect local contexts and policies. And the science education system standards
consist of criteria for judging the performance of the overall science education
The Standards rest on the premise that science is an active process. Learning
science is something that students do, not something that is done to them.
"Hands-on" activities, although essential, are not enough. Students must have
"minds-on" experiences as well.
The Standards make inquiry a central part of science learning. When engaging in
inquiry, students describe objects and events, ask questions, construct
explanations, test those explanations against current scientific knowledge, and
communicate their ideas to others. They identify their assumptions, use critical
and logical thinking, and consider alternative explanations. In this way,
students actively develop their understanding of science by combining scientific
knowledge with reasoning and thinking skills.
The importance of inquiry does not imply that all teachers should pursue a
single approach to teaching science. Just as inquiry has many different facets,
so too do teachers need to use many different strategies to develop the
understandings and abilities described in the Standards.
Nor should the Standards be seen as requiring a specific curriculum. A
curriculum is the way content is organized and presented in the classroom. The
content embodied in the Standards can be organized and presented with different
emphases and perspectives in many different curricula.
Evolution and the Nature of Science in the
National Science Education Standards
Evolution and the nature of science are major topics in the content standards.
The first mention of evolution is in the initial content standard, entitled
"Unifying Concepts and Processes." This standard points out that conceptual and
procedural schemes unify science disciplines and provide students with powerful
ideas to help them understand the natural world. It is the only standard that
extends across all grades, because the understanding and abilities associated
with this standard need to be developed over an entire education.
The standard is as follows:
As a result of activities in grades K—12, all students should develop
understanding and abilities aligned with the following concepts and processes:
Systems, order, and organization
Evidence, models, and explanation
Constancy, change, and measurement
Evolution and equilibrium
Form and function
The guidance offered for the standard is to establish a broad context for
thinking about evolution:
Evolution is a series of changes, some gradual and some sporadic, that
accounts for the present form and function of objects, organisms, and natural
and designed systems. The general idea of evolution is that the present arises
from materials and forms of the past. Although evolution is most commonly
associated with the biological theory explaining the process of descent with
modification of organisms from common ancestors, evolution also describes
changes in the universe.
With this unifying standard as a basis, the remaining content standards are
organized by age group and discipline.
Grades K—4
The life science standard for grades K—4 is organized into the categories of
characteristics of organisms, life cycles of organisms, and organisms and their
environments. Evolution is not explicitly mentioned in these standards, but the
text explains the basic things in life science that elementary school children
ought to be able to understand and do:
During the elementary grades, children build understanding of biological
concepts through direct experience with living things, their life cycles, and
their habitats. These experiences emerge from the sense of wonder and natural
interests of children who ask questions such as: "How do plants get food? How
many different animals are there? Why do some animals eat other animals? What
is the largest plant? Where did the dinosaurs go?" An understanding of the
characteristics of organisms, life cycles of organisms, and of the complex
interactions among all components of the natural environment begins with
questions such as these and an understanding of how individual organisms
maintain and continue life.
The intention of the K—4 standard is to develop the knowledge base that will be
needed when the fundamental concepts of evolution are introduced in the middle
and high school years.
Grades 5—8
For grades 5—8, the life science standard is the following:
As a result of their activities in grades 5—8, all students should develop
understanding of:

Structure and function in living systems
Reproduction and heredity
Regulation and behavior
Populations and ecosystems
Diversity and adaptations of organisms
The guidance for this standard defines regulation and behavior as follows:
An organism's behavior evolves through adaptation to its environment. How a
species moves, obtains food, reproduces, and responds to danger are based in
the species' evolutionary history.
The text discusses diversity and adaptations as follows:
Diversity and Adaptations of Organisms
Millions of species of animals, plants, and microorganisms are alive today.
Although different species might look dissimilar, the unity among organisms
becomes apparent from an analysis of internal structures, the similarity of
their chemical processes, and the evidence of common ancestry.
Biological evolution accounts for the diversity of species developed through
gradual processes over many generations. Species acquire many of their unique
characteristics through biological adaptation, which involves the selection of
naturally occurring variations in populations. Biological adaptations include
changes in structures, behaviors, or physiology that enhance survival and
reproductive success in a particular environment.
Extinction of a species occurs when the environment changes and the adaptive
characteristics of a species are insufficient to allow its survival. Fossils
indicate that many organisms that lived long ago are extinct. Extinction of
species is common; most of the species that have lived on the earth no longer
The text accompanying the standard also discusses some of the difficulties
encountered in teaching about adaptation:
Understanding adaptation can be particularly troublesome at this level. Many
students think adaptation means that individuals change in major ways in
response to environmental changes (that is, if the environment changes,
individual organisms deliberately adapt).
In fact, as described in Chapter 2 of this book, adaptation occurs through
natural selection, a topic described under the life science standards for grades
The content standards also treat evolution in grades 5—8 in the section on
earth's history. The standard reads as follows:
As a result of their activities in grades 5—8, all students should develop an
understanding of:
Structure of the earth system
Earth's history
Earth in the solar system
The text discusses the importance of teaching students about earth systems and
their interactions.
A major goal of science in the middle grades is for students to develop an
understanding of earth and the solar system as a set of closely coupled
systems. The idea of systems provides a framework in which students can
investigate the four major interacting components of the earth
system—geosphere (crust, mantle, and core), hydrosphere (water), atmosphere
(air), and the biosphere (the realm of all living things). In this holistic
approach to studying the planet, physical, chemical, and biological processes
act within and among the four components on a wide range of time scales to
change continuously earth's crust, oceans, atmosphere, and living organisms.
Their study of earth's history provides students with some evidence about
co-evolution of the planet's main features—the distribution of land and sea,
features of the crust, the composition of the atmosphere, global climate, and
populations of living organisms in the biosphere.
The material offering guidance for the standard explicitly ties the earth's
history to the history of life:
Earth's History
The earth processes we see today, including erosion, movement of lithospheric
plates, and changes in atmospheric composition, are similar to those that
occurred in the past. Earth's history is also influenced by occasional
catastrophes, such as the impact of an asteroid or comet.
Fossils provide important evidence of how life and environmental conditions
have changed.
The standards for grades 5—8 cover the nature of science in the section on the
history and nature of science:
As a result of activities in grades 5—8, all students should develop an
understanding of:
Science as a human endeavor
Nature of science
History of science
The guidance accompanying this standard offers the following discussion of these
Nature of Science
Scientists formulate and test their explanations of nature using observation,
experiments, and theoretical and mathematical models. Although all scientific
ideas are tentative and subject to change and improvement in principle, for
most major ideas in science, there is much experimental and observational
confirmation. Those ideas are not likely to change greatly in the future.
Scientists do and have changed their ideas about nature when they encounter
new experimental evidence that does not match their existing explanations.
In areas where active research is being pursued and in which there is not a
great deal of experimental or observational evidence and understanding, it is
normal for scientists to differ with one another about the interpretation of
the evidence or theory being considered. Different scientists might publish
conflicting experimental results or might draw different conclusions from the
same data. Ideally, scientists acknowledge such conflict and work towards
finding evidence that will resolve their disagreement.
It is part of scientific inquiry to evaluate the results of scientific
investigations, experiments, observations, theoretical models, and the
explanations proposed by other scientists. Evaluation includes reviewing the
experimental procedures, examining the evidence, identifying faulty reasoning,
pointing out statements that go beyond the evidence, and suggesting
alternative explanations for the same observations. Although scientists may
disagree about explanations of phenomena, about interpretations of data, or
about the value of rival theories, they do agree that questioning, response to
criticism, and open communication are integral to the process of science. As
scientific knowledge evolves, major disagreements are eventually resolved
through such interactions between scientists.
History of Science
Many individuals have contributed to the traditions of science. Studying some
of these individuals provides further understanding of scientific inquiry,
science as a human endeavor, the nature of science, and the relationships
between science and society.
In historical perspective, science has been practiced by different individuals
in different cultures. In looking at the history of many peoples, one finds
that scientists and engineers of high achievement are considered to be among
the most valued contributors to their culture.
Tracing the history of science can show how difficult it was for scientific
innovators to break through the accepted ideas of their time to reach the
conclusions that we currently take for granted.
Grades 9—12
The life science standard for grades 9—12 directly addresses biological
evolution. The standard reads as follows:
As a result of their activities in grades 9—12, all students should develop an
understanding of:

The cell
Molecular basis of heredity
Biological evolution
Interdependence of organisms
Matter, energy, and organization in living systems
Behavior of organisms
The guidance for the life science standard describes the major themes of
evolutionary theory:
Biological Evolution
Species evolve over time. Evolution is the consequence of the interactions of
(1) the potential for a species to increase its numbers, (2) the genetic
variability of offspring due to mutation and recombination of genes, (3) a
finite supply of the resources required for life, and (4) the ensuing
selection by the environment of those offspring better able to survive and
leave offspring.
The great diversity of organisms is the result of more than 3.5 billion years
of evolution that has filled every available niche with life forms.
Natural selection and its evolutionary consequences provide a scientific
explanation for the fossil record of ancient life forms, as well as for the
striking molecular similarities observed among the diverse species of living
The millions of different species of plants, animals, and microorganisms that
live on earth today are related by descent from common ancestors.
Biological classifications are based on how organisms are related. Organisms
are classified into a hierarchy of groups and subgroups based on similarities
which reflect their evolutionary relationships. Species is the most
fundamental unit of classification.
The text following the standard describes some of the difficulties that students
can have in comprehending the basic concepts of evolution.
Students have difficulty with the fundamental concepts of evolution. For
example, students often do not understand natural selection because they fail
to make a conceptual connection between the occurrence of new variations in a
population and the potential effect of those variations on the long-term
survival of the species. One misconception that teachers may encounter
involves students attributing new variations to an organism's need,
environmental conditions, or use. With some help, students can understand
that, in general, mutations occur randomly and are selected because they help
some organisms survive and produce more offspring. Other misconceptions center
on a lack of understanding of how a population changes as a result of
differential reproduction (some individuals producing more offspring), as
opposed to all individuals in a population changing. Many misconceptions about
the process of natural selection can be changed through instruction.
Finally, evolution is discussed again in the guidance following the earth and
space science standard:
As a result of their activities in grades 9—12, all students should develop an
understanding of:
Energy in the earth system
Geochemical cycles
Origin and evolution of the earth system
Origin and evolution of the universe
The discussions of the origin and evolution of the earth system and the universe
relate evolution to universal physical processes:
The Origin and Evolution of the Earth System
The sun, the earth, and the rest of the solar system formed from a nebular
cloud of dust and gas 4.5 billion years ago. The early earth was very
different from the planet we live on today.
Geologic time can be estimated by observing rock sequences and using fossils
to correlate the sequences at various locations. Current methods include using
the known decay rates of radioactive isotopes present in rocks to measure the
time since the rock was formed.
Interactions among the solid earth, the oceans, the atmosphere, and organisms
have resulted in the ongoing evolution of the earth system. We can observe
some changes such as earthquakes and volcanic eruptions on a human time scale,
but many processes such as mountain building and plate movements take place
over hundreds of millions of years.
Evidence for one-celled forms of life—the bacteria—extends back more than 3.5
billion years. The evolution of life caused dramatic changes in the
composition of the earth's atmosphere, which did not originally contain
The Origin and Evolution of the Universe
The origin of the universe remains one of the greatest questions in science.
The "big bang" theory places the origin between 10 and 20 billion years ago,
when the universe began in a hot dense state; according to this theory, the
universe has been expanding ever since.
Early in the history of the universe, matter, primarily the light atoms
hydrogen and helium, clumped together by gravitational attraction to form
countless trillions of stars. Billions of galaxies, each of which is a
gravitationally bound cluster of billions of stars, now form most of the
visible mass in the universe.
Stars produce energy from nuclear reactions, primarily the fusion of hydrogen
to form helium. These and other processes in stars have led to the formation
of all the other elements.
The standard for the history and nature of science elaborates on the knowledge
established in previous years:
As a result of activities in grades 9—12, all students should develop an
understanding of:
Science as a human endeavor
Nature of scientific knowledge
Historical perspectives
The discussion of this standard relates the nature of science explicitly to many
of the problems that arise in the teaching of evolution.
Nature of Scientific Knowledge
Science distinguishes itself from other ways of knowing and from other bodies
of knowledge through the use of empirical standards, logical arguments, and
skepticism, as scientists strive for the best possible explanations about the
natural world.
Scientific explanations must meet certain criteria. First and foremost, they
must be consistent with experimental and observational evidence about nature,
and must make accurate predictions, when appropriate, about systems being
studied. They should also be logical, respect the rules of evidence, be open
to criticism, report methods and procedures, and make knowledge public.
Explanations on how the natural world changes based on myths, personal
beliefs, religious values, mystical inspiration, superstition, or authority
may be personally useful and socially relevant, but they are not scientific.
Because all scientific ideas depend on experimental and observational
confirmation, all scientific knowledge is, in principle, subject to change as
new evidence becomes available. The core ideas of science such as the
conservation of energy or the laws of motion have been subjected to a wide
variety of confirmations and are therefore unlikely to change in the areas in
which they have been tested. In areas where data or understanding are
incomplete, such as the details of human evolution or questions surrounding
global warming, new data may well lead to changes in current ideas or resolve
current conflicts. In situations where information is still fragmentary, it is
normal for scientific ideas to be incomplete, but this is also where the
opportunity for making advances may be greatest.
Historical Perspectives
In history, diverse cultures have contributed scientific knowledge and
technologic inventions. Modern science began to evolve rapidly in Europe
several hundred years ago. During the past two centuries, it has contributed
significantly to the industrialization of Western and non-Western cultures.
However, other, non-European cultures have developed scientific ideas and
solved human problems through technology.
Usually, changes in science occur as small modifications in extant knowledge.
The daily work of science and engineering results in incremental advances in
our understanding of the world and our ability to meet human needs and
aspirations. Much can be learned about the internal workings of science and
the nature of science from study of individual scientists, their daily work,
and their efforts to advance scientific knowledge in their area of study.
The material addressing evolution in the National Science Education Standards is
embedded within the full range of content standards describing what students
should know, understand, and be able to do in the natural sciences. Used in
conjunction with standards for other parts of the science education system, the
content standards—and their treatment of evolution—point toward the levels of
scientific literacy needed to meet the challenges of the twenty-first century.
National Research Council. 1996. National Science Education Standards.
Washington, DC: National Academy Press. www.nap.edu/readingroom/books/nses
American Association for the Advancement of Science. 1993. Benchmarks for
Science Literacy. Project 2061. New York: Oxford University Press.
National Science Teachers Association. 1993. Scope, Sequence, and Coordination
of Secondary School Science. Vol. 1. The Content Core: A Guide for Curriculum
Designers. rev. ed. Arlington, VA: NSTA. www.nsta.org

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Copyright 1998 National Academy Press

Chapter 5

Frequently Asked Questions
About Evolution and the Nature of Science

Teachers often face difficult questions about evolution, many from parents and
others who object to evolution being taught. Science has good answers to these
questions, answers that draw on the evidence supporting evolution and on the
nature of science. This chapter presents short answers to some of the most
commonly asked questions.


What is evolution?

Evolution in the broadest sense explains that what we see today is different
from what existed in the past. Galaxies, stars, the solar system, and earth have
changed through time, and so has life on earth.

Biological evolution concerns changes in living things during the history of
life on earth. It explains that living things share common ancestors. Over time,
evolutionary change gives rise to new species. Darwin called this process
"descent with modification," and it remains a good definition of biological
evolution today.

What is "creation science"?

The ideas of "creation science" derive from the conviction that God created the
universe—including humans and other living things—all at once in the relatively
recent past. However, scientists from many fields have examined these ideas and
have found them to be scientifically insupportable. For example, evidence for a
very young earth is incompatible with many different methods of establishing the
age of rocks. Furthermore, because the basic proposals of creation science are
not subject to test and verification, these ideas do not meet the criteria for
science. Indeed, U.S. courts have ruled that ideas of creation science are
religious views and cannot be taught when evolution is taught.

The Supporting Evidence

How can evolution be scientific when no one was there to see it happen?
This question reflects a narrow view of how science works. Things in science can
be studied even if they cannot be directly observed or experimented on.
Archaeologists study past cultures by examining the artifacts those cultures
left behind. Geologists can describe past changes in sea level by studying the
marks ocean waves left on rocks. Paleontologists study the fossilized remains of
organisms that lived long ago.

Something that happened in the past is thus not "off limits" for scientific
study. Hypotheses can be made about such phenomena, and these hypotheses can be
tested and can lead to solid conclusions. Furthermore, many key aspects of
evolution occur in relatively short periods that can be observed directly—such
as the evolution in bacteria of resistance to antibiotics.

Isn't evolution just an inference?

No one saw the evolution of one-toed horses from three-toed horses, but that
does not mean that we cannot be confident that horses evolved. Science is
practiced in many ways besides direct observation and experimentation. Much
scientific discovery is done through indirect experimentation and observation in
which inferences are made, and hypotheses generated from those inferences are

For instance, particle physicists cannot directly observe subatomic particles
because the particles are too small. They must make inferences about the weight,
speed, and other properties of the particles based on other observations. A
logical hypothesis might be something like this: If the weight of this particle
is Y, when I bombard it, X will happen. If X does not happen, then the
hypothesis is disproved. Thus, we can learn about the natural world even if we
cannot directly observe a phenomenon —and that is true about the past, too.
In historical sciences like astronomy, geology, evolutionary biology, and
archaeology, logical inferences are made and then tested against data. Sometimes
the test cannot be made until new data are available, but a great deal has been
done to help us understand the past. For example, scorpionflies (Mecoptera) and
true flies (Diptera) have enough similarities that entomologists consider them
to be closely related. Scorpionflies have four wings of about the same size, and
true flies have a large front pair of wings but the back pair is replaced by
small club-shaped structures. If Diptera evolved from Mecoptera, as comparative
anatomy suggests, scientists predicted that a fossil fly with four wings might
be found—and in 1976 this is exactly what was discovered. Furthermore,
geneticists have found that the number of wings in flies can be changed through
mutations in a single gene.

Evolution is a well-supported theory drawn from a variety of sources of data,
including observations about the fossil record, genetic information, the
distribution of plants and animals, and the similarities across species of
anatomy and development. Scientists have inferred that descent with modification
offers the best scientific explanation for these observations.

Is evolution a fact or a theory?

The theory of evolution explains how life on earth has changed. In scientific
terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage.
Scientific theories are explanations of natural phenomena built up logically
from testable observations and hypotheses. Biological evolution is the best
scientific explanation we have for the enormous range of observations about the
living world.

Scientists most often use the word "fact" to describe an observation. But
scientists can also use fact to mean something that has been tested or observed
so many times that there is no longer a compelling reason to keep testing or
looking for examples. The occurrence of evolution in this sense is a fact.
Scientists no longer question whether descent with modification occurred because
the evidence supporting the idea is so strong.

Why isn't evolution called a law?

Laws are generalizations that describe phenomena, whereas theories explain
phenomena. For example, the laws of thermodynamics describe what will happen
under certain circumstances; thermodynamics theories explain why these events

Laws, like facts and theories, can change with better data. But theories do not
develop into laws with the accumulation of evidence. Rather, theories are the
goal of science.

Don't many famous scientists reject evolution?

No. The scientific consensus around evolution is overwhelming. Those opposed to
the teaching of evolution sometimes use quotations from prominent scientists out
of context to claim that scientists do not support evolution. However,
examination of the quotations reveals that the scientists are actually disputing
some aspect of how evolution occurs, not whether evolution occurred. For
example, the biologist Stephen Jay Gould once wrote that "the extreme rarity of
transitional forms in the fossil record persists as the trade secret of
paleontology." But Gould, an accomplished paleontologist and eloquent educator
about evolution, was arguing about how evolution takes place. He was discussing
whether the rate of change of species is constant and gradual or whether it
takes place in bursts after long periods when little change occurs—an idea known
as punctuated equilibrium. As Gould writes in response, "This quotation,
although accurate as a partial citation, is dishonest in leaving out the
following explanatory material showing my true purpose—to discuss rates of
evolutionary change, not to deny the fact of evolution itself."

Gould defines punctuated equilibrium as follows:

Punctuated equilibrium is neither a creationist idea nor even a non-Darwinian
evolutionary theory about sudden change that produces a new species all at
once in a single generation. Punctuated equilibrium accepts the conventional
idea that new species form over hundreds or thousands of generations and
through an extensive series of intermediate stages. But geological time is so
long that even a few thousand years may appear as a mere "moment" relative to
the several million years of existence for most species. Thus, rates of
evolution vary enormously and new species may appear to arise "suddenly" in
geological time, even though the time involved would seem long, and the change
very slow, when compared to a human lifetime.

Isn't the fossil record full of gaps?

Though significant gaps existed in the fossil record in the 19th century, many
have been filled in. In addition, the consistent pattern of ancient to modern
species found in the fossil record is strong evidence for evolution. The plants
and animals living today are not like the plants and animals of the remote past.
For example, dinosaurs were extinct long before humans walked the earth. We know
this because no human remains have ever been found in rocks dated to the
dinosaur era.

Some changes in populations might occur too rapidly to leave many transitional
fossils. Also, many organisms were very unlikely to leave fossils, either
because of their habitats or because they had no body parts that could easily be
fossilized. However, in many cases, such as between primitive fish and
amphibians, amphibians and reptiles, reptiles and mammals, and reptiles and
birds, there are excellent transitional fossils.

Can evolution account for new species?

One argument sometimes made by supporters of "creation science" is that natural
selection can produce minor changes within species, such as changes in color or
beak size, but cannot generate new species from pre-existing species. However,
evolutionary biologists have documented many cases in which new species have
appeared in recent years (some of these cases are discussed in Chapter 2). Among
most plants and animals, speciation is an extended process, and a single human
observer can witness only a part of this process. Yet these observations of
evolution at work provide powerful confirmation that evolution forms new

If humans evolved from apes, why are there still apes?

Humans did not evolve from modern apes, but humans and modern apes shared a
common ancestor, a species that no longer exists. Because we shared a recent
common ancestor with chimpanzees and gorillas, we have many anatomical, genetic,
biochemical, and even behavioral similarities with the African great apes. We
are less similar to the Asian apes—orangutans and gibbons—and even less similar
to monkeys, because we shared common ancestors with these groups in the more
distant past.

Evolution is a branching or splitting process in which populations split off
from one another and gradually become different. As the two groups become
isolated from each other, they stop sharing genes, and eventually genetic
differences increase until members of the groups can no longer interbreed. At
this point, they have become separate species. Through time, these two species
might give rise to new species, and so on through millennia.

Doesn't the sudden appearance of all the "modern groups" of animals during the
Cambrian explosion prove creationism?

During the Cambrian explosion, primitive representatives of the major phyla of
invertebrate animals appeared—hard-shelled organisms like mollusks and
arthropods. More modern representatives of these invertebrates appeared
gradually through the Cambrian and the Ordovician periods. "Modern groups" like
terrestrial vertebrates and flowering plants were not present. It is not true
that "all the modern groups of animals" appeared during this period.

Also, Cambrian fossils did not appear spontaneously. They had ancestors in the
Precambrian period, but because these Precambrian forms were soft-bodied, they
left fewer fossils. A characteristic of the Cambrian fossils is the evolution of
hard body parts, which greatly improved the chance of fossilization. And even
without fossils, we can infer relationships among organisms from biochemical

Religious Issues

Can a person believe in God and still accept evolution?

Many do. Most religions of the world do not have any direct conflict with the
idea of evolution. Within the Judeo-Christian religions, many people believe
that God works through the process of evolution. That is, God has created both a
world that is ever-changing and a mechanism through which creatures can adapt to
environmental change over time.

At the root of the apparent conflict between some religions and evolution is a
misunderstanding of the critical difference between religious and scientific
ways of knowing. Religions and science answer different questions about the
world. Whether there is a purpose to the universe or a purpose for human
existence are not questions for science. Religious and scientific ways of
knowing have played, and will continue to play, significant roles in human

No one way of knowing can provide all of the answers to the questions that
humans ask. Consequently, many people, including many scientists, hold strong
religious beliefs and simultaneously accept the occurrence of evolution.
Aren't scientific beliefs based on faith as well?

Usually "faith" refers to beliefs that are accepted without empirical evidence.
Most religions have tenets of faith. Science differs from religion because it is
the nature of science to test and retest explanations against the natural world.
Thus, scientific explanations are likely to be built on and modified with new
information and new ways of looking at old information. This is quite different
from most religious beliefs.

Therefore, "belief" is not really an appropriate term to use in science, because
testing is such an important part of this way of knowing. If there is a
component of faith to science, it is the assumption that the universe operates
according to regularities—for example, that the speed of light will not change
tomorrow. Even the assumption of that regularity is often tested—and thus far
has held up well. This "faith" is very different from religious faith.

Science is a way of knowing about the natural world. It is limited to explaining
the natural world through natural causes. Science can say nothing about the
supernatural. Whether God exists or not is a question about which science is

Legal Issues

Why can't we teach creation science in my school?

The courts have ruled that "creation science" is actually a religious view.
Because public schools must be religiously neutral under the U.S. Constitution,
the courts have held that it is unconstitutional to present creation science as
legitimate scholarship.

In particular, in a trial in which supporters of creation science testified in
support of their view, a district court declared that creation science does not
meet the tenets of science as scientists use the term (McLean v. Arkansas Board
of Education). The Supreme Court has held that it is illegal to require that
creation science be taught when evolution is taught (Edwards v. Aguillard). In
addition, district courts have decided that individual teachers cannot advocate
creation science on their own (Peloza v. San Juan Capistrano School District and
Webster v. New Lennox School District).

Teachers' organizations such as the National Science Teachers Association, the
National Association of Biology Teachers, the National Science Education
Leadership Association, and many others also have rejected the science and
pedagogy of creation science and have strongly discouraged its presentation in
the public schools. (Statements from some of these organizations appear in
Appendix C.) In addition, a coalition of religious and other organizations has
noted in "A Joint Statement of Current Law" (see Appendix B) that "in science
class, [schools] may present only genuinely scientific critiques of, or evidence
for, any explanation of life on earth, but not religious critiques (beliefs
unverifiable by scientific methodology)."

Some argue that "fairness" demands the teaching of creationism along with
evolution. But a science curriculum should cover science, not the religious
views of particular groups or individuals.

Educational Issues

If evolution is taught in schools, shouldn't creationism be given equal time?
Some religious groups deny that microorganisms cause disease, but the science
curriculum should not therefore be altered to reflect this belief. Most people
agree that students should be exposed to the best possible scholarship in each
field. That scholarship is evaluated by professionals and educators in those
fields. In science, scientists as well as educators have concluded that
evolution—and only evolution—should be taught in science classes because it is
the only scientific explanation for why the universe is the way it is today.
Many people say that they want their children to be exposed to creationism in
school, but there are thousands of different ideas about creation among the
world's people. Comparative religions might comprise a worthwhile field of study
but not one appropriate for a science class. Furthermore, the U.S. Constitution
states that schools must be religiously neutral, so legally a teacher could not
present any particular creationist view as being more "true" than others.
Why should teachers teach evolution when they already have so many things to
teach and can cover biology without mentioning evolution?

Teachers face difficult choices in deciding what to teach in their limited time,
but some ideas are of central importance in each discipline. In biology,
evolution is such an idea. Biology is sometimes taught as a list of facts, but
if evolution is introduced early in a class and in an uncomplicated manner, it
can tie many disparate facts together. Most important, it offers a way to
understand the astonishing complexity, diversity, and activity of the modern
world. Why are there so many different types of organisms? What is the response
of a species or community to a changing environment? Why is it so difficult to
develop antibiotics and insecticides that are useful for more than a decade or
two? All of these questions are easily discussed in terms of evolution but are
difficult to answer otherwise.

A lack of instruction about evolution also can hamper students when they need
that information to take other classes, apply for college or medical school, or
make decisions that require a knowledge of evolution.

Should students be given lower grades for not believing in evolution?

No. Children's personal views should have no effect on their grades. Students
are not under a compulsion to accept evolution. A grade reflects a teacher's
assessment of a student's understanding. If a child does not understand the
basic ideas of evolution, a grade could and should reflect that lack of
understanding, because it is quite possible to comprehend things that are not

Can evolution be taught in an inquiry-based fashion?

Any science topic can be taught in an inquiry-oriented manner, and evolution is
particularly amenable to this approach. At the core of inquiry-oriented
instruction is the provision for students to collect data (or be given data when
collection is not possible) and to analyze the data to derive patterns,
conclusions, and hypotheses, rather than just learning facts. Students can use
many data sets from evolution (such as diagrams of anatomical differences in
organisms) to derive patterns or draw connections between morphological forms
and environmental conditions. They then can use their data sets to test their

Students also can collect data in real time. For example, they can complete
extended projects involving crossbreeding of fruit flies or plants to illustrate
the genetic patterns of inheritance and the influence of the environment on
survival. In this way, students can develop an understanding of evolution,
scientific inquiry, and the nature of science.

Chapter 6

Activities for Teaching About
Evolution and the Nature of Science

Prior chapters in this volume answer the what and why questions of teaching
about evolution and the nature of science. As every educator knows, such
discussions only set a stage. The actual play occurs when science teachers act
on the basic content and well-reasoned arguments for inclusion of evolution and
the nature of science in school science programs.

This chapter goes beyond discussions of content and rationales. It presents, as
examples of investigative teaching exercises, eight activities that science
teachers can use as they begin developing students' understandings and abilities
of evolution and the nature of science. The following descriptions briefly
introduce each activity.

* ACTIVITY 1: Introducing Inquiry and the Nature of Science

This activity introduces basic procedures involved in inquiry and concepts
describing the nature of science. In the first portion of the activity the
teacher uses a numbered cube to involve students in asking a question—what is
on the unseen bottom of the cube?—and the students propose an explanation
based on their observations. Then the teacher presents the students with a
second cube and asks them to use the available evidence to propose an
explanation for what is on the bottom of this cube. Finally, students design a
cube that they exchange and use for an evaluation. This activity provides
students with opportunities to learn the abilities and understandings aligned
with science as inquiry and the nature of science as described in the National
Science Education Standards.1 Designed for grades 5 through 12, the activity
requires a total of four class periods to complete. Lower grade levels might
only complete the first cube and the evaluation where students design a
problem based on the cube activity.

* ACTIVITY 2: The Formulation of Explanations: An Invitation to Inquiry on
Natural Selection

This activity uses the concept of natural selection to introduce the idea of
formulating and testing a scientific hypothesis. Through a focused discussion
approach, the teacher provides information and allows students time to think,
interact with peers, and propose explanations for observations described by
the teacher. The teacher then provides more information, and the students
continue their discussion based on the new information. This activity will
help students in grades 5 through 8 develop abilities related to scientific
inquiry and formulate understandings about the nature of science.

* ACTIVITY 3: Investigating Natural Selection

In this activity, the students investigate one mechanism for evolution through
a simulation that models the principles of natural selection and helps answer
the question: How might biological change have occurred and been reinforced
over time? The activity is designed for grades 9 through 12 and requires three
class periods.

* ACTIVITY 4: Investigating Common Descent: Formulating Explanations and Models

In this activity, students formulate explanations and models that simulate
structural and biochemical data as they investigate the misconception that
humans evolved from apes. The investigations require two 45-minute periods.
They are designed for use in grades 9 through 12.

* ACTIVITY 5: Proposing Explanations for Fossil Footprints

In this investigation, students observe and interpret "fossil footprint"
evidence. From the evidence, they are asked to construct defensible hypotheses
or explanations for events that took place in the geologic past. Estimated
time requirements for this activity: two class periods. This activity is
designed for grades 5 through 8.

* ACTIVITY 6: Understanding Earth's Changes Over Time

Comparing the magnitude of geologic time to spans of time within a person's
own lifetime is difficult for many students. In this activity, students use a
long paper strip and a reasonable scale to represent visually all of geologic
time, including significant events in the development of life on earth as well
as recent human events. The investigation requires two class periods and is
appropriate for grades 5 through 12.

* ACTIVITY 7: Proposing the Theory of Biological Evolution: Historical

This activity uses historical perspectives and the theme of evolution to
introduce students to the nature of science. The teacher has students read
short excerpts of original statements on evolution from Jean Lamarck, Charles
Darwin, and Alfred Russel Wallace. These activities are intended as either
supplements to other investigations or core activities. Designed for grades 9
through 12, the activities should be used as part of three class periods.

* ACTIVITY 8: Connecting Population Growth and Biological Evolution

In this activity, students develop a model of the mathematical nature of
population growth. The investigation provides an excellent opportunity for
consideration of population growth of plant and animal species and the
relationship to mechanisms promoting natural selection. This activity will
require two class periods and is appropriate for grades 5 through 12.

The activities in this chapter do not represent a curriculum. They are directed,
instead, toward other purposes.

First, they present examples of standards-based instructional materials. In this
case, the level of organization is an activity—one to five days of lessons—and
not a larger level of organization such as a unit of several weeks, a semester,
or a year. Also, these exercises generally do not use biological materials, such
as fruit flies, or computer simulations. The use of these instructional
materials in the curriculum greatly expands the range of possible

Second, these activities demonstrate how existing exercises can be recast to
emphasize the importance of inquiry and the fundamental concepts of evolution.
Each of these exercises was derived from already existing activities that were
revised to reflect the National Science Education Standards. For each exercise,
student outcomes drawn from the Standards are listed to focus attention on the
concepts and abilities that students are meant to develop.

Third, the activities demonstrate some, but not all, of the criteria for
curricula to be described in Chapter 7. For example, several of the activities
emphasize inquiry and the nature of science while others focus on concepts
related to evolution. All activities use an instructional model, described in
the next section, that increases coherence and enhances learning.

Finally, there remains a paucity of instructional materials for teaching
evolution and the nature of science. Science teachers who recognize this need
are encouraged to develop new materials and lessons to introduce the themes of
evolution and the nature of science. (See http://www4.nas.edu/opus/evolve.nsf)
Developing Students' Understanding and Abilities: The Curriculum Perspective

An Instructional Model

ENGAGE — This phase of the instructional model initiates the learning
task. The activity should (1) make connections between past and present
learning experiences and (2) anticipate activities and focus students'
thinking on the learning outcomes of current activities. Students should
become mentally engaged in the concept, process, or skill to be explored.

EXPLORE — This phase of the teaching model provides students with a
common base of experiences within which they identify and develop
current concepts, processes, and skills. During this phase, students
actively explore their environment or manipulate materials.

EXPLAIN — This phase of the instructional model focuses students'
attention on a particular aspect of their engagement and exploration
experiences and provides opportunities for them to develop explanations
and hypotheses. This phase also provides opportunities for teachers to
introduce a formal label or definition for a concept, process, skill, or

ELABORATE — This phase of the teaching model challenges and extends
students' conceptual understanding and allows further opportunity for
students to test hypotheses and practice desired skills and behaviors.
Through new experiences, the students develop a deeper and broader
understanding, acquire more information, and develop and refine skills.

EVALUATE — This phase of the teaching model encourages students to
assess their understanding and abilities and provides opportunities for
teachers to evaluate student progress toward achieving the educational

For students to develop an understanding of evolution and the nature of science
requires many years and a variety of educational experiences. Teachers cannot
rely on single lessons, chapters, or biology and earth science courses for
students to integrate the ideas presented in this document into their own
understanding. In early grades (K—4) students might learn the fundamental
concepts associated with "characteristics of organisms," "life cycles," and
"organisms and environments." In middle grades they learn more about
"reproduction and heredity" and "diversity and adaptation of organisms." Such
learning experiences, as described in the National Science Education Standards,
set a firm foundation for the study of biological evolution in grades 9—12.

The slow and steady development of concepts such as evolution and related ideas
such as natural selection and common descent requires careful consideration of
the overall structure and sequence of learning experiences. Although this
chapter does not propose a curriculum or a curriculum framework, current efforts
by Project 2061 of the American Association for the Advancement of Science
(AAAS) demonstrate the interrelated nature of students' understanding of science
concepts and emphasize the importance of well-designed curricula at several
levels of organization (for example, activities, units, and school science
programs). The figure on the next page presents the "Growth-of-Understanding Map
for Evolution and Natural Selection" based on Benchmarks for Science Literacy.2

Developing Student Understanding and Abilities: The Instructional Perspective
The activities in the chapter incorporate an instructional model, summarized in
the accompanying box, that includes five steps: engagement, exploration,
explanation, elaboration, and evaluation. Just as scientific investigations
originate with a question that engages a scientist, so too must students engage
in the activities of learning. The activities therefore begin with a strategic
question that gets students thinking about the content of the lesson.

Once engaged, students need time to explore ideas before concepts begin to make
sense. In this exploration phase, students try their ideas, ask questions, and
look for possible answers to questions. Students use inquiry strategies; they
try to relate their ideas to those of other students and to what scientists
already know about evolution.

In the third step, students can propose answers and develop hypotheses. Also in
this step, the teacher explains what scientists know about the questions. This
is the step when teachers should make the major concepts explicit and clear to
the students.

Educators understand that informing students about a concept does not
necessarily result in their immediate comprehension and understanding of the
idea. These activities therefore provide a step referred to as elaboration in
which students have opportunities to apply their ideas in new and slightly
different situations.

Finally, how well do students understand the concepts, or how successful are
they at applying the desired skills? These are the questions to be answered
during the evaluation phase. Ideally, evaluations are more than tests. Students
should have opportunities to see if their ideas can be applied in new situations
and to compare their understanding with scientific explanations of the same


National Research Council. 1996. National Science Education Standards.
Washington, DC: National Academy Press. www.nap.edu/readingroom/books/nses
A Draft Growth-of-Understanding Map derived from Benchmarks for Science
Literacy (Jan. 1998), AAAS (American Association for the Advancement of
Science) Project 2061.

ACTIVITY 1: Introducing Inquiry and the Nature of Science
ACTIVITY 2: The Formulation of Explanations: An Invitation to Inquiry on
Natural Selection
ACTIVITY 3: Investigating Natural Selection
ACTIVITY 4: Investigating Common Descent: Formulating Explanations and Models
ACTIVITY 5: Proposing Explanations for Fossil Footprints
ACTIVITY 6: Understanding Earth's Changes Over Time
ACTIVITY 7: Proposing the Theory of Biological Evolution: Historical
ACTIVITY 8: Connecting Population Growth and Biological Evolution

PDF Activities and Worksheets

Cube #1
Cube #2
Cube #3
Footprint Puzzle
Student Investigation Sheet A
Student Sheet: Zoological Philosophy
Student Sheet: On the Tendency of Varieties to Depart Indefinitely from the
Original Type
Student Sheet: On the Origin of Species
Student Sheet: On the Origin of Species (continued)

Chapter 7

Selecting Instructional Materials

Quality instructional materials are essential in teaching about evolution and
the nature of science.

It also is important to consider the context within which specific materials
will be used. This chapter therefore begins with brief discussions of school
science programs and the criteria used to design curricula.

Criteria for Contemporary Science Curriculum

Before selecting specific materials to teach evolution and the nature of
science, it is important to identify criteria that can help evaluate school
science programs and the design of instructional materials. Chapter seven in the
National Science Education Standards, "Science Education Program Standards,"
describes the conditions needed for quality school science programs. These
conditions focus on six areas:

Consistency across all elements of the science program and across the K-12
Quality in the program of studies
Coordination with mathematics
Quality resources
Equitable opportunities for achievement

Collaboration within the school community to support a quality program
Similarly, educators need to consider criteria against which to judge
instructional materials. Teachers, curriculum designers, and other school
personnel can use the following criteria to evaluate the design of a new
curriculum, to select instructional materials, or to adapt instructional
materials through professional development. No set of instructional materials
will meet all the following criteria. You will have to make a judgment about the
degree to which materials meet criteria and about acceptable and unacceptable
omissions. These criteria are adapted from earlier discussions of
standards-based curriculum.1

Criterion 1: A Coherent, Consistent, and Coordinated Framework for Science
Content. Science content should be consistent with national, state, and local
standards and benchmarks. Whether for lessons, units, or a complete elementary,
middle, or high school program, the content should be well-thought-out,
coordinated, and conceptually, procedurally, and coherently organized. The roles
of science concepts, inquiry, science in personal and social contexts, and the
history and nature of science should be clear and explicit.

Criterion 2: An Organized and Systematic Approach to Instruction. Most
contemporary science curricula incorporate an instructional model. The
instructional model should (1) provide for different forms of interaction among
students and between the teachers and students, (2) incorporate a variety of
teaching strategies, such as inquiry-oriented investigations, cooperative
groups, use of technology, and (3) allow adequate time and opportunities for
students to acquire knowledge, skills, and attitudes.

Criterion 3: An Integration of Psychological Principles Relative to Cognition,
Motivation, Development, and Social Psychology. Psychological principles such as
those found in the American Psychological Association publication How Students
Learn: Reforming School Through Learner-Centered Education2 should be applied to
the framework for content, teaching, and assessment. These psychological
principles include more than learning theory. They include providing for
motivation, development, and social interactions.

Criterion 4: Varied Curriculum Emphases. The idea of curriculum emphases can be
expressed by thinking about the foreground and background in a painting. An
artist decides what will be in the foreground, and that subject is emphasized.
Science curricula can, for example, emphasize science concepts, inquiry, or the
history and nature of science, while other goals may be evident but not
emphasized. No one curriculum emphasis is best for all students; probably, a
variety of emphases accommodates the interests, strengths, and demands of
science content.

Criterion 5: An Array of Opportunities to Develop Knowledge, Understanding, and
Abilities Associated with Different Dimensions of Scientific Literacy.
Contemporary science curricula should provide a balance among the different
dimensions of science literacy, which include an understanding of scientific
concepts, the ability to engage in inquiry, and a capacity to apply scientific
information in making decisions.3

Criterion 6: Teaching Methods and Assessment Strategies Consistent with the Goal
of Science Literacy. Approaches to teaching and assessment ought to be
consistent with the goals of teaching evolution, inquiry, and the history and
nature of science. This can be accomplished by using inquiry-oriented teaching
methods and by assessing students during investigative activities.

Criterion 7: Professional Development for Science Teachers Who Implement the
Curriculum. Curricula need to provide opportunities that support teachers as
they develop the knowledge and skills associated with implementing and
institutionalizing the science program.

Criterion 8: An Inclusion of Appropriate Educational Technologies. The use of
computers and various types of software enhances learning when students use the
technologies in meaningful ways. The use of educational technologies should be
consistent with other features of the curriculum—for instance, the dimensions of
scientific literacy and an instructional model.

Criterion 9: Thorough Field Testing and Review for Scientific Accuracy and
Pedagogic Quality. One important legacy of the 1960s curriculum reform is the
field testing of materials in a variety of science classrooms. Field testing and
reviewing a program identify problems that developers did not recognize and fine
tune the materials to the varied needs of teachers, learners, and schools.
Scientists should review materials for accuracy. Developers can miss the
subtleties of scientific concepts, inquiry, and design. In addition, educators
who review materials can provide valuable insights about teaching and assessment
that help developers improve materials and enhance learning.

Criterion 10: Support from the Educational System. Research on the adoption,
implementation, and change associated with curricula indicates the importance of
intellectual, financial, and moral support from those within the larger
educational system.4 This support includes science teachers, administrators,
school boards, and communities. Although a curriculum cannot ensure support, it
should address the need for support and provide indicators of support, such as
provision of materials and equipment for laboratory investigations, budget
allocations for professional development, and proclamations by the school board.
Clearly, no one curriculum thoroughly incorporates all ten criteria. There are
always trade-offs when developing, adapting, or adopting a science curriculum.
However, the criteria should provide assistance to those who have the
responsibility of improving the science curriculum.

Analyzing Instructional Materials

The process of selecting quality materials includes determining the degree to
which they are consistent with the goals, principles, and criteria developed in
the National Science Education Standards. Well-defined selection criteria help
ensure a thoughtful and effective process. To be both usable and defensible, the
selection criteria must be few in number and embody the critical tenets of
accurate science content, effective teaching strategies, and appropriate
assessment techniques.

The process described in the following pages can help teachers, curriculum
designers, or other school personnel complete a thorough and accurate evaluation
of instructional materials. To help make this examination both thorough and
usable, references to specific pages and sections in the National Science
Education Standards have been provided, as have worksheets to keep track of the
information needed to analyze and select the best instructional materials.

Analysis Procedures

The procedures outlined in this section include:

Overview of instructional materials
Analysis of science subject matter
Analysis of pedagogy
Analysis of assessment process
Evaluating the teacher's guide
Analysis of use and management

The extent to which instructional materials meet the criteria outlined in this
chapter determines their usefulness for classroom teachers and the degree of
alignment with the Standards. A thorough analysis of instructional materials
requires considerable time and collaboration with others and attention to
detail. Good working notes are helpful in this process. We recommend using the
analysis worksheets provided at the end of this chapter.

Overview of Instructional Materials

The following overview of instructional materials introduces the review process
and provides a general context for analysis and subsequent selection of specific

1. The first consideration is whether the key concepts of evolution and the
nature of science are being emphasized. To help make this determination, locate
the table of contents, index, and glossary in the material you are evaluating.
The box below contains terms related to fundamental concepts in evolution and
the nature of science taken from the Standards. Record page numbers where each
is found for future reference. (See Worksheet 1 on page 112 in the back of this
chapter.) These terms will give you a preliminary indication of coverage on
these fundamental topics.


Nature of Science

evolution, diversity, adaptation, interpreting fossil evidence, techniques
for age determination, natural selection, descent from common ancestors
explanation, experiment, evidence, inquiry, model, theory, skepticism

2. Look through both student and teacher materials. Are student outcomes listed?

Note page numbers for several outcomes related to evolution and the nature of

3. Look for student investigations or activities. Where are they located? Note
that in some materials, student investigations are integrated within the reading
material. In others they are located in a separate section—sometimes at the back
of a chapter or book or in a separate laboratory manual.

4. Read several relevant paragraphs of student text material. What is your
judgment about the concepts? Are the concepts in the students' text consistent
with the fundamental concepts in the Standards? Does the text include more,
fewer, or different concepts?

5. Do the photographs and illustrations provide further understanding of the
fundamental concepts?

Analysis of Instructional Materials for Science Subject Matter


The following procedures for content analysis will help you examine
instructional materials for fundamental concepts of evolution, science as
inquiry, and the nature of science. Look for evidence in discussions in the text
and in the student investigations to determine the degree to which the
fundamental concepts are addressed. Fundamental concepts underlying specific
standards on evolution and the nature of science are referenced below. (Note:
You will need a copy of the National Science Education Standards or access to it
through the World Wide Web at www.nap.edu/readingroom/books/nses.)

Content Standard C—Life Science: grades 5-8, "Diversity and Adaptations of
Organisms," p. 158; grades 9-12, "Biological Evolution," p. 185; also read
"Developing Student Understanding" grades 5-8, pp. 155-156; and grades
9-12, p. 181.

Content Standard D—Earth and Space Science: grades 5-8, "Earth's History,"
p. 160; grades 9-12, "The Origin and Evolution of the Earth System," pp.
189-190; also read "Developing Student Understanding," grades 5-8, pp.
158-159; grades 9-12, pp. 187-188.

1. Choose a lesson or representative section of the student instructional
materials on the topic of evolution. Make a preliminary list of the fundamental
concepts from the Standards that are included in the lesson and place them on
your worksheet. (See Worksheet 2 on page 114 in the back of this chapter.)
2. Select one of these fundamental concepts and list all sections of the
materials that deal with this idea. Determine whether the materials focus on the
fundamental concepts, or if they represent only a superficial match. For
example, Life Science Standard C in the Standards5 specifies: "Biological
evolution accounts for the diversity of species developed through gradual
processes over many generations. Species acquire many of their unique
characteristics through biological adaptation, which involves the selection of
naturally occurring variations in populations." The instructional materials
should provide opportunities for students to develop an understanding of
biodiversity and evolution as described in the Standards. A negative example
would be defining the term biodiversity only in reference to the fact that wide
varieties of plants and animals populate particular environments.
You should complete this analysis for all fundamental concepts associated with a
particular standard. The more fundamental concepts you analyze using this
process, the more confidence you will have in the quality of the instructional
materials and their alignment with the Standards. Identify the fundamental
concepts that are not developed and the variation of treatment among those that
are included in the materials.
3. If appropriate, select one of the student investigations for analysis of
subject matter. On what fundamental concepts from Life Science Standard C or
Earth and Space Science Standard D is the investigation focused? To what degree
does the activity fulfill the intent of the fundamental concepts? For example,
making and comparing model casts and molds of sea shells does not necessarily
contribute to an understanding of how fossils are formed or provide important
evidence of how life and environmental conditions have changed. It is
recommended that you analyze a second student investigation.
1. You should develop some understanding of scientific inquiry in the Standards.
Read Standard A, Science as Inquiry, referenced on the following page.

Standard A—Science as Inquiry: grades 5-8, pp. 145-148; grades 9-12, pp.
175-176; also read "Developing Student Understanding," grades 5-8, pp.
143-144; grades 9-12, pp. 173-174.

Note that Standard A specifies two separate aspects of science as inquiry:
abilities necessary to do scientific inquiry, and fundamental understandings
about scientific inquiry. Examine several lessons in the student and teacher
materials to answer the following question: To what degree do the lessons
provide students the opportunity to develop the abilities and understandings of
scientific inquiry?
2. Read through the text narrative, looking for student investigations and
examining any suggestions for activities outside of class time. Are
opportunities provided for students to develop abilities of scientific inquiry
such as posing their own relevant questions, planning and conducting
investigations, using appropriate tools and techniques to gather data, using
evidence to communicate defensible explanations of cause and effect
relationships, or using scientific criteria to analyze alternative explanations
to determine a preferred explanation? Record page numbers where examples are
found and make notes of explanation.
3. What opportunities are provided for students to develop a fundamental
understanding of scientific inquiry? In addition to the language of the text,
examine the teacher's guide for suggestions that teachers can use to discuss the
role and limitations of scientific skills such as making observations,
organizing and interpreting data, and constructing defensible explanations based
on evidence. Can you find a discussion of how science advances through
legitimate skepticism? Can you find a discussion of how scientists evaluate
proposed explanations of others by examining and comparing evidence, identifying
reasoning that goes beyond the evidence, and suggesting alternative explanations
for the same evidence? Are there opportunities for students to demonstrate these
same understandings as a part of their investigations? Make notes where this
evidence is found for later reference.
1. Are history and the nature of science incorporated into the treatment of
evolution? Read Standard G, History and Nature of Science, referenced in the
following box.

Content Standard G—History and Nature of Science: grades 5-8, pp. 170-171;
grades 9-12, pp. 200-201 and p. 204; also read "Developing Student
Understanding," grades 5-8, p. 170; grades 9-12, p. 200.

2. Read through several lessons in the student and teacher materials. Can you
find examples describing the roles of scientists, human insight, and scientific
reasoning in the historical and contemporary development of explanations for
evolution? Can you find specific references to historical contributions of
scientists in the development of fundamental concepts of evolution? What
evidence can you find in the text narrative or student investigations that
demonstrates how scientific explanations are developed, reviewed by peers, and
revised in light of new evidence and thinking?
Analysis of Pedagogy
What students learn about evolution and the nature of science depends on many
things, including the accuracy and developmental appropriateness of content and
its congruence with the full intent of the content standards. Opportunities to
learn should be consistent with contemporary models of learning. The criteria in
this section are based on characteristics of effective teaching proposed in
Teaching Standards A, B, and E.

Teaching Standard A—Teachers of science plan an inquiry-based science
program for their students, pp. 30-32.

Teaching Standard B—Teachers of science guide and facilitate learning, pp.
32-33 and 36-37.

Teaching Standard E—Teachers of science develop communities of science
learners that reflect the intellectual rigor of scientific inquiry and the
attitudes and social values conducive to science learning, pp. 45-46 and

Using the following sequence of questions, examine several lessons in the
student materials and the teacher's guide. (See Worksheet 3 on page 117 in the
back of this chapter.)
Do the materials identify specific learning goals or outcomes for students
that focus on one or more of the fundamental concepts of evolution and the
nature of science?

Study the opening pages of a relevant chapter or section. Does the material on
the opening pages of the chapter or section on evolution engage and focus
student thinking on interesting questions, problems, or relevant issues?

Does the material provide a sequence of learning activities connected in such
a way as to help students build understanding of a fundamental concept? Are
suggestions provided to help the teacher keep students focused on the purpose
of the lesson?

Does the teacher's guide present common student misconceptions related to the
fundamental concepts of evolution and the nature of science? Are suggestions
provided for teachers to find out what their students already know? Are there
learning activities designed to help students confront their misconceptions
and encourage conceptual change?

Analysis of Assessment Process
Assessment criteria in this section are grounded in the Assessment Standards.

Assessment Standards A to E, Chapter 5, pp. 78-87.

Examine several lessons in the student and teacher materials for evidence to
answer the following questions. (See Worksheet 4 on page 118 in the back of this
Is there consistency between learning goals and assessment? For example, if
instruction focuses on building understanding of fundamental concepts, do
assessments focus on explanations and not on vocabulary?

Do assessments stress application of concepts to new or different situations?
For example, are the students asked to explain new situations with concepts
they have learned?

Are assessment tasks fair for all students? For example, does success on
assessment tasks depend too heavily on the student's ability to read complex
items or write explanations as opposed to understanding the fundamental

Are suggestions for scoring criteria or rubrics provided for the teacher?

Evaluating the Teacher's Guide
Examine several lessons in the teacher's guide to help answer the following
Does the teacher's guide present appropriate and sufficient background on

Are the suggested teaching strategies usable by most teachers?

Are suggestions provided for pre- and post-investigation discussions focusing
on concept development, inquiry, and the nature of science?

Does the teacher's guide recommend additional professional development?

Does the teacher's guide indicate the types of support teachers will need for
the instructional materials?

Analysis of Use and Management
A high degree of alignment with Standards content, pedagogy, and assessment
criteria does not necessarily guarantee that instructional materials will be
easy to manage. The Standards address the importance of professional
development, and some aspects of the program standards apply as well.6
How many different types of materials must be managed and orchestrated during
a typical chapter, unit, or teaching sequence (e.g., student text, teacher's
guide, transparencies, handouts, videos, and software)? (See Worksheet 5 on
page 119 in the back of this chapter.)

Does the teacher's guide contain suggestions for effectively managing

Do the instructional materials call for equipment, supplies, and technology
that teachers may not have?

Do the instructional materials identify safety issues and provide adequate

Is the cost for materials and replacements reasonable? Are there special

Rodger Bybee. 1997. Achieving Scientific Literacy: From Purposes to Practices.
Portsmouth, NH: Heinemann. Rodger Bybee, 1996. National Standards and the
Science Curriculum. Dubuque, IA: Kendall/Hunt Publishing Co.
N. M. Lambert and B. L. McCombs. 1998. How Students Learn: Reforming Schools
Through Learner-Centered Education. Washington, DC: American Psychological
National Research Council. 1996. National Science Education Standards.
Washington, DC: National Academy Press, p. 22.
M.G. Fullan and S. Stiegelbauer. 1991. The New Meaning of Educational Change,
2nd ed. New York: Teachers College Press, Columbia University.
G.E. Hall and S.M. Hord. 1987. Change in Schools: Facilitating the Process.
Albany: State University of New York Press.
S. Loucks-Horsley and S. Stiegelbauer. 1991. Using Knowledge of Change to
Guide Staff Development. In Staff Development for Education in the 90s: New
Demands, New Realities, New Perspectives. A. Lieberman and L. Miller, eds. New
York: Teachers College Press, Columbia University.
See National Science Education Standards, p. 158.
See National Science Education Standards, pp. 55-73.

PDF Worksheets
Worksheet 1: General Overview
Worksheet 1 (Continued)
Worksheet 2: Analysis of Science Subject Matter
Worksheet 2: (Continued)
Worksheet 2: (Continued)
Worksheet 3: Analysis of Pedagogy
Worksheet 4: Analysis of Assessment Process
Worksheet 5: Analysis of Use and Management

[Table of Contents] — [Previous Section] — [Next Section]

Copyright 1998 National Academy Press

Appendix A

Six Significant Court Decisions Regarding Evolution and Creationism Issues1

The following are excerpts from important court decisions regarding evolution
and creationism issues. The reader is encouraged to read the full statements as
need and time allows.

In 1968, in Epperson v. Arkansas, the United States Supreme Court invalidated
an Arkansas statute that prohibited the teaching of evolution. The Court held
the statute unconstitutional on grounds that the First Amendment to the U.S.
Constitution does not permit a state to require that teaching and learning
must be tailored to the principles or prohibitions of any particular religious
sect or doctrine. (Epperson v. Arkansas, 393 U.S. 97. (1968))

In 1981, in Segraves v. State of California, the Court found that the
California State Board of Education's Science Framework, as written and as
qualified by its anti-dogmatism policy, gave sufficient accommodation to the
views of Segraves, contrary to his contention that class discussion of
evolution prohibited his and his children's free exercise of religion. The
anti-dogmatism policy provided that class distinctions of origins should
emphasize that scientific explanations focus on "how," not "ultimate cause,"
and that any speculative statements concerning origins, both in texts and in
classes, should be presented conditionally, not dogmatically. The court's
ruling also directed the Board of Education to widely disseminate the policy,
which in 1989 was expanded to cover all areas of science, not just those
concerning issues of origins. (Segraves v. California, No. 278978 Sacramento
Superior Court (1981))

In 1982, in McLean v. Arkansas Board of Education, a federal court held that a
"balanced treatment" statute violated the Establishment Clause of the U.S.
Constitution. The Arkansas statute required public schools to give balanced
treatment to "creation-science" and "evolution-science." In a decision that
gave a detailed definition of the term "science," the court declared that
"creation science" is not in fact a science. The court also found that the
statute did not have a secular purpose, noting that the statute used language
peculiar to creationist literature in emphasizing origins of life as an aspect
of the theory of evolution. While the subject of life's origins is within the
province of biology, the scientific community does not consider the subject as
part of evolutionary theory, which assumes the existence of life and is
directed to an explanation of how life evolved after it originated. The theory
of evolution does not presuppose either the absence or the presence of a
creator. (McLean v. Arkansas Board of Education, 529 F. Supp. 1255, 50 (1982)
U.S. Law Week 2412)

In 1987, in Edwards v. Aguillard, the U.S. Supreme Court held unconstitutional
Louisiana's "Creationism Act." This statute prohibited the teaching of
evolution in public schools, except when it was accompanied by instruction in
"creation science." The Court found that, by advancing the religious belief
that a supernatural being created humankind, which is embraced by the term
creation science, the act impermissibly endorses religion. In addition, the
Court found that the provision of a comprehensive science education is
undermined when it is forbidden to teach evolution except when creation
science is also taught. (Edwards v. Aguillard, 482, U.S. 578, 55 (1987) U.S.
Law Week 4860, S. CT. 2573, 96 L. Ed. 2d510)

In 1990, in Webster v. New Lennox School District, the Seventh Circuit Court
of Appeals found that a school district may prohibit a teacher from teaching
creation science in fulfilling its responsibility to ensure that the First
Amendment's establishment clause is not violated, and religious beliefs are
not injected into the public school curriculum. The court upheld a district
court finding that the school district had not violated Webster's free speech
rights when it prohibited him from teaching "creation science," since it is a
form of religious advocacy. (Webster v. New Lennox School District #122, 917
F.2d 1004 (7th. Cir., 1990))

In 1994, in Peloza v. Capistrano Unified School District, the Ninth Circuit
Court of Appeals upheld a district court finding that a teacher's First
Amendment right to free exercise of religion is not violated by a school
district's requirement that evolution be taught in biology classes. Rejecting
plaintiff Peloza's definition of a "religion" of "evolutionism," the Court
found that the district had simply and appropriately required a science
teacher to teach a scientific theory in biology class. (Peloza v. Capistrano
Unified School District, 37 F.3d 517 (9th Cir., 1994))


Matsumura, M., ed. 1995. Pp. 2-3 in Voices for Evolution. 2nd ed. Berkeley,
CA: National Center for Science Education.

Appendix B

Excerpt from "Religion in the Public Schools:
A Joint Statement of Current Law"1

Schools may teach about explanations of life on earth, including religious ones
(such as "creationism"), in comparative religion or social studies classes. In
science class, however, they may present only genuinely scientific critiques of,
or evidence for, any explanation of life on earth, but not religious critiques
(beliefs unverifiable by scientific methodology). Schools may not refuse to
teach evolutionary theory in order to avoid giving offense to religion nor may
they circumvent these rules by labeling as science an article of religious
faith. Public schools must not teach as scientific fact or theory any religious
doctrine, including "creationism," although any genuinely scientific evidence
for or against any explanation of life may be taught. Just as they may neither
advance nor inhibit any religious doctrine, teachers should not ridicule, for
example, a student's religious explanation for life on earth.


1. Excerpt from the brochure, "Religion in the Public Schools: A Joint
Statement of Current Law." April 1995. Full copy available by contacting
Religion in the Public Schools, 15 East 84th Street, Suite 501, New York, NY
10028 or by the World Wide Web at www.ed.gov./Speeches/04-1995/prayer.html.
Drafting Committee: American Jewish Congress, Chair; American Civil Liberties
Union; American Jewish Committee; American Muslim Council; Anti-Defamation
League; Baptist Joint Committee; Christian Legal Society; General Conference
of Seventh-Day Adventists; National Association of Evangelicals; National
Council of Churches; People for the American Way; Union of American Hebrew
Congregations. Endorsing Organizations: American Ethical Union; American
Humanist Association; Americans for Religious Liberty; Americans United for
Separation of Church and State; B'nai B'rith International; Christian Science
Church; Church of the Brethren, Washington Office; Church of Scientology
International; Evangelical Lutheran Church in America, Lutheran Office of
Governmental Affairs; Federation of Reconstructionist Congregations and
Havurot; Friends Committee on National Legislation; Guru Gobind Singh
Foundation; Hadassah, The Women's Zionist Organization of America; Interfaith
Alliance; Interfaith Impact for Justice and Peace; National Council of Jewish
Women; National Jewish Community Relations Advisory Council (NJCRAC); National
Ministries, American Baptist Churches, USA; National Sikh Center; North
American Council for Muslim Women; Presbyterian Church (USA); Reorganized
Church of Jesus Christ of Latter Day Saints; Unitarian Universalist
Association of Congregations; United Church of Christ, Office for Church in

Appendix C

Three Statements in Support of Teaching Evolution from
Science and Science Education Organizations

1. A NSTA (National Science Teachers Association) Position Statement on the
Teaching of Evolution1

Approved by the NSTA Board of Directors, July 1997

Introductory Remarks

The National Science Teachers Association supports the position that evolution
is a major unifying concept of science and should be included as part of
K—College science frameworks and curricula. NSTA recognizes that evolution has
not been emphasized in science curricula in a manner commensurate to its
importance because of official policies, intimidation of science teachers, the
general public's misunderstanding of evolutionary theory, and a century of

Furthermore, teachers are being pressured to introduce creationism, creation
"science," and other nonscientific views, which are intended to weaken or
eliminate the teaching of evolution.

Within this context, NSTA recommends that:

Science curricula and teachers should emphasize evolution in a manner
commensurate with its importance as a unifying concept in science and its
overall explanatory power.

Policy-makers and administrators should not mandate policies requiring the
teaching of creation science or related concepts such as so-called
"intelligent design," "abrupt appearance," and "arguments against evolution."
Science teachers should not advocate any religious view about creation, nor
advocate the converse: that there is no possibility of supernatural influence
in bringing about the universe as we know it. Teachers should be nonjudgmental
about the personal beliefs of students.

Administrators should provide support to teachers as they design and implement
curricula that emphasize evolution. This should include inservice education to
assist teachers to teach evolution in a comprehensive and professional manner.
Administrators also should support teachers against pressure to promote
nonscientific views or to diminish or eliminate the study of evolution.

Parental and community involvement in establishing the goals of science
education and the curriculum development process should be encouraged and
nurtured in our democratic society. However, the professional responsibility
of science teachers and curriculum specialists to provide students with
quality science education should not be bound by censorship, pseudoscience,
inconsistencies, faulty scholarship, or unconstitutional mandates.
Science text books shall emphasize evolution as a unifying concept. Publishers
should not be required or volunteer to include disclaimers in textbooks
concerning the nature and study of evolution.

NSTA offers the following background information:

The Nature of Science and Scientific Theories

Science is a method of explaining the natural world. It assumes the universe
operates according to regularities and that through systematic investigation we
can understand these regularities. The methodology of science emphasizes the
logical testing of alternate explanations of natural phenomena against empirical
data. Because science is limited to explaining the natural world by means of
natural processes, it cannot use supernatural causation in its explanations.
Similarly, science is precluded from making statements about supernatural forces
because these are outside its provenance. Science has increased our knowledge
because of this insistence on the search for natural causes.

The most important scientific explanations are called "theories." In ordinary
speech, "theory" is often used to mean "guess," or "hunch," whereas in
scientific terminology, a theory is a set of universal statements which explain
the natural world. Theories are powerful tools. Scientists seek to develop
theories that

are internally consistent and compatible with the evidence
are firmly grounded in and based upon evidence
have been tested against a diverse range of phenomena
possess broad and demonstrable effectiveness in problem solving
explain a wide variety of phenomena.

The body of scientific knowledge changes as new observations and discoveries are
made. Theories and other explanations change. New theories emerge and other
theories are modified or discarded. Through-out this process, theories are
formulated and tested on the basis of evidence, internal consistency, and their
explanatory power.

Evolution as a Unifying Concept

Evolution in the broadest sense can be defined as the idea that the universe has
a history: that change through time has taken place. If we look today at the
galaxies, stars, the planet earth, and the life on planet earth, we see that
things today are different from what they were in the past: galaxies, stars,
planets, and life forms have evolved. Biological evolution refers to the
scientific theory that living things share ancestors from which they have
diverged: Darwin called it "descent with modification." There is abundant and
consistent evidence from astronomy, physics, biochemistry, geochronology,
geology, biology, anthropology, and other sciences that evolution has taken

As such, evolution is a unifying concept for science. The National Science
Education Standards recognizes that conceptual schemes such as evolution "unify
science disciplines and provide students with powerful ideas to help them
understand the natural world," and recommends evolution as one such scheme. In
addition, the Benchmarks for Science Literacy from the American Association for
the Advancement of Science's Project 2061 and NSTA's Scope, Sequence, and
Coordination Project, as well as other national calls for science reform, all
name evolution as a unifying concept because of its importance across the
discipline of science. Scientific disciplines with a historical component, such
as astronomy, geology, biology, and anthropology, cannot be taught with
integrity if evolution is not emphasized.

There is no longer a debate among scientists over whether evolution has taken
place. There is considerable debate about how evolution has taken place: the
processes and mechanisms producing change, and what has happened during the
history of the universe. Scientists often disagree about their explanations. In
any science, disagreements are subject to rules of evaluation. Errors and false
conclusions are confronted by experiment and observation, and evolution, as in
any aspect of science, is continually open to and subject to experimentation and


The word "creationism" has many meanings. In its broadest meaning, creationism
is the idea that a supernatural power or powers created. Thus to Christians,
Jews, and Muslims, God created; to the Navajo, the Hero Twins created. In a
narrower sense, "creationism" has come to mean "special creation": the doctrine
that the universe and all that is in it was created by God in essentially its
present form, at one time. The most common variety of special creationism
asserts that

the earth is very young
life was originated by a creator
life appeared suddenly
kinds of organisms have not changed
all life was designed for certain functions and purposes.

This version of special creation is derived from a literal interpretation of
Biblical Genesis. It is a specific, sectarian religious belief that is not held
by all religious people. Many Christians and Jews believe that God created
through the process of evolution. Pope John Paul II, for example, issued a
statement in 1996 that reiterated the Catholic position that God created, but
that the scientific evidence for evolution is strong.

"Creation science" is an effort to support special creationism through methods
of science. Teachers are often pressured to include it or synonyms such as
"intelligent design theory," "abrupt appearance theory," "initial complexity
theory," or "arguments against evolution" when they teach evolution. Special
creationist claims have been discredited by the available evidence. They have no
power to explain the natural world and its diverse phenomena. Instead,
creationists seek out supposed anomalies among many existing theories and
accepted facts. Furthermore, creation science claims do not provide a basis for
solving old or new problems or for acquiring new information.

Nevertheless, as noted in the National Science Education Standards,
"Explanations on how the natural world changed based on myths, personal beliefs,
religious values, mystical inspiration, superstition, or authority may be
personally useful and socially relevant, but they are not scientific." Because
science can only use natural explanations and not supernatural ones, science
teachers should not advocate any religious view about creation, nor advocate the
converse: that there is no possibility of supernatural influence in bringing
about the universe as we know it.

Legal Issues

Several judicial rulings have clarified issues surrounding the teaching of
evolution and the imposition of mandates that creation science be taught when
evolution is taught. The First Amendment of the Constitution requires that
public institutions such as schools be religiously neutral; because special
creation is a specific, sectarian religious view, it cannot be advocated as
"true," accurate scholarship in the public schools. When Arkansas passed a law
requiring "equal time" for creationism and evolution, the law was challenged in
Federal District Court. Opponents of the bill included the religious leaders of
the United Methodist, Episcopalian, Roman Catholic, African Methodist Episcopal,
Presbyterian, and Southern Baptist churches, and several educational
organizations. After a full trial, the judge ruled that creation science did not
qualify as a scientific theory (McLean v. Arkansas Board of Education, 529 F.
Supp. 1255 (ED Ark. 1982)).

Louisiana's equal time law was challenged in court and eventually reached the
Supreme Court. In Edwards v. Aguillard 482 U.S. 578 (1987), the court determined
that creationism was inherently a religious idea and to mandate or advocate it
in the public schools would be unconstitutional. Other court decisions have
upheld the right of a district to require that a teacher teach evolution and not
teach creation science: (Webster v. New Lennox School District #122, 917 F.2d
1003 (7th Cir. 1990); Peloza v. Capistrano Unified School District, 37 F.3d 517
(9th Cir. 1994)).

Some legislatures and policy-makers continue attempts to distort the teaching of
evolution through mandates that would require teachers to teach evolution as
"only a theory," or that require a textbook or lesson on evolution to be
preceded by a disclaimer. Regardless of the legal status of these mandates, they
are bad educational policy. Such policies have the effect of intimidating
teachers, which may result in the de-emphasis or omission of evolution. The
public will only be further confused about the special nature of scientific
theories, and if less evolution is learned by students, science literacy itself
will suffer.


American Association for the Advancement of Science (AAAS). 1993. Benchmarks for
Science Literacy. Project 2061. New York: Oxford University Press.
Daniel v. Waters. 515 F.2d 485 (6th Cir., 1975).
Edwards v. Aguillard. 482 U.S. 578 (1987).
Epperson v. Arkansas. 393 U.S. 97 (1968)
Laudan, Larry. 1996. Beyond Positivism and Relativism: Theory, Method, and
Evidence. Boulder, CO: Westview Press.
McLean v. Arkansas Board of Education. 529 F. Supp. 1255 (D. Ark. 1982).
National Research Council (NRC). 1996. National Science Education Standards.
Washington, DC: National Academy Press.
National Science Teachers Association (NSTA). 1996. A Framework for High School
Science Education. Arlington, VA: National Science Teachers Association.
NSTA. 1993. The Content Core: Vol. I. Rev. ed. Arlington, VA: National Science
Teachers Association.
Peloza v. Capistrano Unified School District. 37 F.3d 517 (9th Cir. 1994).
Ruse, Michael. 1996. But Is It Science? The Philosophical Question in the
Creation/Evolution Controversy. Amherst, NY: Prometheus Books.
Webster v. New Lennox School District #122. 917 F.2d 1003 (7th Cir. 1990).

Task Force Members

Gerald Skoog, Chair, College of Education, Texas Tech University, Lubbock, Texas
Randy Cielen, Joseph Teres School, Winnipeg, Manitoba, Canada
Linda Jordan, Science Consultant, Franklin, Tennessee
Janis Lariviere, Westlake Alternative Learning Center, Austin, Texas
Larry Scharmann, Kansas State University, Manhattan, Kansas
Eugenie Scott, National Center for Science Education, Berkeley, California

2. National Association of Biology Teachers Statement on Teaching Evolution2

As stated in The American Biology Teacher by the eminent scientist Theodosius
Dobzhansky (1973), "Nothing in biology makes sense except in the light of
evolution."3 This often-quoted assertion accurately illuminates the central,
unifying role of evolution in nature, and therefore in biology. Teaching biology
in an effective and scientifically-honest manner requires classroom discussions
and laboratory experiences on evolution.

Modern biologists constantly study, ponder and deliberate the patterns,
mechanisms and pace of evolution, but they do not debate evolution's occurrence.
The fossil record and the diversity of extant organisms, combined with modern
techniques of molecular biology, taxonomy and geology, provide exhaustive
examples and powerful evidence for genetic variation, natural selection,
speciation, extinction and other well-established components of current
evolutionary theory. Scientific deliberations and modifications of these
components clearly demonstrate the vitality and scientific integrity of
evolution and the theory that explains it.

The same examination, pondering and possible revision have firmly established
evolution as an important natural process explained by valid scientific
principles, and clearly differentiate and separate science from various kinds of
nonscientific ways of knowing, including those with a supernatural basis such as
creationism. Whether called "creation science," "scientific creationism,"
"intelligent-design theory," "young-earth theory" or some other synonym,
creation beliefs have no place in the science classroom. Explanations employing
nonnaturalistic or supernatural events, whether or not explicit reference is
made to a supernatural being, are outside the realm of science and not part of a
valid science curriculum. Evolutionary theory, indeed all of science, is
necessarily silent on religion and neither refutes nor supports the existence of
a deity or deities.

Accordingly, the National Association of Biology Teachers, an organization of
science teachers, endorses the following tenets of science, evolution and
biology education:

The diversity of life on earth is the outcome of evolution: an unpredictable
and natural process of temporal descent with genetic modification that is
affected by natural selection, chance, historical contingencies and changing

Evolutionary theory is significant in biology, among other reasons, for its
unifying properties and predictive features, the clear empirical testability
of its integral models, and the richness of new scientific research it

The fossil record, which includes abundant transitional forms in diverse
taxonomic groups, establishes extensive and comprehensive evidence for organic

Natural selection, the primary mechanism for evolutionary changes, can be
demonstrated with numerous, convincing examples, both extant and extinct.
Natural selection—a differential, greater survival and reproduction of some
genetic variants within a population under an existing environmental state—has
no specific direction or goal, including survival of a species.

Adaptations do not always provide an obvious selective advantage. Furthermore,
there is no indication that adaptations—molecular to organismal—must be
perfect: adaptations providing a selective advantage must simply be good
enough for survival and increased reproductive fitness.

The model of punctuated equilibrium provides another account of the tempo of
speciation in the fossil record of many lineages: it does not refute or
overturn evolutionary theory, but instead adds to its scientific richness.

Evolution does not violate the second law of thermodynamics: producing order
from disorder is possible with the addition of energy, such as from the sun.
Although comprehending deep time is difficult, the earth is about 4.5 billion
years old. Homo sapiens has occupied only a minuscule moment of that immense
duration of time.

When compared with earlier periods, the Cambrian explosion evident in the
fossil record reflects at least three phenomena: the evolution of animals with
readily fossilized hard body parts; Cambrian environment (sedimentary rock)
more conducive to preserving fossils; and the evolution from pre-Cambrian
forms of an increased diversity of body patterns in animals.

Radiometric and other dating techniques, when used properly, are highly
accurate means of establishing dates in the history of the planet and in the
history of life.

In science, a theory is not a guess or an approximation but an extensive
explanation developed from well-documented, reproducible sets of
experimentally-derived data from repeated observations of natural processes.

The models and the subsequent outcomes of a scientific theory are not decided
in advance, but can be, and often are, modified and improved as new empirical
evidence is uncovered. Thus, science is a constantly self-correcting endeavor
to understand nature and natural phenomena.

Science is not teleological: the accepted processes do not start with a
conclusion, then refuse to change it, or acknowledge as valid only those data
that support an unyielding conclusion. Science does not base theories on an
untestable collection of dogmatic proposals. Instead, the processes of science
are characterized by asking questions, proposing hypotheses, and designing
empirical models and conceptual frameworks for research about natural events.

Providing a rational, coherent and scientific account of the taxonomic history
and diversity of organisms requires inclusion of the mechanisms and principles
of evolution.

Similarly, effective teaching of cellular and molecular biology requires
inclusion of evolution.

Specific textbook chapters on evolution should be included in biology
curricula, and evolution should be a recurrent theme throughout biology
textbooks and courses.

Students can maintain their religious beliefs and learn the scientific
foundations of evolution.

Teachers should respect diverse beliefs, but contrasting science with
religion, such as belief in creationism, is not a role of science. Science
teachers can, and often do, hold devout religious beliefs, accept evolution as
a valid scientific theory, and teach the theory's mechanisms and principles.
Science and religion differ in significant ways that make it inappropriate to
teach any of the different religious beliefs in the science classroom.

Opposition to teaching evolution reflects confusion about the nature and
processes of science. Teachers can, and should, stand firm and teach good
science with the acknowledged support of the courts. In Epperson v. Arkansas
(1968), the U.S. Supreme Court struck down a 1928 Arkansas law prohibiting the
teaching of evolution in state schools. In McLean v. Arkansas (1982), the
federal district court invalidated a state statute requiring equal classroom
time for evolution and creationism.

Edwards v. Aguillard (1987) led to another Supreme Court ruling against
so-called "balanced treatment" of creation science and evolution in public
schools. In this landmark case, the Court called the Louisiana equal-time
statute "facially invalid as violative of the Establishment Clause of the First
Amendment, because it lacks a clear secular purpose." This decision—"the Edwards
restriction"—is now the controlling legal position on attempts to mandate the
teaching of creationism: the nation's highest court has said that such mandates
are unconstitutional. Subsequent district court decisions in Illinois and
California have applied "the Edwards restriction" to teachers who advocate
creation science, and to the right of a district to prohibit an individual
teacher from promoting creation science, in the classroom.

Courts have thus restricted school districts from requiring creation science in
the science curriculum and have restricted individual instructors from teaching
it. All teachers and administrators should be mindful of these court cases,
remembering that the law, science and NABT support them as they appropriately
include the teaching of evolution in the science curriculum.

References and Suggested Reading

Clough, M. 1994. Diminish students' resistance to biological evolution. American
Biology Teacher 56(Oct.):409-415. Futuyma, D. 1997. Evolutionary Biology. 3rd
ed. Sunderland, MA: Sinauer Associates, Inc.
Gillis, A. 1994. Keeping creationism out of the classroom. BioScience
Gould, S. 1994. The evolution of life on the earth. Scientific American
Gould, S. 1977. Ever Since Darwin: Reflections in Natural History. New York:
W.W. Norton.
Mayr, E. 1991. One Long Argument: Charles Darwin and the Genesis of Modern
Evolutionary Thought. Cambridge, MA: Harvard University Press.
McComas, W., ed. 1994. Investigating Evolutionary Biology in the Laboratory.
Reston, VA: National Association of Biology Teachers.
Moore, J. 1993. Science as a Way of Knowing: The Foundation of Modern Biology.
Cambridge, MA: Harvard University Press.
National Center for Science Education, P.O. Box 9477, Berkeley, CA 94709.
Numerous publications such as Facts, faith and fairness: Scientific creationism
clouds scientific literacy by S. Walsh and T. Demere.
Numbers, R. 1993. The Creationists: The Evolution of Scientific Creationism.
Berkeley, CA: University of California Press.
Weiner, J. 1994. The Beak of the Finch: A Story of Evolution in Our Time. New
York: Alfred A. Knopf.

3. Resolution passed by the American Association for the Advancement of Science
Commission on Science Education4

The Commission on Science Education of the American Association for the
Advancement of Science, is vigorously opposed to attempts by some boards of
education, and other groups, to require that religious accounts of creation be
taught in science classes.

During the past century and a half, the earth's crust and the fossils preserved
in it have been intensively studied by geologists and paleontologists.
Biologists have intensively studied the origin, structure, physiology, and
genetics of living organisms. The conclusion of these studies is that the living
species of animals and plants have evolved from different species that lived in
the past. The scientists involved in these studies have built up the body of
knowledge known as the biological theory of the origin and evolution of life.
There is no currently acceptable alternative scientific theory to explain the

The various accounts of creation that are part of the religious heritage of many
people are not scientific statements or theories. They are statements that one
may choose to believe, but if he does, this is a matter of faith, because such
statements are not subject to study or verification by the procedures of
science. A scientific statement must be capable of test by observation and
experiment. It is acceptable only if, after repeated testing, it is found to
account satisfactorily for the phenomena to which it is applied.

Thus the statements about creation that are part of many religions have no place
in the domain of science and should not be regarded as reasonable alternatives
to scientific explanations for the origin and evolution of life.

Resolution on Inclusion of the Theory of Creation in Science Curricula5

WHEREAS some State Boards of Education and State Legislatures have required or
are considering requiring inclusion of the theory of creation as an alternative
to evolutionary theory in discussions of origins of life, and
WHEREAS the requirement that the theory of creation be included in textbooks as
an alternative to evolutionary theory represents a constraint upon the freedom
of the science teacher in the classroom, and
WHEREAS its inclusion also represents dictation by a lay body of what shall be
considered within the corpus of a science,
THEREFORE the American Association for the Advancement of Science strongly urges
that reference to the theory of creation, which is neither scientifically
grounded nor capable of performing the roles required of scientific theories,
not be required in textbooks and other classroom materials intended for use in
science curricula.

Statement on Forced Teaching of Creationist Beliefs in Public School Science

WHEREAS it is the responsibility of the American Association for the Advancement
of Science to preserve the integrity of science, and
WHEREAS science is a systematic method of investigation based on continuous
experimentation, observation, and measurement leading to evolving explanations
of natural phenomena, explanations which are continuously open to further
testing, and
WHEREAS evolution fully satisfies these criteria, irrespective of remaining
debates concerning its detailed mechanisms, and
WHEREAS the Association respects the right of people to hold diverse beliefs
about creation that do not come within the definitions of science, and
WHEREAS creationist groups are imposing beliefs disguised as science upon
teachers and students to the detriment and distortion of public education in the
United States,
THEREFORE be it resolved that because "creationist science" has no scientific
validity it should not be taught as science, and further, that the AAAS views
legislation requiring "creationist science" to be taught in public schools as a
real and present threat to the integrity of education and the teaching of
science, and
Be it further resolved that the AAAS urges citizens, educational authorities,
and legislators to oppose the compulsory inclusion in science education
curricula of beliefs that are not amenable to the process of scrutiny, testing,
and revision that is indispensable to science.


Reprinted with permission from NSTA Publications, copyright 1997 from NSTA
Handbook, 1997-98, National Science Teachers Association, 1840 Wilson
Boulevard, Arlington, VA 22201-3000.
Statement on Teaching Evolution, National Association of Biology Teachers
(NABT). Adopted by the NABT Board of Directors on March 15, 1995.
Dobzhansky, T. 1973. Nothing in biology makes sense except in the light of
evolution. American Biology Teacher 35:125-129.
American Association for the Advancement of Science (AAAS), Commission on
Science Education. October 13, 1972.
Adopted by AAAS Council on December 30, 1972.
Adopted by the AAAS Board of Directors on January 4, 1982, and by the AAAS
Council on January 7, 1982.

Appendix D

References for Further Reading and Other Resources

The following list of references represents a sampling of the vast literature
available on education, biology, and evolution. The reader is encouraged to
explore the literature further as need and time allow.

Please visit our World Wide Web address at http://www4.nas.edu/opus/evolve.nsf
for more extensive resource listings for these subjects.

Publications on Education

AAAS (American Association for the Advancement of Science). 1993. Benchmarks for
Science Literacy. Project 2061. New York: Oxford University Press.
Bybee, R. 1997. Achieving Scientific Literacy: From Purposes to Practices.
Portsmouth, NH: Heinemann

Educational Books.

Bybee, R. 1996. National Standards and the Science Curriculum: Challenges,
Opportunities, and Recommendations. Dubuque, IA: Kendall/Hunt Publishing Co.
NRC (National Research Council). 1996. National Science Education Standards.
Washington, DC: National Academy Press.
NSRC (National Science Resources Center). 1997. Science for All Children: A
Guide to Improving Elementary Science Education in Your School District.
Washington, DC: National Academy Press.
NSTA (National Science Teachers Association). 1996. A Framework for High School
Science Education. Arlington, VA: National Science Teachers Association.
NSTA. 1993. Scope, Sequence, and Coordination of Secondary School Science. Vol.
I. The Content Core: A Guide for Curriculum Designers. rev. ed. Arlington, VA:

National Science Teachers Association.
Publications on Biology and Other Sciences
Berg, P., and M. Singer. 1992. Dealing with Genes: The Language of Heredity.
Mill Valley, CA: University Science Books.
BSCS (Biological Sciences Curriculum Study). 1998.
BSCS Biology: An Ecological Approach. 8th ed. Dubuque, IA: Kendall/Hunt
Publishing Co.
BSCS. 1997. BSCS Biology: A Human Approach. Dubuque, IA: Kendall/Hunt Publishing
BSCS. 1996. Biological Science: A Molecular Approach. 7th ed. Lexington, MA:
D.C. Heath.
BSCS. 1993. Developing Biological Literacy: A Guide to Developing Secondary and
Post-secondary Biology Curricula. Colorado Springs, CO: BSCS.
BSCS. 1983. Biological Science: Interaction of Experiments and Ideas. Englewood
Cliffs, NJ: Prentice Hall.
BSCS. 1978. Biology Teachers' Handbook. 3rd ed. William V. Mayer, ed. New York:
John Wiley and Sons.
Campbell, N. 1996. Biology. 4th ed. Menlo Park, CA: Benjamin-Cummings.
ESCP (Earth Science Curriculum Project). 1973. Investigating the Earth. rev. ed.
Boston, MA: Houghton Mifflin.
Jacob, F. 1982. The Possible and the Actual. New York: Pantheon Books.
Mayr, E. 1997. This Is Biology: The Science of the Living World. Cambridge, MA:
Belknap Press of Harvard University Press.
Moore, J.A. 1993. Science as a Way of Knowing: The Foundations of Modern
Biology. Cambridge, MA: Harvard University Press.
Oosterman, M., and M. Schmidt, eds. 1990. Earth Science Investigations.
Alexandria, VA: American Geological Institute.
Raven, P.H., and G.B. Johnson. 1992. Biology. 3rd ed. St. Louis, MO: Mosby Year
Book, Inc.
Scientific American. 1994. Life in the universe: special issue. 271(Oct.).
Trefil, J., and R.M. Hazen. 1998. The Sciences: An Integrated Approach. 2nd ed.
New York: John Wiley and Sons.
Publications on Evolution
Berra, T. 1990. Evolution and the Myth of Creationism: A Basic Guide to the
Facts in the Evolution Debate. Stanford, CA: Stanford University Press.
Clough, M. 1994. Diminish students' resistance to biological evolution. American
Biology Teacher 56:409—415. Darwin, C. 1934. Charles Darwin's Diary of the
Voyage of H.M.S. Beagle, Nora Barlow, ed. Cambridge, UK: The University Press.
Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. London:
J. Murray.
Dawkins, R. 1996. Climbing Mount Improbable. New York: W.W. Norton.
Dawkins, R. 1986. The Blind Watchmaker: Why Evidence of Evolution Reveals a
Universe Without Design. New York: W.W. Norton.
de Duve, C. 1995. Vital Dust: Life as a Cosmic Imperative. New York: Basic
Dennett, D.C. 1995. Darwin's Dangerous Idea: Evolution and the Meanings of Life.
New York: Simon and Schuster.
Diamond, J. 1997. Guns, Germs, and Steel: The Fates of Human Societies. New
York: W.W. Norton.
Diamond, J. 1992. The Third Chimpanzee: The Evolution and Future of the Human
Animal. New York: HarperCollins.
Diamond, J., and M.L. Cody, eds. 1975. Ecology and Evolution of Communities.
Cambridge, MA: Belknap Press of Harvard University Press.
Ewald, P. 1994. The Evolution of Infectious Disease. New York: Oxford University
Futuyma, D. 1997. Evolutionary Biology. 3rd ed. Sunderland, MA: Sinauer
Associates, Inc.
Futuyma, D. 1995. Science on Trial: The Case for Evolution. 2nd ed. Sunderland,
MA.: Sinauer Associates, Inc.
Gillis, A. 1994. Keeping creationism out of the classroom. BioScience
Goldschmidt, T. 1996. Darwin's Dreampond: Drama in Lake Victoria. Cambridge, MA:
MIT Press.
Goldsmith, T. H. 1991. The Biological Roots of Human Nature: Forging Links
Between Evolution and Behavior. New York: Oxford University Press.
Gould, S.J. 1997. This view of life: Nonoverlapping magisteria. Natural History
Gould, S.J. 1994. The evolution of life on the earth. Scientific American
Gould, S.J. 1989. Wonderful Life: The Burgess Shale and the Nature of History.
New York: W.W. Norton.
Gould, S.J. 1980. The Panda's Thumb: More Reflections in Natural History. New
York: W.W. Norton.
Gould, S.J. 1977. Ever Since Darwin: Reflections in Natural History. New York:
W.W. Norton.
Kitcher, P. 1982. Abusing Science: The Case Against Creationism. Cambridge, MA:
MIT Press.
Matsumura, M., ed. 1995. Voices for Evolution. 2nd ed. Berkeley, CA: National
Center for Science Education.
Mayr, E. 1991. One Long Argument: Charles Darwin and the Genesis of Modern
Evolutionary Thought. Cambridge, MA: Harvard University Press.
Mayr, E. 1972. The nature of the Darwinian revolution. Science 176:981-989.
McComas, W., ed. 1994. Investigating Evolutionary Biology in the Laboratory.
Reston, VA: National Association of Biology Teachers.
McKinney, M.L. 1993. Evolution of Life: Processes, Patterns, and Prospects.
Englewood Cliffs, NJ: Prentice Hall.
Moore, J.R. 1979. The Post-Darwinian Controversies: A Study of the Protestant
Struggle to Come to Terms with Darwin in Great Britain and America, 1870-1900.
Cambridge, UK: Cambridge University Press.
Nesse, R., and G. Williams. 1995. Why We Get Sick: The New Science of Darwinian
Medicine. New York: Times Books.
Newell, N.D. 1982. Creation and Evolution: Myth or Reality? New York: Columbia
University Press.
Numbers, R. 1993. The Creationists: The Evolution of Scientific Creationism.
Berkeley, CA: University of California Press.
Quammen, D. 1996. The Song of the Dodo: Island Biogeography in an Age of
Extinctions. New York: Scribner.
Ruse, M. 1996. But Is It Science? The Philosophical Question in the
Creation/Evolution Controversy. Amherst, NY: Prometheus Books.
Ruse, M. 1982. Darwinism Defended: A Guide to the Evolution Controversies.
Reading, MA: Addison-Wesley.
Ruse, M. 1979. The Darwinian Revolution: Science Red in Tooth and Claw. Chicago:
University of Chicago Press.
Tiffin, L. 1994. Creationism's Upside-down Pyramid: How Science Refutes
Fundamentalism. Amherst, NY: Prometheus Books.
Walsh, S., and T. Demere. 1993. Facts, Faith and Fairness: Scientific
Creationism Clouds Scientific Literacy. Berkeley, CA: National Center for
Science Education.
Weiner, J. 1994. The Beak of the Finch: A Story of Evolution in Our Time. New
York: Alfred A. Knopf.
Wills, C. 1989. The Wisdom of the Genes: New Pathways in Evolution. New York:
Basic Books.
Wilson, E. 1992. The Diversity of Life. Cambridge, MA: Harvard University Press.
Publications on the Nature of Science
Aicken, F. 1991. The Nature of Science. 2nd ed. Portsmouth, NH: Heinemann
Educational Books.
Bronowski, J. 1965. Science and Human Values. New York: Harper.
Chalmers, A. 1995. What Is This Thing Called Science? 2nd ed. Indianapolis:
Chalmers, A. 1990. Science and Its Fabrication. Minneapolis, MN: University of
Minnesota Press.
Daedalus. 1978. Limits of scientific inquiry. 107 (Spring).
Hull, D. 1988. Science as a Process: An Evolutionary Account of the Social and
Conceptual Development of Science. Chicago: University of Chicago Press.
Kuhn, T.S. 1970. The Structure of Scientific Revolutions. Chicago: University of
Chicago Press.
Laudan, Larry. 1996. Beyond Positivism and Relativism: Theory, Method, and
Evidence. Boulder, CO: Westview Press.
Popper, K. 1994. The Myth of the Framework: In Defense of Science and
Rationality. London: Routledge.
Wolpert, L. 1992. The Unnatural Nature of Science. Cambridge, MA: Harvard
University Press.
Woolgar, S. 1988. Science: The Very Idea. London: Routledge.
The Day the Universe Changed (episode #10, Worlds Without End). 1986. Owings
Mills, MD: MPT-TV.
The Pleasure of Finding Things Out. 1982. Video interview with Richard Feynman.
New York: Time/Life video.
Darwin's Revolution in Thought. Talk given by Stephen Jay Gould (No. 126).
Available from Into the Classroom Video, 351 Pleasant Street, Northhampton, MA
God, Darwin and the Dinosaurs. 1989. Boston: WGBH Educational Foundation.
In the Beginning: The Creationist Controversy. 1994. Chicago: WTTW.

Appendix E


This report has been reviewed by individuals chosen for their diverse
perspectives and technical expertise, in accordance with procedures approved by
the NRC's Report Review Committee. The purpose of this independent review is to
provide candid and critical comments that will assist the authors and the NRC in
making their published report as sound as possible and to ensure that the report
meets institutional standards for objectivity, evidence, and responsiveness to
the study charge. The content of the review comments and draft manuscript remain
confidential to protect the integrity of the deliberative process. We wish to
thank the following individuals for their participation in the review of this

Paul Baker
Evan Pugh Professor of Anthropology, Emeritus
Pennsylvania State University
Kaneohe, Hawaii

Howard Berg
Professor of Biology
Harvard University
Cambridge, Massachusetts

Donald Brown
Department of Embryology
Carnegie Institution of Washington
Washington, DC

Wayne Carley
Executive Director
National Association of Biology Teachers
Reston, Virginia

Betty Carvellas
Biology Teacher
Essex High School
Essex Junction, Vermont

Wilford Gardner
Adjunct Professor of Soil Physics
University of California at Berkeley
Berkeley, California

Robert Griffiths
Professor of Physics
Carnegie Mellon University
Pittsburgh, Pennsylvania

Dudley Herschbach
Professor of Science
Harvard University
Cambridge, Massachusetts

Ken Miller
Professor of Biology
Brown University
Providence, Rhode Island

Nancy Ridenour
Biology Teacher and
Science Department Chair
Ithaca High School
Ithaca, New York

Martin Rodbell
Scientist Emeritus
National Institute of Environmental
Health Sciences
Research Triangle Park, North Carolina

Robert Sinsheimer
Professor of Biology, Emeritus
University of California at Santa Barbara
Santa Barbara, California

Gerald Skoog
Helen DeVitt Jones Professor of Curriculum and Instruction
Texas Technology University
Lubbock, Texas

George Wertherill
Department of Terrestrial Magnetism
Carnegie Institution of Washington
Washington, DC

And other anonymous reviewers.

While the individuals listed above have provided many constructive comments and suggestions, responsibility for the final content of this report rests solely
with the authoring committee and the NRC.

Promoting an Understanding of the Intelligent Design of the Universe