Teaching About Evolution and the Nature of Science
National Academy Press
WORKING GROUP ON TEACHING EVOLUTION
Donald Kennedy (Chairman)
Bing Professor of Environmental Studies
Stanford, California John Moore
Professor Emeritus of Biology
University of California
National Academy of Sciences
Washington, DC Eugenie Scott
National Center for Science Education
El Cerrito, California
Bell Junior High School
San Diego, California Maxine Singer
Carnegie Institution of Washington
Department of Biology
New Haven, Connecticut Mike Smith
Associate Professor of Medical Education
Mercer University School of Medicine
Staff Scientist, Geophysical Laboratory
Carnegie Institution of Washington
Washington, DC Marilyn Suiter
Director, Education and Human Resources
American Geological Institute
Professor, College of Science
Science and Mathematics Education
Oregon State University
Corvallis, Oregon Rachel Wood
Delaware State Department of Public Instruction
Biological Sciences Curriculum Study
Colorado Springs, Colorado
STAFF OF THE CENTER FOR SCIENCE, MATHEMATICS, AND ENGINEERING
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
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
The National Academy of Sciences gratefully acknowledges
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
knowledge among the people. No other sure foundation can be devised
preservation of freedom and happiness."1 Jefferson saw clearly what
increasingly evident since then: the fortunes of a nation rest on
the ability of
its citizens to understand and use information about the world
We are about to enter a century in which the United States will be
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
considerable extent on the education that people receive from
through high school. Yet the teaching of science in the nation's
often is marred by a serious omission. Many students receive little
exposure to the most important concept in modern biology, a concept
understanding key aspects of living things—biological evolution.
groups opposed to the teaching of evolution in the public schools
teachers and administrators to present ideas that conflict with
evolution or to
teach evolution as a "theory, not a fact." They have persuaded some
publishers to downplay or eliminate treatments of evolution and have
legislation and policies at the state and local levels meant to
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
one-half of American adults believe that humans evolved from earlier
More than one half of Americans say that they would like to have
taught in public school classrooms3—even though the Supreme Court
has ruled that
"creation science" is a religious idea and that its teaching cannot
in the public schools.4
The widespread misunderstandings about evolution and the conviction
creationism should be taught in science classes are of great concern
National Academy of Sciences, a private nonpartisan group of 1,800
dedicated to the use of science and technology for the general
Academy and its affiliated institutions—the National Academy of
Institute of Medicine, and the National Research Council—have all
counter misinformation about evolution because of the enormous body
supporting evolution and because of the importance of evolution as a
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
policy makers who design, deliver, and oversee classroom instruction
It summarizes the overwhelming observational evidence for evolution
effective ways of teaching the subject. It explains the nature of
describes how science differs from other human endeavors. It
provides answers to
frequently asked questions about evolution and the nature of science
guidance on how to analyze and select teaching materials.
This publication does not attempt specifically to refute the ideas
those who oppose the teaching of evolution in public schools. A
document, Science and Creationism: A View from the National Academy
discusses evolution and "creation science" in detail.5 This
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
evolution conflicts with religious principles. But accepting
evolution as an
accurate description of the history of life on earth does not mean
religion. On the contrary, most religious communities do not hold
concept of evolution is at odds with their descriptions of creation
Nevertheless, religious faith and scientific knowledge, which are
and important, are different. This publication is designed to help
students receive an education in the sciences that reflects this
The book is divided into seven chapters and five appendices, plus
interspersed "dialogues" in which several fictional teachers discuss
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
terms, such as "theory" and "fact," used throughout the book.
The first dialogue, "The Challenge to Teachers," follows the
three teachers as they discuss some of the problems that can arise
evolution and the nature of science.
Chapter 2, "Major Themes in Evolution," provides a general overview
evolutionary processes, describes the evidence supporting evolution,
how evolutionary theory is related to other areas of biology.
The second dialogue, "Teaching About the Nature of Science," follows
teachers as they engage in a teaching exercise designed to
prominent features of science.
Chapter 3, "Evolution and the Nature of Science," uses several
theories, including evolution, to highlight important
The third dialogue, "Teaching Evolution Through Inquiry," presents a
using an exercise designed to interest and educate her students in
the mechanisms of evolution.
Chapter 4, "Evolution and the National Science Education Standards,"
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
sections from the 1996 National Science Education Standards released
National Research Council that relate to biological evolution and
and history of science.
Chapter 5, "Frequently Asked Questions About Evolution and the
Science," gives short answers to some of the questions asked most
by students, parents, educators, and others.
Chapter 6, "Activities for Teaching About Evolution and the Nature
Science," provides eight sample activities that teachers can use to
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
The appendices summarize significant court decisions regarding
creationism issues, reproduce statements from a number of
regarding the teaching of evolution, provide references for further
and other resources, and conclude with a list of reviewers.
Teaching About Evolution and the Nature of Science was produced by
Group on Teaching Evolution under the Council of the National
Sciences. The Working Group consists of 13 scientists and educators
been extensively involved in research and education on evolution and
scientific subjects. The group worked closely with teachers, school
administrators, state officials, and others in preparing this
soliciting suggestions for what would be most useful, and responding
on draft materials. We welcome additional input and guidance from
we can incorporate into future versions of this publication. Please
World Wide Web site at www4.nas.edu/opus/evolve.nsf for additional
1. Thomas Jefferson, To George Wythe, "Crusade Against Ignorance" in
Jefferson on Education, ed. Gordon C. Lee. 1961. New York: Teachers
Press, pp. 99-100.
2. National Science Board. 1996. Science and Engineering
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
the 1982 decision of a federal district court that the teaching of
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
Why Teach Evolution?
Why is it so important to teach evolution? After all, many questions
can be answered without mentioning evolution: How do birds fly? How
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
all the time.
The answers to these questions often raise deeper questions that are
asked by students: How did things come to be that way? What is the
birds of flying? How did desert plants come to differ from others?
How did an
individual organism come to have its particular genetic endowment?
questions like these requires a historical context—a framework of
that recognizes change through time.
People who study nature closely have always asked these kinds of
time, two observations have proved to be especially perplexing. The
these has to do with the diversity of life: Why are there so many
kinds of plants and animals? The more we explore the world, the more
we are with the multiplicity of kinds of organisms. In the
century, when Charles Darwin was writing On the Origin of Species,
recognized several tens of thousands of different plant and animal
the middle of the twentieth century, biologists had paid more
attention to less
conspicuous forms of life, from insects to microorganisms, and the
up to 1 or 2 million. Since then, investigations in tropical rain
center of much of the world's biological diversity—have multiplied
estimates at least tenfold. What process has created this
The second question involves the inverse of life's diversity. How
similarities among organisms be explained? Humans have always
similarities among closely related species, but it gradually became
that even distantly related species share many anatomical and
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
cells into embryos, they pass through many similar developmental
Furthermore, as paleontologists studied the fossil record, they
countless extinct species that are clearly related in various ways
This question has emerged with even greater force as modern
has focused on processes at the cellular and molecular level. From
yeast to mice to humans, all living things use the same biochemical
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
species. Certain human genes that code for proteins differ little
corresponding genes in fruit flies, mice, and primates. All living
the same biochemical system to pass genetic information from one
From a scientific standpoint, there is one compelling answer to
life's commonalities. Different kinds of organisms share so many
of structure and function because they are related to one another.
Solving the Puzzle
Investigations of forest
ecosystems have helped
reveal the incredible
diversity of earth's
The concept of biological evolution addresses both of these
questions. It accounts for the relatedness among organisms by
the millions of different species of plants, animals, and
live on earth today are related by descent from common
cousins. Organisms in nature typically produce more offspring than
and reproduce given the constraints of food, space, and other
resources in the
environment. These offspring often differ from one another in ways
heritable—that is, they can pass on the differences genetically to
offspring. If competing offspring have traits that are advantageous
in a given
environment, they will survive and pass on those traits. As
to accumulate over generations, populations of organisms diverge
This straightforward process, which is a natural consequence of
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
animals, and microorganisms that exist today. Though humans, fish,
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.
organisms with characteristics enabling them to occupy ecological
occupied by similar organisms have a greater chance of surviving.
the next chapter discusses in more detail—species have diversified
occupied more and more ecological niches to take advantage of new
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
life has been on earth, it has played an increasingly important role
the planet's physical environment. For example, the composition of
atmosphere is partly a consequence of living systems. During
which is a product of evolution, green plants absorb carbon dioxide
produce organic compounds, and release oxygen. This process has
continues to maintain an atmosphere rich in oxygen. Living
profoundly affect weather and the movement of water among the
atmosphere, and land. Much of the rainfall in the forests of the
basin consists of water that has already made one or more recent
trips through a
living plant. In addition, plants and soil microorganisms exert
controls over global temperature by absorbing or emitting
(such as carbon dioxide and methane) that increase the earth's
Living things have altered the earth's oceans, land surfaces, and
atmosphere. For example,
photosynthetic organisms are responsible for the oxygen that makes
about a fifth of the
earth's atmosphere. The rapid accumulation of atmospheric oxygen
billion years ago
led to the evolution of more structured eucaryotic cells, which in
gave rise to multicellular
plants and animals.
In short, biological evolution accounts for three of the most
features of the world around us: the similarities among living
diversity of life, and many features of the physical world we
Explanations of these phenomena in terms of evolution draw on
physics, chemistry, geology, many areas of biology, and other
evolution is the central organizing principle that biologists use to
the world. To teach biology without explaining evolution deprives
students of a
powerful concept that brings great order and coherence to our
The teaching of evolution also has great practical value for
or indirectly, evolutionary biology has made many contributions to
Evolution explains why many human pathogens have been developing
formerly effective drugs and suggests ways of confronting this
serious problem (this issue is discussed in greater detail in
Evolutionary biology has also contributed to many important
advances by explaining the relationships among wild and domesticated
animals and their natural enemies. An understanding of evolution has
essential in finding and using natural resources, such as fossil
fuels, and it
will be indispensable as human societies strive to establish
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
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
within the scientific community over whether evolution occurred, and
there is no
evidence that evolution has not occurred. Some of the details of how
occurs are still being investigated. But scientists continue to
debate only the
particular mechanisms that result in evolution, not the overall
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
offers educators a superb opportunity to illuminate the nature of
science and to
differentiate science from other forms of human endeavor and
Chapter 3 describes the nature of science in detail. However, it is
from the outset to understand how the meanings of certain key words
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
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
the natural world that can incorporate facts, laws, inferences, and
Think, for example, of how people usually use the word "theory."
refer to an idea and then add, "But that's only a theory." Or
preface a remark by saying, "My theory is . . . ." In common usage,
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
the sun rather than vice versa. Such concepts are supported by such
observational and experimental evidence that they are no longer
Sometimes scientists themselves use the word "theory" loosely and
apply it to
tentative explanations that lack well-established evidence. But it
to distinguish these casual uses of the word "theory" with its use
concepts such as evolution that are supported by overwhelming
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
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
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
revolution around the sun. However, ideas are not referred to as
science unless they are supported by bodies of evidence that make
subsequent abandonment very unlikely. When a theory is supported by
evidence as evolution, it is held with a very high degree of
In science, the word "hypothesis" conveys the tentativeness inherent
common use of the word "theory." A hypothesis is a testable
statement about the
natural world. Through experiment and observation, hypotheses can be
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
over and over. However, observations are gathered by our senses,
which can never
be trusted entirely. Observations also can change with better
with better ways of looking at data. For example, it was held as a
fact for many years that human cells have 24 pairs of chromosomes,
improved techniques of microscopy revealed that they actually have
Ironically, facts in science often are more susceptible to change
theories—which is one reason why the word "fact" is not much used in
Finally, "laws" in science are typically descriptions of how the
behaves under certain circumstances. For example, the laws of motion
how objects move when subjected to certain forces. These laws can be
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
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
science. They are understandings that develop from extensive
experimentation, and creative reflection. They incorporate a large
scientific facts, laws, tested hypotheses, and logical inferences.
sense, evolution is one of the strongest and most useful scientific
Evolution and Everyday Life
The concept of evolution has an importance in education that goes
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
different conditions than their parents or teachers have had to face
The story of evolution is one chapter—perhaps the most important
scientific revolution that has occupied much of the past four
central feature of this revolution has been the abandonment of one
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
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
see change as an agent of opportunity rather than as a threat—is a
message and challenge in the lesson of evolution.
The following dialogue dramatizes some of the problems educators
teaching evolution and demonstrates ways of overcoming these
2 returns to the basic themes that characterize evolutionary theory,
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
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
for third period, Karen, the newest teacher on the faculty, walks
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,
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
didn't see it there."
"You didn't see it on my syllabus because it's not a separate
Barbara says. "I use evolution as a theme to tie the course
it comes into just about every unit. You'll see a section called
of Life' on the second page, and there's a section called 'Natural
Selection.' But I don't treat evolution separately because it is
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
just don't get around to teaching it. I don't. You do, but you're
than most of us."
"It's not a matter of bravery, Doug," Barbara replies. "It's a
what needs to be taught if we want students to understand biology.
Teaching biology without evolution would be like teaching civics and
mentioning the United States Constitution."
"But how can you be sure that evolution is all that important.
there a lot of scientists who don't believe in evolution? Say it's
"The debate in science is over some of the details of how evolution
occurred, not whether evolution happened or not. A lot of science
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
school district or state that put a disclaimer against evolution in
their biology textbooks. It said that students didn't need to
evolution because it wasn't a fact, only a theory. The argument was
no one really knows how life began or how it evolved because no one
there to see it happen."3
"If I taught evolution, I'd sure teach it as a theory—not a fact,"
"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
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
a 'fact' is an observation that has been made so many times that
assumed to be okay. How facts are explained is where theories come
theories are explanations of what we observe. One place where
confused about evolution is that they think of 'theory' as meaning
or 'hunch.' But evolution isn't a hunch. It's a scientific
and a very good one."
"But how good a theory is it?" asks Doug. "We don't know everything
"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
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
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
amphibians and between reptiles and mammals," Barbara says. "Our
of fossil intermediates is actually pretty good.4 And, Doug, it
like you know more about evolution than you're letting on. Why don't
"I don't want any trouble. Every time I teach evolution, I have a
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
mine in the middle school started out her Life Science unit by
students interview their ministers or priests or rabbis about their
religion's views on evolution. She said that most of her students
back really surprised. 'Hey,' they said, 'evolution is okay.' It
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
sure he would not come back saying evolution was okay."
"That can be a hard situation," says Barbara. "But even if Stanley
back to class saying that his religion does not accept evolution, it
help a teacher show that there are many different religious views
evolution. That's the point: religious people can still accept
"Stanley will never believe in evolution."
"We talk about 'believing' in evolution, but that's not necessarily
right word. We accept evolution as the best scientific explanation
lot of observations—about fossils and biochemistry and evolutionary
changes we can actually see, like how bacteria become resistant to
medicines. That's why people accepted the idea that the earth goes
the sun—because it accounted for many different observations that we
In science, when a better explanation comes around, it replaces
"Does that mean that evolution will be replaced by a better theory
day?" asks Karen.
"It's not likely. Not all old theories are replaced, and evolution
been tested and has a lot of evidence to support it. The point is
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
"I had Stanley's sister in AP biology one year," Barbara replies.
raised a fuss about evolution, and I told her that I wasn't going to
her on her opinion of evolution but on her knowledge of the facts
concepts. She seemed satisfied with that and actually got an A in
"I still think that if you teach evolution, it's only fair to teach
"What do you mean by both?" asks Barbara. "If you mean both
creationism, what kind of creationism do you want to teach? Will you
evolution and the Bible? What about other religions like Buddhism or
views of Native Americans? It's hard to argue for 'both' evolution
creationism when people have many different ideas about creation."
"I can't teach a whole bunch of creation stories in my Bio class,"
"That's the point. We can't add subjects to the science curriculum
fair to groups that hold certain beliefs. Teaching ecology isn't
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.
creationism be made scientific?"
"That's an interesting story. 'Creation science' is the idea that
scientific evidence can support a literal interpretation of
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
not supported by verifiable data. Still, back in the early 1980s,
states passed laws requiring that 'creation science' be taught
evolution was taught. But the Supreme Court threw out 'equal time'
saying that because creationism was inherently a religious and not a
scientific idea, it couldn't be presented as 'truth' in science
the public schools."5
"Well, I'm willing to teach evolution," says Karen, "and I'd like to
it your way, Barbara, as a theme that ties biology together. But I
don't know enough about evolution to do it. Do you have any
about where I can get information?"
"Sure, I'd be glad to share what I have. But an important part of
evolution has to do with explaining the nature of science. I'm
a demonstration after school today that I'm going to use with my Bio
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
scientists present as a scientific explanation for the origin of
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.
(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
the 1982 decision of a federal district court that the teaching of
"creation science" in public schools violates the First Amendment of
Major Themes in Evolution
The world around us changes. This simple fact is obvious everywhere
Streams wash dirt and stones from higher places to lower places.
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
thereafter, when rock, dust, and gas circling the sun condensed into
of our solar system. Fossils of primitive microorganisms show that
emerged on earth by about 3.8 billion years ago.
Similarly, the fossil record reveals profound changes in the kinds
things that have inhabited our planet over its long history.
populated the seas hundreds of millions of years ago no longer crawl
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
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
Many kinds of cumulative change through time have been described by
"evolution," and the term is used in astronomy, geology, biology,
and other sciences. This document focuses on the changes in living
the long history of life on earth—on what is called biological
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
thought that a gradual evolution had created the world's organic
a formless condition, and he had a fairly modern view of the
aquatic species into terrestrial ones. Following the rise of
Westerners generally accepted the explanation provided in Genesis,
book of the Judeo-Christian-Muslim Bible, that God created
everything in its
present form over the course of six days. However, other
even then. Among Christian theologians, for example, Saint Thomas
to 1274) stated that the earth had received the power to produce
criticized the idea that species had originated in accordance with
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
especially as geological investigations revealed the rich story laid
out in the
fossilized remains of extinct creatures. But although ideas about
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
generation to another. Then, in 1858, two English
naturalists—Charles Darwin and
Alfred Russel Wallace—simultaneously issued papers proposing such a
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
different ways. (Humans— including any class of high school
such differences.) Both men further observed that many of these
inherited and can be passed on to offspring. This conclusion was
the experiences of plant and animal breeders.
Alfred Russel Wallace (1823-1913)
Darwin and Wallace were both deeply influenced by the realization
though most species produce an abundance of offspring, the size of
population usually remains about the same. Thus, an oak tree might
thousands of acorns each year, but few, if any, will survive to
Darwin—who conceived of his ideas in the 1830s but did not publish
Wallace came to similar conclusions—presented the case for evolution
in his 1859 book On the Origin of Species by Natural Selection.
that there will be differences between offspring that survive and
those that do not. In particular, individuals that have heritable
characteristics making them more likely to survive and reproduce in
particular environment will, on average, have a better chance of
characteristics on to their own offspring. In this way, as many
pass, nature would select those individuals best suited to
environments, a process Darwin called natural selection. Over very
Darwin argued, natural selection acting on varying individuals
population of organisms could account for all of the great variety
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
what is the ultimate source of this variation? This problem plagued
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
peas, Mendel demonstrated that organisms acquire traits through
of heredity which later came to be known as genes. The variation
through these inherited traits is the raw material on which natural
Mendel's paper was all but forgotten until 1890, when it was
contributed to a growing wave of interest and research in genetics.
But it was
not immediately clear how to reconcile new findings about the
inheritance with evolution through natural selection. Then, in the
group of biologists demonstrated how the results of genetics
research could both
buttress and extend evolutionary theory. They showed that all
slight and dramatic, arose through changes, or mutations, in genes.
mutation enabled an organism to survive or reproduce more
mutation would tend to be preserved and spread in a population
selection. Evolution was thus seen to depend both on genetic
mutations and on
natural selection. Mutations provided abundant genetic variation,
selection sorted out the useful changes from the deleterious ones.
Selection by natural processes of favored variants explained many
on the geography of species differences—why, for example, members of
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
explained by the advantages of large size and dark coloration in
regions. And, if the species occupied the entire range continuously,
favoring light color and small size would be able to flow into the
population, and vice versa—prohibiting their separation into
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
members of other such groups.
Variation: Genetically determined differences in the characteristics
members of the same species.
Natural selection: Greater reproductive success among particular
of a species arising from genetically determined characteristics
confer an advantage in a particular environment.
How new species are formed was a mystery that eluded biologists
information about genetics and the geographical distribution of
plants could be put together. As a result, it became clear that the
important source of new species is the process of geographical
which barriers to gene flow can be created. In the earlier example,
interposition of a major mountain barrier, or the origin of an
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
circumstances for creating reproductive barriers and allowing
populations of the
same species to diverge from one another under the influence of
selection. After a time, the species become sufficiently different
when reunited they remain reproductively isolated. They have become
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
things—that is, the actual sequences of the amino acids that make up
protein. Almost immediately, it became clear that certain proteins
the same function in different species have very similar amino acid
The protein evidence was completely consistent with the idea of a
evolutionary history for the planet's living things. Even more
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
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
missing link of major importance in the study of evolution. We did
many fossils of ants dating back 50 million years. These were
species from those existing today, but their bodies still possessed
basic body form of modern ants. The missing link of ant evolution
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
fossilized but few insects. And that is exactly what happened. In
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
solitary wasps and the highly social modern ants, and so I gave them
scientific name Sphecomyrma, meaning "wasp ant." Since that time
more Sphecomyrma specimens of similar age have been found in the
States, Canada, and Siberia, but none belonging to the modern type.
each passing year, such fossils and other kinds of evidence tighten
conception of the evolutionary origin of this important group of
—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
the chemical bases in DNA both specifies the order of amino acids in
and determines which proteins are synthesized in which cells. In
this way, DNA
is the ultimate source of both change and continuity in evolution.
modification of DNA through occasional changes or rearrangements in
sequences underlies the emergence of new traits, and thus of new
evolution. At the same time, all organisms use the same molecular
translate DNA base sequences into protein amino acid sequences. This
in the genetic code is powerful evidence for the interrelatedness of
things, suggesting that all organisms presently alive share a common
that can be traced back to the origins of life on earth.
One common misconception among students is that individual organisms
their characteristics in response to the environment. In other
often think that the environment acts on individual organisms to
physical characteristics that can then be passed on genetically to
But selection can work only on the genetic variation that already is
any new generation, and genetic variation occurs randomly, not in
the needs of a population or organism. In this sense, as Francois
written, evolution is a "tinkerer, not an engineer."2 Evolution does
new organisms; rather, new organisms emerge from the inherent
that occurs in organisms.
Genetic variation is random, but natural selection is not. Natural
tests the combinations of genes represented in the members of a
allows to proliferate those that confer the greatest ability to
reproduce. In this sense, evolution is not the simple product of
The booklet Science and Creationism: A View from the National
Sciences3 summarizes several compelling lines of evidence that
beyond any reasonable doubt that evolution occurred as a historical
continues today. In brief:
Fossils found in rocks of increasing age attest to the interrelated
living things, from the single-celled organisms that lived billions
ago to Homo sapiens. The most recent fossils closely resemble the
alive today, whereas increasingly older fossils are progressively
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
similarities are more than skin deep. All vertebrates, for example,
to humans, have a common body plan characterized by a segmented body
hollow main nerve cord along the back. The best available scientific
explanation for these common structures is that all vertebrates are
from a common ancestor species and that they have diverged through
In the past, evolutionary relationships could be studied only by
consequences of genetic information, such as the anatomy,
embryology of living organisms. But the advent of molecular biology
it possible to read the history of evolution that is written in
organism's DNA. This information has allowed organisms to be placed
common evolutionary family tree in a much more detailed way than
previous evidence. For example, as described in Chapter 3,
comparisons of the
differences in DNA sequences among organisms provides evidence for
evolutionary events that cannot be found in the fossil record.
Evolution is the only plausible scientific explanation that accounts
extensive array of observations summarized above. The concept of
through random genetic variation and natural selection makes sense
of what would
otherwise be a huge body of unconnected observations. It is no
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
evolutionary theory. The first concerns the occurrence of evolution
time"—how changes come about and result in new kinds of species. The
the ecological framework that underlies evolution, which is needed
the expansion of biological diversity.
Evolution as a Contemporary Process
Evolution by natural selection is not only a historical process—it
operates today. For example, the continual evolution of human
pathogens has come
to pose one of the most serious public health problems now facing
societies. Many strains of bacteria have become increasingly
once-effective antibiotics as natural selection has amplified
that arose through naturally occurring genetic variation. The
that cause malaria, gonorrhea, tuberculosis, and many other diseases
demonstrated greatly increased resistance to the antibiotics and
used to treat them in the past. The continued use and overuse of
had the effect of selecting for resistant populations because the
give these strains an advantage over nonresistant strains.4
Similar episodes of rapid evolution are occurring in many different
Rats have developed resistance to the poison warfarin. Many hundreds
species and other agricultural pests have evolved resistance to the
used to combat them—and even to chemical defenses genetically
plants. Species of plants have evolved tolerance to toxic metals and
reduced their interbreeding with nearby nontolerant plants—an
initial step in
the formation of separate species. New species of plants have arisen
crossbreeding of native plants with plants introduced from elsewhere
The North American species Chrysoperla carnea and Chrysoperla
separated from a common ancestor species recently in evolutionary
are very similar. But they are different in color, reflecting their
and they breed at different times of the year.
The creation of a new species from a pre-existing species generally
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
example, many closely related species have been identified that
split from a
common ancestor very recently in evolutionary terms. An example is
the North American lacewings Chrysoperla carnea and Chrysoperla
former lives in deciduous woodlands and is pale green in summer and
winter. The latter lives among evergreen conifers and is dark green
round. The two species are genetically and morphologically very
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
example comes from recently discovered fossil evidence documenting
of whales and dolphins. The fossil record shows that these cetaceans
from a primitive group of hoofed mammals called Mesonychids. Some of
mammals crushed and ate turtles, as evidenced by the shape of their
mammal gave rise to a species with front forelimbs and powerful hind
large feet that were adapted for paddling. This animal, known as
could have moved between sea and land. Its fossilized vertebrae also
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
Pakistan shows an animal with smaller functional hind limbs and even
back flexibility. This species, Rodhocetus, probably did not venture
very often, if at all. Finally, Basilosaurus fossils from Egypt and
States present a recognizable whale, with front flippers for
steering and a
completely flexible backbone. But this animal still has hind limbs
have been nonfunctional), which have become further reduced in
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
through many intermediate forms into modern humans. Homo sapiens,
one of 185
known living species in the primate order, is a member of the
category that includes orangutans, gorillas, and chimpanzees. The
species that would give rise to humans seems to have separated from
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
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
modern humans evolved in Africa about 150,000 years ago and spread
the world, replacing archaic populations of Homo sapiens.
Early hominids had smaller brains and larger faces than species
to the genus Homo, including our own species, Homo sapiens. White
the skulls are reconstructions, and the skulls are not all on the
Evolution and Ecology
Animals and plants do not live in isolation, nor do they evolve in
Indeed, much of the pressure toward diversification comes not only
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
as competitors for space and light, can be a host for parasites, and
food for herbivores. The interactions within the complex
ecosystems, in which organisms live can generate powerful
Ongoing Evolution Among Darwin's Finches
A particularly interesting example of contemporary evolution
the 13 species of finches studied by Darwin on the Galapagos
now known as Darwin's finches. A research group led by Peter and
Rosemary Grant of Princeton University has shown that a single year
drought on the islands can drive evolutionary changes in the
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
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
Evolution in natural communities arises from both constraints and
The constraints come from competitors, primarily among the same
are only so many nest holes for bluebirds and so much food for mice.
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
resource free of competition. As a result, the trait that opened up
opportunity will be favored by natural selection because the
possessing it are able to survive and reproduce better than other
their species in the new environment.
An ecologist would say that the variant had occupied a new niche—a
defines the "job description" of an organism. (For example, a
have the niche of insect- and fruit-eater, inhabitant of forest
meadows, tree-hole nester, and so on.) One often finds closely
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
advantage over the other and eventually drive the less fit species
or to a different niche. That leads to a tentative hypothesis: where
such a situation, careful observation should reveal subtle niche
of the apparently competing species.
This hypothesis has been tested by many biologists. For example, in
Robert MacArthur carefully studied three North American warblers of
genus that were regularly seen feeding on insects in coniferous
trees in the
same areas—indeed, often in the same trees. MacArthur's painstaking
revealed that the three were actually specialists: one fed on
insects on the
major branches near the trunk; another occupied the mid-regions of
ate from different parts of the foliage; and the third fed on
the finest needles near the periphery of the tree. Although the
occurred together, they were in fact not competitors for the same
A Chemical Distress Signal
J. H. Tumlinson and colleagues at the U.S. Department of
Research Service Laboratories in Gainesville, Florida, have explored
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
involves the caterpillar itself, the tissues of the plant, and a
participant, a wasp that preys on the caterpillar. When the
chews on the cotton plant leaf, a reaction occurs that causes the
to synthesize and release a class of volatile chemicals that escape
the air and travel rapidly downwind. The chemicals are detected by
wasps, who follow the scent and are able to find the caterpillars
deposit eggs within them. The eggs hatch, and the wasp larvae
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
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
with defenses against predation; in turn, predators experience
traits that overcome those defenses. Such coevolutionary
competitions are common
in nature. Many plants manufacture and store chemicals that deter
insects; but usually one or more insect species will have evolved
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
the myxomatosis virus into Australia. After rabbits were brought to
they multiplied rapidly and threatened the wool industry because
they grazed on
the same plants as sheep. To control the rabbit population, a
of rabbits, the myxomatosis virus, also was introduced into
Australia. Within a
decade, rabbits had become more resistant to the virus, and the
evolved into a less virulent form, allowing both the host and
As the examples in this chapter demonstrate, evolutionary biology
extremely active and rich source of new insights into the world. By
the history of life on earth and shedding light on how evolution
evolutionary biology is linking fundamental scientific research to
needed to meet important societal needs, including the preservation
environment. Few other ideas in science have had such a far-reaching
our thinking about ourselves and how we relate to the world.
Biological Sciences Curriculum Study. 1978. Biology Teachers'
ed. William V. Mayer, ed. New York: John Wiley and Sons.
Francois Jacob. June 10, 1977. Evolution and tinkering. Science
National Academy of Sciences. (in press). Science and Creationism: A
the National Academy of Sciences. Washington, DC: National Academy
P. Ewald. 1994. The Evolution of Infectious Disease. New York:
"Evolution, Science, and Society: A White Paper on Behalf of the
Evolutionary Biology," Draft, June 4, 1997.
Jonathan Weiner. 1994. The Beak of the Finch. New York: Alfred A.
Peter R. Grant. 1991. Natural selection and Darwin's finches.
American, October, pp. 82-87.
James H. Tumlinson, W. Joe Lewis, and Louise E. M. Vet. 1993. How
wasps find their hosts. Scientific American, March, pp. 100-106.
F. Fenner and F.N. Ratcliffe. 1965. Myxomatosis. Cambridge:
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
"Oh, I had that in college," says Karen. "The scientific method is
identify a question, gather information about it, develop a
that answers the question, and then do an experiment that either
or disproves the hypothesis."
"But that was one of my points about evolution," Doug says. "No one
there when evolution happened and we can't do any experiments about
happened in the past. So by your definition, Karen, evolution isn't
"Science is a lot more than just supporting or rejecting
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
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,
asks Karen and Doug to predict what will happen when she pulls on
the ropes. Both are able to predict that if Barbara pulls rope A,
will move. Barbara then asks if there are additional manipulations
would like to see, and she follows their requests.
Barbara then asks Doug and Karen to sketch a model of what is inside
tube that could explain their observations.
When Karen and Doug show their sketches to each other, they realize
they have come up with different models. Barbara asks them if they
to make any changes to their sketches based on the comparison, and
of them make modifications, although their final models are still
Barbara then gives them their own cardboard tubes and some string
asks them to build the model they proposed. When their models are
Barbara holds up her tube and asks Doug and Karen to follow her
with their own models, to see if the two models behave in the same
as Barbara's tube. But when Barbara pulls string A in her tube,
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
same way as Barbara's. Doug's model consistently behaves the same
"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
about. You observed what happened when I pulled these ropes. Then,
on your initial observations, you made a prediction about what would
happen if we manipulated the system in a specific way. How accurate
"We were right," Doug responds.
"And why were you able to predict what would happen before I pulled
"I used what I observed in the first few pulls to help me predict
would happen later."
"Basically what each of you did was to speculate about how my tube
working on the basis of some limited observations. Scientists do
type of thing all the time. They make observations and try to
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
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
heard from Karen?"
"Well, yes. I first thought that ropes A and C were the two ends of
same rope and B and D were two ends of another rope. Karen had A and
as ends of the same rope and C and D as ends of another rope, and
explanation seemed to fit better than mine."
"Right. Communication about observations and interpretations is very
important among scientists because different scientists may
data in different ways. Hearing someone else's views can help a
scientist revise his or her interpretation. In essence that was what
were doing when you shared your diagrams. Karen, when your model
work, what did you do?"
"All I did was adjust the length of one rope, and then it worked
"So as a result of your formal testing of the predictions from your
model, you revised your explanation of the system. Your
improved. In scientific terms, you revised your model to make it
consistent with your further observations. In science, the validity
any explanation is determined by its coherence with observations in
natural world and by its ability to predict further observations."
"But we still have different models," Karen observes. "How do we
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
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
happened, just as we are confident that some things have happened."
"And if different scientists can have different explanations, like
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,"
"Good science always attempts to be objective, but it also relies on
individual insights of scientists. And the questions they choose to
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
other scientists and must be consistent with the natural world and
future experiments, so there are checks on subjectivity. What we
science books is a combination of observations and inferred
of those observations that can change with new research."
"Still, I'm wondering," says Karen, "how can we find out which model
"Let's just open up Barbara's tube," says Doug.
"We could do that," Barbara says. "But let's assume in this analogy
opening the tube is not possible. Sometimes scientists figure out
open up the natural world and look inside, but sometimes they can't.
not opening up the tube is a good metaphor for how science often
Science involves coming up with explanations that are based on
With time, additional evidence might require changing the
so that at any time what we have is the best explanation possible
how things work. In the future, with additional data, we may change
original explanation—just like you did, Karen.
"Remember when we were talking this morning about evolution being
or theory? That conversation is very relevant to what we have been
with the tubes. As scientists started to notice patterns in nature,
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
each other, and you used these patterns to develop a model to
the patterns. The model you created is analogous to the beginning of
scientific theory. Except in science, theories are only formalized
many years of testing the predictions that come from the model.
"Because of our human limitations in collecting complete data,
necessarily contain some judgments about what is important.
aren't a weakness of scientific theory. They are a basic part of how
"I always thought of science as a bunch of absolute facts," says
"I never thought about how knowledge is developed by scientists."
"Creativity and insight are what help make science such a powerful
of understanding the natural world.
"There's another important thing that I try to teach my students
this activity," Barbara continues. "It's important for them to be
to distinguish questions that can be answered by science from those
cannot be answered by science. Here's a list of questions that I use
get them talking. I ask them if a question can be answered by
cannot be answered by science, or has some parts that belong to
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
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
"I can see that these would get students thinking," says Karen. "I
understanding the nature of science really is relevant to real
"That's what this exercise is about."
Evolution and the Nature of Science
Science is a particular way of knowing about the world. In science,
are restricted to those that can be inferred from confirmable
obtained through observations and experiments that can be
substantiated by other
scientists. Anything that can be observed or measured is amenable to
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
through observations made today, and these observations have led to
accounts of how organisms have changed over time. The best available
suggests that life on earth began more than three and a half billion
For more than two billion years after that, life was housed in the
many kinds of tiny, single-celled organisms, some of which produced
that now makes up more than a fifth of the earth's atmosphere. Less
billion years ago, much more complex organisms appeared. By about
half a billion
years ago, evolution had resulted in a wide variety of multicellular
plants living in the sea that are the clear ancestors of many of the
of organisms that continue to live to this day. Somewhat more than
years ago, some marine plants and animals began one of the greatest
innovations in evolution—they invaded dry land. For our own phylum,
Chordata, this move away from the nurturing sea led to the
amphibians, reptiles, birds, and mammals—the latter including, of
own species, Homo sapiens.
This chapter looks at how science works in the context of our
understanding of how biological evolution occurred. It begins,
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
of night and day. Some of the earliest surviving human records left
tablets relate to the movements of the sun and other celestial
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
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
synchronized with those of the sun. The moon is slower by about one
day. The stars remain almost the same on successive nights, but
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
In fact, it takes a full year for the stars to return to their
position, an interval of time that defines our year.
The ancient observers realized that not all stars move in unison.
move in majestic unity, a few others are "wanderers"—appearing now
group of stars and a week later somewhere else. The majority were
stars," the wanderers were called "planets."
Nicolaus Copernicus (1473-1543)
Nicolaus Copernicus, Johannes Kepler, Galileo Galilei, and Isaac
led the way to a new understanding of the relationship between the
and the sun and initiated an age of scientific progress that
During the late Middle Ages, and especially in the Renaissance,
models known as orreries were made to show the relative positions
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
celestial bodies were positioned on rings of metal, each moving by
its own rate. The fixed stars required a simple solution—they could
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
of a "solar system," and the earth revolves around it. That simple
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
religious authorities. The view of Christianity over most of its
on a literal interpretation of the Bible, was that the earth is the
the universe around which the celestial bodies revolve. Copernicus
book describing the theory of heliocentricism, De revolutionibus
coelestium, to Pope Paul III and promptly died. That saved him the
were to beset Galileo (1564 to 1642), whose astronomical
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
The Church held that his views were dangerous to faith.
Isaac Newton (1642-1727)
Continued study and ever more careful measurements of the movements
planets and sun continued to support the heliocentric hypothesis.
Then, in the
latter half of the 17th century, Isaac Newton (1642 to 1727) showed
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
accumulation of evidence, the theological interpretation of
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
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
European seafarers used the more accurate understanding of celestial
to assist in navigation.
Looking at the night sky remains a powerful experience. But that
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
This triumph of the human mind says a great deal about the nature of
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
phenomenon, and one of them, the rotation of the earth on its axis,
responsible for day and night. A concept based on observation proved
extensive modification as new observations accumulated.
Second, the statements of science should never be accepted as "final
Instead, over time they generally form a sequence of increasingly
statements. Nevertheless, in the case of heliocentricism as in
data are so convincing that the accuracy of the theory is no longer
Third, scientific progress depends on individuals, but the
contributions of one
individual could be made by others. If Copernicus had kept his ideas
the discovery of heliocentricism would have been postponed, but it
have been blocked, since other astronomers eventually would have
come to the
same conclusion. Similarly, had Darwin and Wallace not published
hypotheses, the concept of biological evolution would nevertheless
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
published, we would most assuredly never have had his plays. The
scientists, unlike those of playwrights, are a means to an end—they
are not the
Science Requires Careful Description
What are the scientific methods that have led to our current
the history of life over vast eons of time? They begin with careful
of the material being studied.
The material for the study of biological evolution is life itself.
aspect of life is that individuals can be grouped as similar kinds,
Another important observation is that many species seem to be
closely related to
each other. The scientific classification of species and their
groups began with the publication in 1758 of Systema Naturae, or
nature, by the Swedish naturalist Carolus Linnaeus (1707 to 1778).
Linnaeus knew seven dog-like species, and he gave each a double
Subsequently many more species were discovered and some of the names
changed—and continue to be changed as more information is obtained.
dog is Canis familiaris; the coyote of North America is Canis
Australian dingo is Canis dingo; and the wolf of the northern
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
Generations of scientists have discovered new species, described
arranged them into the system first suggested by Linnaeus. Whereas
recognized about 9,000 species, systematists now have recognized
million. The task of categorizing and describing species is still
complete. Most species of smaller invertebrates, and many bacteria
microscopic organisms, remain to be discovered. The plant kingdom is
incompletely known. Though the flowering plants of many areas, such
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
have caused biologists to raise their estimates of the number of
species. For example, a very high proportion of insects collected
forest canopy are "new" species to science. It is now believed that
of different species of plants and animals in the world may be ten
The scientific methods used in classifying organisms have been
over time. The process begins with the intensive field work in which
animals, plants, and microorganisms are collected and carefully
will be known to a specialist, but there might be some unusual
However, none is likely to be a complete stranger, since the
probably recognize that any puzzling specimen is similar to some
species. Next the specialist must check all that has been published
on the group
of organisms that contains the similar species. If, after an
there is no record of a described species that corresponds to the
examined, the specimen is probably a new species. The specialist
prepare a careful description of the new species and publish it in a
journal. There is a permanent reward for being the describer of a
thereafter monographs that deal with the classification of the group
the new species belongs will add the describer's name at the end of
scientific name. Thus, for example, "Homo sapiens Linnaeus" is our
identification, because Linnaeus was the first to give us our
birds, bats are
This example makes it clear that not all scientific data are derived
result of experimentation. The conventional classification of
seemingly natural groups involved the careful observation of a
different species, followed by the use of selected characteristics
in an attempt
to define groups of species thought to be related. But the groupings
always obvious. For example, it might have seemed reasonable to
with birds, since the most conspicuous characteristic of each is the
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
by milk from the mother's mammary glands.
Although most of the species we know today were described after the
Linnaeus, we continue to use his basic system of hierarchical
For example, similar genera are united in families, similar families
similar orders in classes, and similar classes in phyla. The
listed above (the genus Canis), plus a number of similar but more
dog-like animals, are grouped as the family Canidae. This family
families of cats, bears, seals, and weasels form the order
carnivores and all other animals with hair are combined as the class
Mammals are combined with the birds, reptiles, amphibians, fishes,
small marine animals in the phylum Chordata. Today, many
organisms according to a system known as cladistics. By determining
of a species evolved earlier and which evolved later, this system
classify organisms according to their evolutionary history.
Science as Explanation
In the quest for understanding, science involves a great deal of
observation that eventually produces an elaborate written
description of the
natural world. This description is communicated to scientists in
journals or at scientific meetings, so that others can build on
work. In this way, the accuracy and sophistication of the
description tends to
increase with time, as subsequent generations of scientists correct
the observations of their predecessors. Because the total sum of
knowledge increases relentlessly, scientific progress is something
scientists take for granted.
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
showing how different
species evolved from
species (represented in
this diagram by the
letters A through E).
But science is not just description. Even as observations are being
human mind attempts to sort, or organize, the observations in a way
some underlying order in the objects or phenomena being observed.
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
to an intensified examination of the similarities between bats and
at the anatomical level, and later with respect to the genes and
molecules that form their cells. In this case, new evidence was
confirmed the suggested relationship. In other cases, the further
inspired by a tentative grouping have caused the rejection of a new
The realization that species can be arranged in a hierarchy of
seemingly similar forms raised an obvious question: What accounts
relatedness of different groups of organisms? The mechanism that was
Darwin directly addressed this question. It suggested that all
classified as belonging to the same group had a common ancestor
is, dogs, wolves, coyotes and all members of the genus Canis are
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
are formed when
carried by wind or
in layers and then
are compressed by
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
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
This observation might not be obvious in the lifetime of an
individual, but it
is dramatic over thousands of years. Relatively flat surfaces are
form mountains, and then the mountains slowly erode to form
produce powerful waves that erode cliffs at the seashore. These
the common feature of moving solid materials, and the subsequent
settling out of
these materials makes possible the formation of a special form of
contains a record of the earth's past.
Consider the case of a river with a source in the mountains. As the
downstream, it erodes the slopes of the mountains. Tiny grains
produced by the
erosion, called silt, are relatively easy to move. When the river
flatlands, a lake, or the ocean, the solid material being carried by
is deposited—often reaching great thicknesses over long periods of
the pressure of the sediments on top can cause the sediments beneath
into "sedimentary rocks."
The river may carry things other than silt, sand, and rocks. Hard
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
circumstances, these remains of organisms undergo a chemical change
in which the
original material is replaced by molecules that form stone. In this
organic remains of living things are fossilized (changed into
the evidence of ancient life studied by scientists.
Because of the order in which the sediments are deposited, the most
of rocks normally will be on top and the oldest layer will be on the
(though sometimes sediments are flipped upside down by the geologic
rock layers). Also, the fossils in each layer usually will be of
that lived at the time the layer was formed. Thus, the fossils in
layers will represent species that lived earlier than those found in
The relative position of fossils tells only which are older and
One can estimate the difference in the ages of the two fossils by
thickness of the rock that separates them. If the difference is only
one might guess the interval of time is less than if two fossils are
by 50 feet of rock layers. Today, however, far more accurate methods
fossils are available, as described on the next page. Because these
based on the known rates of radioactive decay, they provide valuable
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
species. But in some respects the task of the paleontologist is far
difficult. Many species lack hard parts such as bones and shells,
organisms almost always decay without becoming fossilized. This is
the case for
many groups of soft-bodied invertebrates—such as worms of many
and protozoans. Even for such species as mammals, birds, reptiles,
amphibians, death is usually followed by the skeleton being
dismembered and the
bones scattered. For this reason, whereas isolated bones are often
it is exceptionally rare for an intact skeleton to be found.
Before the start of
the Cambrian period
about 550 million
years ago, multicellular
hard parts like shells
and bones and rarely
left fossils. However, a
organisms left traces
of their existence.
Some ancient rocks
the remnants of bacteria
that grew in columns
like stacked pancakes.
Tiny fossils first reveal the existence of bacteria 3.5 to 3.8
ago, and animals composed of more than a single cell are known from
million years ago. But the organisms that lived between these two
hard parts and, hence, were rarely preserved as fossils. Then, about
years ago, a dramatic change took place. At the beginning of the
period, animals evolved that had calcified shells and other types of
coverings that had a far better chance of becoming fossilized. These
demonstrate that Cambrian seas were populated with a variety of
The earliest vertebrate fossils date from about 500 million years
Thereafter early amphibians and reptiles appeared. Birds and mammals
the fossil record only about 200 million years ago, while dinosaurs
about 225 million years ago and disappear suddenly about 160 million
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
fossil record were known (although absolute dates had not yet been
Many thousands of living species had been described, and it was
recognized that they could be organized into various
groups—suggesting that they
are somehow relatives. In addition, analysis of the fossil record
the organisms on the ancient earth had undergone major changes over
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
and observations. Instead, the natural world to him was a gigantic,
challenging puzzle that demanded an explanation for its otherwise
complexity. Why are different organisms so similar? Why has there
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
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
1) In South America, the only continent where living armadillos were
Darwin discovered fossil evidence for the prior existence of ancient
that had many of the unique features of living armadillos, yet were
different. Such fossils were found nowhere else in the world. Why
living and ancient armadillo-like species confined to the same
Dating the Earth
One of the greatest scientific triumphs of the last two centuries
been the discovery of the vast expanse of geologic time. Early
of calculating the age of the earth relied on measures of the rate
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
unknown phenomenon: radioactivity.
Some elements, such as uranium, undergo radioactive decay to produce
other elements. By measuring the quantities of radioactive elements
the elements into which they decay in rocks, geologists can
how much time has elapsed since the rock cooled from an initially
state. For example, the oldest known rocks are found in Greenland
date from about 3.8 billion years ago. Scientists believe the
age to be about 4.6 billion years because meteorites and rocks of
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
only about 570 million years ago.
Radiometric dating draws on information and insights from many areas
science. For example, it requires that the rate of radioactive decay
constant over time and is not influenced by such factors as
or pressure—conclusions supported by extensive research in physics.
also assumes that the rocks being analyzed have not been altered
time by the migration of atoms into or out of the rocks, which
detailed information from both the geologic and chemical sciences.
2) On the Galapagos Islands, 600 miles off the coast of Ecuador,
many distinct living species of birds and reptiles that closely
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
related mockingbird on an island only 30 miles away? And why were
types of animals on these islands related, but distinct from, the
Ecuador, whereas those on the otherwise very similar islands off the
Africa were related to the animals in Africa instead?
Darwin could not see how these observations could be explained by
view of his time: that each species had been independently created,
species that were best suited to each location on the earth being
each particular site. It looked instead as though species could
evolve from one
into another over time, with each being confined to the particular
region where its ancestors happened to be—particularly if isolated
barriers to migration, such as vast expanses of ocean.
A timeline of evolution demonstrates the tremendous expanse of
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
are refined, the overall sequence demonstrates both the scope and
of evolutionary change.
But how could one species turn into another over the course of time?
constructing his hypothesis of how this occurred, Darwin was struck
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
progeny of their crosses. Pigeon breeders, for example, had observed
differences in colors, beaks, necks, feet, and tails of the
offspring from a
single mating pair. They routinely enhanced their stocks for desired
example, selectively breeding those animals that shared a particular
beak. Through such artificial selection, pigeon fanciers had been
able to create
many different-looking pigeons, known as breeds. A similar type of
selection for mating pairs of dogs had likewise created the whole
shapes and sizes of these common pets—ranging from a Great Dane to a
2) Animals living in the wild can face a tremendous struggle for
some birds, for example, fewer than one in 100 animals born in one
survive over a harsh winter into a second year. Those with
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
process natural selection to distinguish it from the artificial
by dog and pigeon breeders to determine which animals to mate to
At least 20 years elapsed between the time that Darwin conceived of
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
today do: he tested his ideas of how things work with new
experiments. In part, he did this by thinking up every possible
could to his own hypothesis. For each such argument, Darwin tried to
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
successfully counter such objections, he strengthened his theory.
Darwin's ideas readily explained why distant oceanic islands were
devoid of terrestrial mammals, except for flying bats. But how could
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
evolved? By floating snails on salt-water for prolonged periods,
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
This example shows how a hypothesis can drive a scientist to do
would otherwise not be done. Prior to Darwin, the existence of land
bats, but not typical terrestrial mammals, on the oceanic islands
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
water. Even if they had, such an experiment would have had little
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
eliminate individual bias and subjectivity, because others must also
be able to
determine whether a proposed explanation is consistent with the
evidence. It also leads to further observations or to experiments
test hypotheses, which has the effect of advancing science.
Many of the hypotheses advanced by scientists turn out to be
tested by further observations or experiments. But skillful
Darwin tend to have good ideas that end up increasing the amount of
the world. For this reason, the ideas of scientists have been—over
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
in the fossil record between major groups of animals. Guided by the
of evolution, thousands of scientists have spent their lives
evidence that either supports or conflicts with the idea. For
Darwin's time, paleontologists have discovered many ancient
connect major groups—such as Archaeopteryx between ancient reptiles
and Ichthyostega between ancient fish and amphibians. By now, so
has been found that supports the fundamental idea of biological
its occurrence is no longer questioned in science.
Even more striking has been the information obtained during the 20th
from studies on the molecular basis of life. The theory of evolution
that each organism should contain detailed molecular evidence of its
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
cells, it must make a new copy of its DNA.
Continental Drift and Plate Tectonics: A Scientific Revolution of
Past 50 Years
The theory of plate tectonics demonstrates that revolutions in
are not just a thing of the past, thus suggesting that more
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
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
formation of the ocean floor, pursued other lines of reasoning.
suggested that all of earth's continents used to be assembled in a
single ancient super-continent he called Pangea. He hypothesized
Pangea began to break up approximately 200 million years ago, with
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
explanation. Similar plant and animal fossils are found in rock
more than 200 million years old in those regions where he claimed
different continents were once aligned. Wegener attributed this to
migration of plants and animals freely throughout these broad
If 200 million years ago Africa and South America had been separated
the Atlantic Ocean as they are today, their climates, environments,
life forms should have been very different from each other—but they
Despite Wegener's use of evidence and logic to develop his
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
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
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
scientists to gather previously unobtainable data. From the mid
through the early 1970s, new evidence for a mechanism to explain
continental drift became available that the scientific community
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
and pushing apart the plates that form the earth's surface.
In a relatively short time, these new observations, measurements,
interpretations provoked a complete shift in the thinking of the
scientific community. Geologists now accept the idea that the
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
planet's surface, changing the relative positions of the continents.
plate tectonic model provides explanations that are widely accepted
the evolution of crustal features such as folded mountain chains,
of active volcanoes and earthquakes, and deep ocean floor trenches.
Direct measurements using the satellite-based global positioning
(GPS) to measure absolute longitude and latitude verify that the
collide, move apart, and slide past one another in different areas
their adjacent boundaries at speeds comparable to the growth rate of
In copying its DNA nucleotides, however, cells inevitably make a
small number of
mistakes. For this reason, a few nucleotides are changed through
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
divides.) Therefore, the larger number of cell divisions that have
between the time that two organisms diverged from their common
more differences there will be in their DNA sequences due to chance
This molecular divergence allows researchers to track evolutionary
sequencing the DNA of different organisms. For example, the lineage
that led to
humans and to chimpanzees diverged about 5 million years ago—whereas
to look back in time about 80 million years to find the last common
shared by mice and humans. As a result, there is a much smaller
between human and chimpanzee DNA than between human (or chimpanzee)
DNA. In fact, scientists today routinely use the differences they
between the DNA sequences of organisms as "molecular clocks" to
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
DNA. For example, every living cell uses a protein called cytochrome
c in its
energy metabolism. The cytochrome c proteins from humans and
identical. But there is only an 86 percent overlap in the molecules
humans and rattlesnakes, and only a 58 percent overlap between us
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
the past, we and yeast shared a common ancestor—and the molecular
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
Human Genome Project, for example, will produce when completed the
sequence of the 3 billion nucleotides that make up our genetic
complete sequence of the yeast genome (12 million nucleotides) is
as are the genomes for numerous species of bacteria (from 0.5 to 5
nucleotides each, depending on the species). Similar sequencing
soon yield the complete sequences for hundreds of bacteria and other
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
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
The confirmation of Darwin's ideas about "descent with modification"
recent molecular evidence has been one of the most exciting
biology in this century. In fact, as the chromosomes of more and
are sequenced over the next few decades, these data will be used to
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
relating one natural phenomenon to another and recognizing the
effects of phenomena. Thus, scientists develop explanations for the
the seasons, the movements of heavenly bodies, the structure of
shaping of mountains and valleys, the changes in the positions of
over time, and the diversity of living things.
The statements of science must invoke only natural things and
statements of science are those that emerge from the application of
intelligence to data obtained from observation and experiment. These
characteristics of science have demonstrated remarkable power in
allowing us to
describe the natural world accurately and to identify the underlying
natural phenomena. This understanding has great practical value, in
it allows us to better predict future events that rely on natural
Progress in science consists of the development of better
explanations for the
causes of natural phenomena. Scientists can never be sure that a
explanation is complete and final. Yet many scientific explanations
have been so
thoroughly tested and confirmed that they are held with great
The theory of evolution is one of these explanations. An enormous
scientific investigation has converted what was initially a
hypothesis into a
theory that is no longer questioned in science. At the same time,
remains an extremely active field of research, with an abundance of
discoveries that are continually increasing our understanding of
exactly how the
evolution of living organisms actually occurred.
THE CONCERNS OF SCIENCE
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
who are not scientists claim the same. The world and all that is in
are the sphere of interest not only of scientists but also of
theologians, philosophers, poets, and politicians. How can one make
demarcation between their concerns and those of the scientist?
How Science Differs from Theology
The demarcation between science and theology is perhaps easiest,
scientists do not invoke the supernatural to explain how the natural
world works, and they do not rely on divine revelation to understand
When early humans tried to give explanations for natural phenomena,
particularly for disasters, invariably they invoked supernatural
and forces, and even today divine revelation is as legitimate a
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
Another feature of science that distinguishes it from theology is
openness. Religions are characterized by their relative
in revealed religions, a difference in the interpretation of even a
single word in the revealed founding document may lead to the origin
a new religion. This contrasts dramatically with the situation in
active field of science, where one finds different versions of
any theory. New conjectures are made continuously, earlier ones are
refuted, and at all times considerable intellectual diversity
Indeed, it is by a Darwinian process of variation and selection in
formation and testing of hypotheses that science advances.
Despite the openness of science to new facts and hypotheses, it must
said that virtually all scientists—somewhat like theologians—bring a
of what we might call "first principles" with them to the study of
natural world. One of these axiomatic assumptions is that there is a
real world independent of human perceptions. This might be called
principle of objectivity (as opposed to subjectivity) or
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
world exists outside of the influence of subjective human
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
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,
every falsification or refutation strengthens the probability that
opposing theory is correct. One of the most characteristic features
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
whether one is testing a fact or an explanation. The existence of a
continent of Atlantis between Europe and America became doubtful
such continent was discovered during the first few Atlantic
the period of discoveries during the late fifteenth and early
centuries. After complete oceanographic surveys of the Atlantic
were made and, even more convincingly, after photographs from
were taken in this century, the new evidence conclusively proved
such continent exists. Often, in science, the absolute truth of a
can be established. The absolute truth of an explanation or theory
much harder, and usually takes much longer, to gain acceptance. The
"theory" of evolution through natural selection was not fully
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
the material world. Theologians may also be interested in the
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
of all believers after death. Such supernatural constructions are
the scope of science.
Teaching Evolution Through Inquiry
The following dialogue demonstrates a way of teaching about
using inquiry-based learning. High school students are often
interested in fossils and in what fossils indicate about organisms
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
students a set of calipers and two plastic sheets that each contain
100 replicas of carefully selected fossil brachiopods.1 "These two
contain fossils from two different species of a marine animal called
brachiopod. Let's begin with some observations of what they look
"They look like butterflies," replies one student.
"They are kind of triangular with a big middle section and ribs,"
"Can you tell if there are any differences between the fossils in
The students quickly conclude that the fossils have different sizes
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
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
They soon realize that the number of ribs is related to the size of
fossils, so the groups focus on measuring the lengths and widths of
fossils. They enter the data on the two different populations into a
computer data base. Two of the graphs that they generate are shown
Graphs showing characteristics of brachiopod populations.
"Now that we have these graphs of the fossils' lengths and widths,"
says at the beginning of the next class period, "we can begin to
about what these measurements mean. We see from one set of graphs
fossils in the second group tend to be both wider and longer than
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
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,
would have some additional information to work with, so let me give
some of that background. As I mentioned last week, these fossils are
marine animals known as brachiopods. When they die their shells are
buried in sediments and fossilized. What I know about the fossils
is that they were taken from sediments that are about 400 million
old. But the two sets of fossils were separated in time by about 10
"Taking that information, I'd like you to do some research on
and develop some hypotheses about whether or not evolution has
their size. Here are some of the questions you can consider as
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
length and width of the brachiopods? Could efficiency in burrowing
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
papers are presented and discussed. She focuses students on their
to ask skeptical questions, evaluate the use of evidence, assess the
understanding of geological and biological concepts, and review
scientific inquiries. During the discussions, students are directed
address the following questions: What evidence would you look for
might indicate these brachiopods were the same or different species?
could changes in their shapes have affected their ability to
successfully? What would be the likely effects of other changes in
environment on the species?
The materials needed to carry out this investigation are available
Carolina Biological Supply Company, 2700 York Rd., Burlington, NC
Phone: 1-800-334-5551. www.carolina.com
Evolution and the
National Science Education Standards
Over the last six years, several major documents have been released
describe what students from kindergarten through twelfth grade
should know and
be able to do as a result of their instruction in the sciences.
the National Science Education Standards released by the National
Council in 1996,1 the Benchmarks for Science Literacy released by
Association for the Advancement of Science in 1993,2 and The Content
Guide for Curriculum Designers released by the Scope, Sequence,
project of the National Science Teachers Association in 1992.3
These documents agree that all students should leave biology class
understanding of the basic concepts of biological evolution and of
possibilities, and dynamics of science as a way of knowing.
Science Literacy, for example, states that "the educational goal
should be for
all children to understand the concept of evolution by natural
evidence and arguments that support it, and its importance in
biology educators, these documents offer significant support for the
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
Education Standards. The Standards are divided into six broad
first set of standards, the science teaching standards, describes
of science at all grade levels should know and be able to do. The
development standards describe the experiences necessary for
teachers to gain
the knowledge, understanding, and ability to implement the
assessment standards provide criteria against which to judge whether
are contributing fully to the goals outlined in the Standards. The
content standards outline what students should know, understand, and
be able to
do in the natural sciences. The science education program standards
planning and actions needed to translate the Standards into programs
reflect local contexts and policies. And the science education
consist of criteria for judging the performance of the overall
The Standards rest on the premise that science is an active process.
science is something that students do, not something that is done to
"Hands-on" activities, although essential, are not enough. Students
"minds-on" experiences as well.
The Standards make inquiry a central part of science learning. When
inquiry, students describe objects and events, ask questions,
explanations, test those explanations against current scientific
communicate their ideas to others. They identify their assumptions,
and logical thinking, and consider alternative explanations. In this
students actively develop their understanding of science by
knowledge with reasoning and thinking skills.
The importance of inquiry does not imply that all teachers should
single approach to teaching science. Just as inquiry has many
so too do teachers need to use many different strategies to develop
understandings and abilities described in the Standards.
Nor should the Standards be seen as requiring a specific curriculum.
curriculum is the way content is organized and presented in the
content embodied in the Standards can be organized and presented
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
The first mention of evolution is in the initial content standard,
"Unifying Concepts and Processes." This standard points out that
procedural schemes unify science disciplines and provide students
ideas to help them understand the natural world. It is the only
extends across all grades, because the understanding and abilities
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
understanding and abilities aligned with the following concepts and
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
thinking about evolution:
Evolution is a series of changes, some gradual and some sporadic,
accounts for the present form and function of objects, organisms,
and designed systems. The general idea of evolution is that the
from materials and forms of the past. Although evolution is most
associated with the biological theory explaining the process of
modification of organisms from common ancestors, evolution also
changes in the universe.
With this unifying standard as a basis, the remaining content
organized by age group and discipline.
The life science standard for grades K—4 is organized into the
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
ought to be able to understand and do:
During the elementary grades, children build understanding of
concepts through direct experience with living things, their life
their habitats. These experiences emerge from the sense of wonder
interests of children who ask questions such as: "How do plants get
many different animals are there? Why do some animals eat other
is the largest plant? Where did the dinosaurs go?" An understanding
characteristics of organisms, life cycles of organisms, and of the
interactions among all components of the natural environment begins
questions such as these and an understanding of how individual
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
and high school years.
For grades 5—8, the life science standard is the following:
As a result of their activities in grades 5—8, all students should
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
An organism's behavior evolves through adaptation to its
environment. How a
species moves, obtains food, reproduces, and responds to danger are
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
Although different species might look dissimilar, the unity among
becomes apparent from an analysis of internal structures, the
their chemical processes, and the evidence of common ancestry.
Biological evolution accounts for the diversity of species developed
gradual processes over many generations. Species acquire many of
characteristics through biological adaptation, which involves the
naturally occurring variations in populations. Biological
changes in structures, behaviors, or physiology that enhance
reproductive success in a particular environment.
Extinction of a species occurs when the environment changes and the
characteristics of a species are insufficient to allow its survival.
indicate that many organisms that lived long ago are extinct.
species is common; most of the species that have lived on the earth
The text accompanying the standard also discusses some of the
encountered in teaching about adaptation:
Understanding adaptation can be particularly troublesome at this
students think adaptation means that individuals change in major
response to environmental changes (that is, if the environment
individual organisms deliberately adapt).
In fact, as described in Chapter 2 of this book, adaptation occurs
natural selection, a topic described under the life science
standards for grades
The content standards also treat evolution in grades 5—8 in the
earth's history. The standard reads as follows:
As a result of their activities in grades 5—8, all students should
Structure of the earth system
Earth in the solar system
The text discusses the importance of teaching students about earth
A major goal of science in the middle grades is for students to
understanding of earth and the solar system as a set of closely
systems. The idea of systems provides a framework in which students
investigate the four major interacting components of the earth
system—geosphere (crust, mantle, and core), hydrosphere (water),
(air), and the biosphere (the realm of all living things). In this
approach to studying the planet, physical, chemical, and biological
act within and among the four components on a wide range of time
change continuously earth's crust, oceans, atmosphere, and living
Their study of earth's history provides students with some evidence
co-evolution of the planet's main features—the distribution of land
features of the crust, the composition of the atmosphere, global
populations of living organisms in the biosphere.
The material offering guidance for the standard explicitly ties the
history to the history of life:
The earth processes we see today, including erosion, movement of
plates, and changes in atmospheric composition, are similar to those
occurred in the past. Earth's history is also influenced by
catastrophes, such as the impact of an asteroid or comet.
Fossils provide important evidence of how life and environmental
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
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
experiments, and theoretical and mathematical models. Although all
ideas are tentative and subject to change and improvement in
most major ideas in science, there is much experimental and
confirmation. Those ideas are not likely to change greatly in the
Scientists do and have changed their ideas about nature when they
new experimental evidence that does not match their existing
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
the evidence or theory being considered. Different scientists might
conflicting experimental results or might draw different conclusions
same data. Ideally, scientists acknowledge such conflict and work
finding evidence that will resolve their disagreement.
It is part of scientific inquiry to evaluate the results of
investigations, experiments, observations, theoretical models, and
explanations proposed by other scientists. Evaluation includes
experimental procedures, examining the evidence, identifying faulty
pointing out statements that go beyond the evidence, and suggesting
alternative explanations for the same observations. Although
disagree about explanations of phenomena, about interpretations of
about the value of rival theories, they do agree that questioning,
criticism, and open communication are integral to the process of
scientific knowledge evolves, major disagreements are eventually
through such interactions between scientists.
History of Science
Many individuals have contributed to the traditions of science.
of these individuals provides further understanding of scientific
science as a human endeavor, the nature of science, and the
between science and society.
In historical perspective, science has been practiced by different
in different cultures. In looking at the history of many peoples,
that scientists and engineers of high achievement are considered to
the most valued contributors to their culture.
Tracing the history of science can show how difficult it was for
innovators to break through the accepted ideas of their time to
conclusions that we currently take for granted.
The life science standard for grades 9—12 directly addresses
evolution. The standard reads as follows:
As a result of their activities in grades 9—12, all students should
Molecular basis of heredity
Interdependence of organisms
Matter, energy, and organization in living systems
Behavior of organisms
The guidance for the life science standard describes the major
Species evolve over time. Evolution is the consequence of the
(1) the potential for a species to increase its numbers, (2) the
variability of offspring due to mutation and recombination of genes,
finite supply of the resources required for life, and (4) the
selection by the environment of those offspring better able to
The great diversity of organisms is the result of more than 3.5
of evolution that has filled every available niche with life forms.
Natural selection and its evolutionary consequences provide a
explanation for the fossil record of ancient life forms, as well as
striking molecular similarities observed among the diverse species
The millions of different species of plants, animals, and
live on earth today are related by descent from common ancestors.
Biological classifications are based on how organisms are related.
are classified into a hierarchy of groups and subgroups based on
which reflect their evolutionary relationships. Species is the most
fundamental unit of classification.
The text following the standard describes some of the difficulties
can have in comprehending the basic concepts of evolution.
Students have difficulty with the fundamental concepts of evolution.
example, students often do not understand natural selection because
to make a conceptual connection between the occurrence of new
variations in a
population and the potential effect of those variations on the
survival of the species. One misconception that teachers may
involves students attributing new variations to an organism's need,
environmental conditions, or use. With some help, students can
that, in general, mutations occur randomly and are selected because
some organisms survive and produce more offspring. Other
on a lack of understanding of how a population changes as a result
differential reproduction (some individuals producing more
opposed to all individuals in a population changing. Many
the process of natural selection can be changed through instruction.
Finally, evolution is discussed again in the guidance following the
space science standard:
As a result of their activities in grades 9—12, all students should
Energy in the earth system
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
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
cloud of dust and gas 4.5 billion years ago. The early earth was
different from the planet we live on today.
Geologic time can be estimated by observing rock sequences and using
to correlate the sequences at various locations. Current methods
the known decay rates of radioactive isotopes present in rocks to
time since the rock was formed.
Interactions among the solid earth, the oceans, the atmosphere, and
have resulted in the ongoing evolution of the earth system. We can
some changes such as earthquakes and volcanic eruptions on a human
but many processes such as mountain building and plate movements
over hundreds of millions of years.
Evidence for one-celled forms of life—the bacteria—extends back more
billion years. The evolution of life caused dramatic changes in the
composition of the earth's atmosphere, which did not originally
The Origin and Evolution of the Universe
The origin of the universe remains one of the greatest questions in
The "big bang" theory places the origin between 10 and 20 billion
when the universe began in a hot dense state; according to this
universe has been expanding ever since.
Early in the history of the universe, matter, primarily the light
hydrogen and helium, clumped together by gravitational attraction to
countless trillions of stars. Billions of galaxies, each of which is
gravitationally bound cluster of billions of stars, now form most of
visible mass in the universe.
Stars produce energy from nuclear reactions, primarily the fusion of
to form helium. These and other processes in stars have led to the
of all the other elements.
The standard for the history and nature of science elaborates on the
established in previous years:
As a result of activities in grades 9—12, all students should
Science as a human endeavor
Nature of scientific knowledge
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
of knowledge through the use of empirical standards, logical
skepticism, as scientists strive for the best possible explanations
Scientific explanations must meet certain criteria. First and
must be consistent with experimental and observational evidence
and must make accurate predictions, when appropriate, about systems
studied. They should also be logical, respect the rules of evidence,
to criticism, report methods and procedures, and make knowledge
Explanations on how the natural world changes based on myths,
beliefs, religious values, mystical inspiration, superstition, or
may be personally useful and socially relevant, but they are not
Because all scientific ideas depend on experimental and
confirmation, all scientific knowledge is, in principle, subject to
new evidence becomes available. The core ideas of science such as
conservation of energy or the laws of motion have been subjected to
variety of confirmations and are therefore unlikely to change in the
which they have been tested. In areas where data or understanding
incomplete, such as the details of human evolution or questions
global warming, new data may well lead to changes in current ideas
current conflicts. In situations where information is still
fragmentary, it is
normal for scientific ideas to be incomplete, but this is also where
opportunity for making advances may be greatest.
In history, diverse cultures have contributed scientific knowledge
technologic inventions. Modern science began to evolve rapidly in
several hundred years ago. During the past two centuries, it has
significantly to the industrialization of Western and non-Western
However, other, non-European cultures have developed scientific
solved human problems through technology.
Usually, changes in science occur as small modifications in extant
The daily work of science and engineering results in incremental
our understanding of the world and our ability to meet human needs
aspirations. Much can be learned about the internal workings of
the nature of science from study of individual scientists, their
and their efforts to advance scientific knowledge in their area of
The material addressing evolution in the National Science Education
embedded within the full range of content standards describing what
should know, understand, and be able to do in the natural sciences.
conjunction with standards for other parts of the science education
content standards—and their treatment of evolution—point toward the
scientific literacy needed to meet the challenges of the
National Research Council. 1996. National Science Education
Washington, DC: National Academy Press.
American Association for the Advancement of Science. 1993.
Science Literacy. Project 2061. New York: Oxford University Press.
National Science Teachers Association. 1993. Scope, Sequence, and
of Secondary School Science. Vol. 1. The Content Core: A Guide for
Designers. rev. ed. Arlington, VA: NSTA. www.nsta.org
[Table of Contents] — [Previous Section] — [Next Section]
Copyright 1998 National Academy Press
Frequently Asked Questions
About Evolution and the Nature of Science
Teachers often face difficult questions about evolution, many from
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
commonly asked questions.
What is evolution?
Evolution in the broadest sense explains that what we see today is
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
life on earth. It explains that living things share common
ancestors. Over time,
evolutionary change gives rise to new species. Darwin called this
"descent with modification," and it remains a good definition of
What is "creation science"?
The ideas of "creation science" derive from the conviction that God
universe—including humans and other living things—all at once in the
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
age of rocks. Furthermore, because the basic proposals of creation
not subject to test and verification, these ideas do not meet the
science. Indeed, U.S. courts have ruled that ideas of creation
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
This question reflects a narrow view of how science works. Things in
be studied even if they cannot be directly observed or experimented
Archaeologists study past cultures by examining the artifacts those
left behind. Geologists can describe past changes in sea level by
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
study. Hypotheses can be made about such phenomena, and these
hypotheses can be
tested and can lead to solid conclusions. Furthermore, many key
evolution occur in relatively short periods that can be observed
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,
does not mean that we cannot be confident that horses evolved.
practiced in many ways besides direct observation and
scientific discovery is done through indirect experimentation and
which inferences are made, and hypotheses generated from those
For instance, particle physicists cannot directly observe subatomic
because the particles are too small. They must make inferences about
speed, and other properties of the particles based on other
logical hypothesis might be something like this: If the weight of
is Y, when I bombard it, X will happen. If X does not happen, then
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
In historical sciences like astronomy, geology, evolutionary
archaeology, logical inferences are made and then tested against
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
true flies (Diptera) have enough similarities that entomologists
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
small club-shaped structures. If Diptera evolved from Mecoptera, as
anatomy suggests, scientists predicted that a fossil fly with four
be found—and in 1976 this is exactly what was discovered.
geneticists have found that the number of wings in flies can be
mutations in a single gene.
Evolution is a well-supported theory drawn from a variety of sources
including observations about the fossil record, genetic information,
distribution of plants and animals, and the similarities across
anatomy and development. Scientists have inferred that descent with
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
terms, "theory" does not mean "guess" or "hunch" as it does in
Scientific theories are explanations of natural phenomena built up
from testable observations and hypotheses. Biological evolution is
scientific explanation we have for the enormous range of
observations about the
Scientists most often use the word "fact" to describe an
scientists can also use fact to mean something that has been tested
so many times that there is no longer a compelling reason to keep
looking for examples. The occurrence of evolution in this sense is a
Scientists no longer question whether descent with modification
the evidence supporting the idea is so strong.
Why isn't evolution called a law?
Laws are generalizations that describe phenomena, whereas theories
phenomena. For example, the laws of thermodynamics describe what
under certain circumstances; thermodynamics theories explain why
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
the teaching of evolution sometimes use quotations from prominent
of context to claim that scientists do not support evolution.
examination of the quotations reveals that the scientists are
some aspect of how evolution occurs, not whether evolution occurred.
example, the biologist Stephen Jay Gould once wrote that "the
extreme rarity of
transitional forms in the fossil record persists as the trade secret
paleontology." But Gould, an accomplished paleontologist and
about evolution, was arguing about how evolution takes place. He was
whether the rate of change of species is constant and gradual or
takes place in bursts after long periods when little change
occurs—an idea known
as punctuated equilibrium. As Gould writes in response, "This
although accurate as a partial citation, is dishonest in leaving out
following explanatory material showing my true purpose—to discuss
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
evolutionary theory about sudden change that produces a new species
once in a single generation. Punctuated equilibrium accepts the
idea that new species form over hundreds or thousands of generations
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"
the several million years of existence for most species. Thus, rates
evolution vary enormously and new species may appear to arise
geological time, even though the time involved would seem long, and
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
have been filled in. In addition, the consistent pattern of ancient
species found in the fossil record is strong evidence for evolution.
and animals living today are not like the plants and animals of the
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
Some changes in populations might occur too rapidly to leave many
fossils. Also, many organisms were very unlikely to leave fossils,
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
amphibians, amphibians and reptiles, reptiles and mammals, and
birds, there are excellent transitional fossils.
Can evolution account for new species?
One argument sometimes made by supporters of "creation science" is
selection can produce minor changes within species, such as changes
in color or
beak size, but cannot generate new species from pre-existing
evolutionary biologists have documented many cases in which new
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
observer can witness only a part of this process. Yet these
evolution at work provide powerful confirmation that evolution forms
If humans evolved from apes, why are there still apes?
Humans did not evolve from modern apes, but humans and modern apes
common ancestor, a species that no longer exists. Because we shared
common ancestor with chimpanzees and gorillas, we have many
biochemical, and even behavioral similarities with the African great
are less similar to the Asian apes—orangutans and gibbons—and even
to monkeys, because we shared common ancestors with these groups in
Evolution is a branching or splitting process in which populations
from one another and gradually become different. As the two groups
isolated from each other, they stop sharing genes, and eventually
differences increase until members of the groups can no longer
this point, they have become separate species. Through time, these
might give rise to new species, and so on through millennia.
Doesn't the sudden appearance of all the "modern groups" of animals
Cambrian explosion prove creationism?
During the Cambrian explosion, primitive representatives of the
major phyla of
invertebrate animals appeared—hard-shelled organisms like mollusks
arthropods. More modern representatives of these invertebrates
gradually through the Cambrian and the Ordovician periods. "Modern
terrestrial vertebrates and flowering plants were not present. It is
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
left fewer fossils. A characteristic of the Cambrian fossils is the
hard body parts, which greatly improved the chance of fossilization.
without fossils, we can infer relationships among organisms from
Can a person believe in God and still accept evolution?
Many do. Most religions of the world do not have any direct conflict
idea of evolution. Within the Judeo-Christian religions, many people
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
ways of knowing. Religions and science answer different questions
world. Whether there is a purpose to the universe or a purpose for
existence are not questions for science. Religious and scientific
knowing have played, and will continue to play, significant roles in
No one way of knowing can provide all of the answers to the
humans ask. Consequently, many people, including many scientists,
religious beliefs and simultaneously accept the occurrence of
Aren't scientific beliefs based on faith as well?
Usually "faith" refers to beliefs that are accepted without
Most religions have tenets of faith. Science differs from religion
because it is
the nature of science to test and retest explanations against the
Thus, scientific explanations are likely to be built on and modified
information and new ways of looking at old information. This is
from most religious beliefs.
Therefore, "belief" is not really an appropriate term to use in
testing is such an important part of this way of knowing. If there
component of faith to science, it is the assumption that the
according to regularities—for example, that the speed of light will
tomorrow. Even the assumption of that regularity is often tested—and
has held up well. This "faith" is very different from religious
Science is a way of knowing about the natural world. It is limited
the natural world through natural causes. Science can say nothing
supernatural. Whether God exists or not is a question about which
Why can't we teach creation science in my school?
The courts have ruled that "creation science" is actually a
Because public schools must be religiously neutral under the U.S.
the courts have held that it is unconstitutional to present creation
In particular, in a trial in which supporters of creation science
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.
of Education). The Supreme Court has held that it is illegal to
creation science be taught when evolution is taught (Edwards v.
addition, district courts have decided that individual teachers
creation science on their own (Peloza v. San Juan Capistrano School
Webster v. New Lennox School District).
Teachers' organizations such as the National Science Teachers
National Association of Biology Teachers, the National Science
Leadership Association, and many others also have rejected the
pedagogy of creation science and have strongly discouraged its
the public schools. (Statements from some of these organizations
Appendix C.) In addition, a coalition of religious and other
noted in "A Joint Statement of Current Law" (see Appendix B) that
class, [schools] may present only genuinely scientific critiques of,
for, any explanation of life on earth, but not religious critiques
unverifiable by scientific methodology)."
Some argue that "fairness" demands the teaching of creationism along
evolution. But a science curriculum should cover science, not the
views of particular groups or individuals.
If evolution is taught in schools, shouldn't creationism be given
Some religious groups deny that microorganisms cause disease, but
curriculum should not therefore be altered to reflect this belief.
agree that students should be exposed to the best possible
scholarship in each
field. That scholarship is evaluated by professionals and educators
fields. In science, scientists as well as educators have concluded
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
Many people say that they want their children to be exposed to
school, but there are thousands of different ideas about creation
world's people. Comparative religions might comprise a worthwhile
field of study
but not one appropriate for a science class. Furthermore, the U.S.
states that schools must be religiously neutral, so legally a
teacher could not
present any particular creationist view as being more "true" than
Why should teachers teach evolution when they already have so many
teach and can cover biology without mentioning evolution?
Teachers face difficult choices in deciding what to teach in their
but some ideas are of central importance in each discipline. In
evolution is such an idea. Biology is sometimes taught as a list of
if evolution is introduced early in a class and in an uncomplicated
can tie many disparate facts together. Most important, it offers a
understand the astonishing complexity, diversity, and activity of
world. Why are there so many different types of organisms? What is
of a species or community to a changing environment? Why is it so
develop antibiotics and insecticides that are useful for more than a
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
that information to take other classes, apply for college or medical
make decisions that require a knowledge of evolution.
Should students be given lower grades for not believing in
No. Children's personal views should have no effect on their grades.
are not under a compulsion to accept evolution. A grade reflects a
assessment of a student's understanding. If a child does not
basic ideas of evolution, a grade could and should reflect that lack
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
particularly amenable to this approach. At the core of
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
conclusions, and hypotheses, rather than just learning facts.
Students can use
many data sets from evolution (such as diagrams of anatomical
organisms) to derive patterns or draw connections between
and environmental conditions. They then can use their data sets to
Students also can collect data in real time. For example, they can
extended projects involving crossbreeding of fruit flies or plants
the genetic patterns of inheritance and the influence of the
survival. In this way, students can develop an understanding of
scientific inquiry, and the nature of science.
Activities for Teaching About
Evolution and the Nature of Science
Prior chapters in this volume answer the what and why questions of
about evolution and the nature of science. As every educator knows,
discussions only set a stage. The actual play occurs when science
on the basic content and well-reasoned arguments for inclusion of
the nature of science in school science programs.
This chapter goes beyond discussions of content and rationales. It
examples of investigative teaching exercises, eight activities that
teachers can use as they begin developing students' understandings
of evolution and the nature of science. The following descriptions
introduce each activity.
* ACTIVITY 1: Introducing Inquiry and the Nature of Science
This activity introduces basic procedures involved in inquiry and
describing the nature of science. In the first portion of the
teacher uses a numbered cube to involve students in asking a
on the unseen bottom of the cube?—and the students propose an
based on their observations. Then the teacher presents the students
second cube and asks them to use the available evidence to propose
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
students with opportunities to learn the abilities and
with science as inquiry and the nature of science as described in
Science Education Standards.1 Designed for grades 5 through 12, the
requires a total of four class periods to complete. Lower grade
only complete the first cube and the evaluation where students
problem based on the cube activity.
* ACTIVITY 2: The Formulation of Explanations: An Invitation to
This activity uses the concept of natural selection to introduce the
formulating and testing a scientific hypothesis. Through a focused
approach, the teacher provides information and allows students time
interact with peers, and propose explanations for observations
the teacher. The teacher then provides more information, and the
continue their discussion based on the new information. This
help students in grades 5 through 8 develop abilities related to
inquiry and formulate understandings about the nature of science.
* ACTIVITY 3: Investigating Natural Selection
In this activity, the students investigate one mechanism for
a simulation that models the principles of natural selection and
the question: How might biological change have occurred and been
over time? The activity is designed for grades 9 through 12 and
* ACTIVITY 4: Investigating Common Descent: Formulating Explanations
In this activity, students formulate explanations and models that
structural and biochemical data as they investigate the
humans evolved from apes. The investigations require two 45-minute
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
evidence. From the evidence, they are asked to construct defensible
or explanations for events that took place in the geologic past.
time requirements for this activity: two class periods. This
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
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
time, including significant events in the development of life on
earth as well
as recent human events. The investigation requires two class periods
appropriate for grades 5 through 12.
* ACTIVITY 7: Proposing the Theory of Biological Evolution:
This activity uses historical perspectives and the theme of
introduce students to the nature of science. The teacher has
short excerpts of original statements on evolution from Jean
Darwin, and Alfred Russel Wallace. These activities are intended as
supplements to other investigations or core activities. Designed for
through 12, the activities should be used as part of three class
* ACTIVITY 8: Connecting Population Growth and Biological Evolution
In this activity, students develop a model of the mathematical
population growth. The investigation provides an excellent
consideration of population growth of plant and animal species and
relationship to mechanisms promoting natural selection. This
require two class periods and is appropriate for grades 5 through
The activities in this chapter do not represent a curriculum. They
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
not a larger level of organization such as a unit of several weeks,
or a year. Also, these exercises generally do not use biological
as fruit flies, or computer simulations. The use of these
materials in the curriculum greatly expands the range of possible
Second, these activities demonstrate how existing exercises can be
emphasize the importance of inquiry and the fundamental concepts of
Each of these exercises was derived from already existing activities
revised to reflect the National Science Education Standards. For
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
curricula to be described in Chapter 7. For example, several of the
emphasize inquiry and the nature of science while others focus on
related to evolution. All activities use an instructional model,
the next section, that increases coherence and enhances learning.
Finally, there remains a paucity of instructional materials for
evolution and the nature of science. Science teachers who recognize
are encouraged to develop new materials and lessons to introduce the
evolution and the nature of science. (See
Developing Students' Understanding and Abilities: The Curriculum
An Instructional Model
ENGAGE — This phase of the instructional model initiates the
task. The activity should (1) make connections between past and
learning experiences and (2) anticipate activities and focus
thinking on the learning outcomes of current activities. Students
become mentally engaged in the concept, process, or skill to be
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
and hypotheses. This phase also provides opportunities for teachers
introduce a formal label or definition for a concept, process,
ELABORATE — This phase of the teaching model challenges and extends
students' conceptual understanding and allows further opportunity
students to test hypotheses and practice desired skills and
Through new experiences, the students develop a deeper and broader
understanding, acquire more information, and develop and refine
EVALUATE — This phase of the teaching model encourages students to
assess their understanding and abilities and provides opportunities
teachers to evaluate student progress toward achieving the
For students to develop an understanding of evolution and the nature
requires many years and a variety of educational experiences.
rely on single lessons, chapters, or biology and earth science
students to integrate the ideas presented in this document into
understanding. In early grades (K—4) students might learn the
concepts associated with "characteristics of organisms," "life
"organisms and environments." In middle grades they learn more about
"reproduction and heredity" and "diversity and adaptation of
learning experiences, as described in the National Science Education
set a firm foundation for the study of biological evolution in
The slow and steady development of concepts such as evolution and
such as natural selection and common descent requires careful
the overall structure and sequence of learning experiences. Although
chapter does not propose a curriculum or a curriculum framework,
by Project 2061 of the American Association for the Advancement of
(AAAS) demonstrate the interrelated nature of students'
understanding of science
concepts and emphasize the importance of well-designed curricula at
levels of organization (for example, activities, units, and school
programs). The figure on the next page presents the
for Evolution and Natural Selection" based on Benchmarks for Science
Developing Student Understanding and Abilities: The Instructional
The activities in the chapter incorporate an instructional model,
the accompanying box, that includes five steps: engagement,
explanation, elaboration, and evaluation. Just as scientific
originate with a question that engages a scientist, so too must
in the activities of learning. The activities therefore begin with a
question that gets students thinking about the content of the
Once engaged, students need time to explore ideas before concepts
begin to make
sense. In this exploration phase, students try their ideas, ask
look for possible answers to questions. Students use inquiry
try to relate their ideas to those of other students and to what
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
is the step when teachers should make the major concepts explicit
and clear to
Educators understand that informing students about a concept does
necessarily result in their immediate comprehension and
understanding of the
idea. These activities therefore provide a step referred to as
which students have opportunities to apply their ideas in new and
Finally, how well do students understand the concepts, or how
they at applying the desired skills? These are the questions to be
during the evaluation phase. Ideally, evaluations are more than
should have opportunities to see if their ideas can be applied in
and to compare their understanding with scientific explanations of
National Research Council. 1996. National Science Education
Washington, DC: National Academy Press.
A Draft Growth-of-Understanding Map derived from Benchmarks for
Literacy (Jan. 1998), AAAS (American Association for the Advancement
Science) Project 2061.
ACTIVITY 1: Introducing Inquiry and the Nature of Science
ACTIVITY 2: The Formulation of Explanations: An Invitation to
ACTIVITY 3: Investigating Natural Selection
ACTIVITY 4: Investigating Common Descent: Formulating Explanations
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
Student Investigation Sheet A
Student Sheet: Zoological Philosophy
Student Sheet: On the Tendency of Varieties to Depart Indefinitely
Student Sheet: On the Origin of Species
Student Sheet: On the Origin of Species (continued)
Selecting Instructional Materials
Quality instructional materials are essential in teaching about
the nature of science.
It also is important to consider the context within which specific
will be used. This chapter therefore begins with brief discussions
science programs and the criteria used to design curricula.
Criteria for Contemporary Science Curriculum
Before selecting specific materials to teach evolution and the
science, it is important to identify criteria that can help evaluate
science programs and the design of instructional materials. Chapter
seven in the
National Science Education Standards, "Science Education Program
describes the conditions needed for quality school science programs.
conditions focus on six areas:
Consistency across all elements of the science program and across
Quality in the program of studies
Coordination with mathematics
Equitable opportunities for achievement
Collaboration within the school community to support a quality
Similarly, educators need to consider criteria against which to
instructional materials. Teachers, curriculum designers, and other
personnel can use the following criteria to evaluate the design of a
curriculum, to select instructional materials, or to adapt
materials through professional development. No set of instructional
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
omissions. These criteria are adapted from earlier discussions of
Criterion 1: A Coherent, Consistent, and Coordinated Framework for
Content. Science content should be consistent with national, state,
standards and benchmarks. Whether for lessons, units, or a complete
middle, or high school program, the content should be
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.
contemporary science curricula incorporate an instructional model.
instructional model should (1) provide for different forms of
students and between the teachers and students, (2) incorporate a
teaching strategies, such as inquiry-oriented investigations,
groups, use of technology, and (3) allow adequate time and
students to acquire knowledge, skills, and attitudes.
Criterion 3: An Integration of Psychological Principles Relative to
Motivation, Development, and Social Psychology. Psychological
principles such as
those found in the American Psychological Association publication
Learn: Reforming School Through Learner-Centered Education2 should
be applied to
the framework for content, teaching, and assessment. These
principles include more than learning theory. They include providing
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
artist decides what will be in the foreground, and that subject is
Science curricula can, for example, emphasize science concepts,
inquiry, or the
history and nature of science, while other goals may be evident but
emphasized. No one curriculum emphasis is best for all students;
variety of emphases accommodates the interests, strengths, and
Criterion 5: An Array of Opportunities to Develop Knowledge,
Abilities Associated with Different Dimensions of Scientific
Contemporary science curricula should provide a balance among the
dimensions of science literacy, which include an understanding of
concepts, the ability to engage in inquiry, and a capacity to apply
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
consistent with the goals of teaching evolution, inquiry, and the
nature of science. This can be accomplished by using
methods and by assessing students during investigative activities.
Criterion 7: Professional Development for Science Teachers Who
Curriculum. Curricula need to provide opportunities that support
they develop the knowledge and skills associated with implementing
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
consistent with other features of the curriculum—for instance, the
scientific literacy and an instructional model.
Criterion 9: Thorough Field Testing and Review for Scientific
Pedagogic Quality. One important legacy of the 1960s curriculum
reform is the
field testing of materials in a variety of science classrooms. Field
reviewing a program identify problems that developers did not
recognize and fine
tune the materials to the varied needs of teachers, learners, and
Scientists should review materials for accuracy. Developers can miss
subtleties of scientific concepts, inquiry, and design. In addition,
who review materials can provide valuable insights about teaching
that help developers improve materials and enhance learning.
Criterion 10: Support from the Educational System. Research on the
implementation, and change associated with curricula indicates the
intellectual, financial, and moral support from those within the
educational system.4 This support includes science teachers,
school boards, and communities. Although a curriculum cannot ensure
should address the need for support and provide indicators of
support, such as
provision of materials and equipment for laboratory investigations,
allocations for professional development, and proclamations by the
Clearly, no one curriculum thoroughly incorporates all ten criteria.
always trade-offs when developing, adapting, or adopting a science
However, the criteria should provide assistance to those who have
responsibility of improving the science curriculum.
Analyzing Instructional Materials
The process of selecting quality materials includes determining the
which they are consistent with the goals, principles, and criteria
the National Science Education Standards. Well-defined selection
ensure a thoughtful and effective process. To be both usable and
selection criteria must be few in number and embody the critical
accurate science content, effective teaching strategies, and
The process described in the following pages can help teachers,
designers, or other school personnel complete a thorough and
of instructional materials. To help make this examination both
usable, references to specific pages and sections in the National
Education Standards have been provided, as have worksheets to keep
track of the
information needed to analyze and select the best instructional
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
alignment with the Standards. A thorough analysis of instructional
requires considerable time and collaboration with others and
detail. Good working notes are helpful in this process. We recommend
analysis worksheets provided at the end of this chapter.
Overview of Instructional Materials
The following overview of instructional materials introduces the
and provides a general context for analysis and subsequent selection
1. The first consideration is whether the key concepts of evolution
nature of science are being emphasized. To help make this
the table of contents, index, and glossary in the material you are
The box below contains terms related to fundamental concepts in
the nature of science taken from the Standards. Record page numbers
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
these fundamental topics.
Nature of Science
evolution, diversity, adaptation, interpreting fossil evidence,
for age determination, natural selection, descent from common
explanation, experiment, evidence, inquiry, model, theory,
2. Look through both student and teacher materials. Are student
Note page numbers for several outcomes related to evolution and the
3. Look for student investigations or activities. Where are they
that in some materials, student investigations are integrated within
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
judgment about the concepts? Are the concepts in the students' text
with the fundamental concepts in the Standards? Does the text
fewer, or different concepts?
5. Do the photographs and illustrations provide further
understanding of the
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,
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
fundamental concepts are addressed. Fundamental concepts underlying
standards on evolution and the nature of science are referenced
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
Organisms," p. 158; grades 9-12, "Biological Evolution," p. 185;
"Developing Student Understanding" grades 5-8, pp. 155-156; and
9-12, p. 181.
Content Standard D—Earth and Space Science: grades 5-8, "Earth's
p. 160; grades 9-12, "The Origin and Evolution of the Earth System,"
189-190; also read "Developing Student Understanding," grades 5-8,
158-159; grades 9-12, pp. 187-188.
1. Choose a lesson or representative section of the student
materials on the topic of evolution. Make a preliminary list of the
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
2. Select one of these fundamental concepts and list all sections of
materials that deal with this idea. Determine whether the materials
focus on the
fundamental concepts, or if they represent only a superficial match.
example, Life Science Standard C in the Standards5 specifies:
evolution accounts for the diversity of species developed through
processes over many generations. Species acquire many of their
characteristics through biological adaptation, which involves the
naturally occurring variations in populations." The instructional
should provide opportunities for students to develop an
biodiversity and evolution as described in the Standards. A negative
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
process, the more confidence you will have in the quality of the
materials and their alignment with the Standards. Identify the
concepts that are not developed and the variation of treatment among
are included in the materials.
3. If appropriate, select one of the student investigations for
subject matter. On what fundamental concepts from Life Science
Standard C or
Earth and Space Science Standard D is the investigation focused? To
does the activity fulfill the intent of the fundamental concepts?
making and comparing model casts and molds of sea shells does not
contribute to an understanding of how fossils are formed or provide
evidence of how life and environmental conditions have changed. It
recommended that you analyze a second student investigation.
B. SCIENTIFIC INQUIRY
1. You should develop some understanding of scientific inquiry in
Read Standard A, Science as Inquiry, referenced on the following
Standard A—Science as Inquiry: grades 5-8, pp. 145-148; grades 9-12,
175-176; also read "Developing Student Understanding," grades 5-8,
143-144; grades 9-12, pp. 173-174.
Note that Standard A specifies two separate aspects of science as
abilities necessary to do scientific inquiry, and fundamental
about scientific inquiry. Examine several lessons in the student and
materials to answer the following question: To what degree do the
provide students the opportunity to develop the abilities and
2. Read through the text narrative, looking for student
examining any suggestions for activities outside of class time. Are
opportunities provided for students to develop abilities of
such as posing their own relevant questions, planning and conducting
investigations, using appropriate tools and techniques to gather
evidence to communicate defensible explanations of cause and effect
relationships, or using scientific criteria to analyze alternative
to determine a preferred explanation? Record page numbers where
found and make notes of explanation.
3. What opportunities are provided for students to develop a
understanding of scientific inquiry? In addition to the language of
examine the teacher's guide for suggestions that teachers can use to
role and limitations of scientific skills such as making
organizing and interpreting data, and constructing defensible
on evidence. Can you find a discussion of how science advances
legitimate skepticism? Can you find a discussion of how scientists
proposed explanations of others by examining and comparing evidence,
reasoning that goes beyond the evidence, and suggesting alternative
for the same evidence? Are there opportunities for students to
same understandings as a part of their investigations? Make notes
evidence is found for later reference.
C. HISTORY AND NATURE OF SCIENCE
1. Are history and the nature of science incorporated into the
evolution? Read Standard G, History and Nature of Science,
referenced in the
Content Standard G—History and Nature of Science: grades 5-8, pp.
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
reasoning in the historical and contemporary development of
evolution? Can you find specific references to historical
scientists in the development of fundamental concepts of evolution?
evidence can you find in the text narrative or student
demonstrates how scientific explanations are developed, reviewed by
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
its congruence with the full intent of the content standards.
learn should be consistent with contemporary models of learning. The
this section are based on characteristics of effective teaching
Teaching Standards A, B, and E.
Teaching Standard A—Teachers of science plan an inquiry-based
program for their students, pp. 30-32.
Teaching Standard B—Teachers of science guide and facilitate
32-33 and 36-37.
Teaching Standard E—Teachers of science develop communities of
learners that reflect the intellectual rigor of scientific inquiry
attitudes and social values conducive to science learning, pp. 45-46
Using the following sequence of questions, examine several lessons
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
that focus on one or more of the fundamental concepts of evolution
nature of science?
Study the opening pages of a relevant chapter or section. Does the
the opening pages of the chapter or section on evolution engage and
student thinking on interesting questions, problems, or relevant
Does the material provide a sequence of learning activities
connected in such
a way as to help students build understanding of a fundamental
suggestions provided to help the teacher keep students focused on
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
provided for teachers to find out what their students already know?
learning activities designed to help students confront their
and encourage conceptual change?
Analysis of Assessment Process
Assessment criteria in this section are grounded in the Assessment
Assessment Standards A to E, Chapter 5, pp. 78-87.
Examine several lessons in the student and teacher materials for
answer the following questions. (See Worksheet 4 on page 118 in the
back of this
Is there consistency between learning goals and assessment? For
instruction focuses on building understanding of fundamental
assessments focus on explanations and not on vocabulary?
Do assessments stress application of concepts to new or different
For example, are the students asked to explain new situations with
they have learned?
Are assessment tasks fair for all students? For example, does
assessment tasks depend too heavily on the student's ability to read
items or write explanations as opposed to understanding the
Are suggestions for scoring criteria or rubrics provided for the
Evaluating the Teacher's Guide
Examine several lessons in the teacher's guide to help answer the
Does the teacher's guide present appropriate and sufficient
Are the suggested teaching strategies usable by most teachers?
Are suggestions provided for pre- and post-investigation discussions
on concept development, inquiry, and the nature of science?
Does the teacher's guide recommend additional professional
Does the teacher's guide indicate the types of support teachers will
the instructional materials?
Analysis of Use and Management
A high degree of alignment with Standards content, pedagogy, and
criteria does not necessarily guarantee that instructional materials
easy to manage. The Standards address the importance of professional
development, and some aspects of the program standards apply as
How many different types of materials must be managed and
a typical chapter, unit, or teaching sequence (e.g., student text,
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
Do the instructional materials call for equipment, supplies, and
that teachers may not have?
Do the instructional materials identify safety issues and provide
Is the cost for materials and replacements reasonable? Are there
Rodger Bybee. 1997. Achieving Scientific Literacy: From Purposes to
Portsmouth, NH: Heinemann. Rodger Bybee, 1996. National Standards
Science Curriculum. Dubuque, IA: Kendall/Hunt Publishing Co.
N. M. Lambert and B. L. McCombs. 1998. How Students Learn: Reforming
Through Learner-Centered Education. Washington, DC: American
National Research Council. 1996. National Science Education
Washington, DC: National Academy Press, p. 22.
M.G. Fullan and S. Stiegelbauer. 1991. The New Meaning of
2nd ed. New York: Teachers College Press, Columbia University.
G.E. Hall and S.M. Hord. 1987. Change in Schools: Facilitating the
Albany: State University of New York Press.
S. Loucks-Horsley and S. Stiegelbauer. 1991. Using Knowledge of
Guide Staff Development. In Staff Development for Education in the
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.
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
Six Significant Court Decisions Regarding Evolution and Creationism
The following are excerpts from important court decisions regarding
and creationism issues. The reader is encouraged to read the full
need and time allows.
In 1968, in Epperson v. Arkansas, the United States Supreme Court
an Arkansas statute that prohibited the teaching of evolution. The
the statute unconstitutional on grounds that the First Amendment to
Constitution does not permit a state to require that teaching and
must be tailored to the principles or prohibitions of any particular
sect or doctrine. (Epperson v. Arkansas, 393 U.S. 97. (1968))
In 1981, in Segraves v. State of California, the Court found that
California State Board of Education's Science Framework, as written
qualified by its anti-dogmatism policy, gave sufficient
accommodation to the
views of Segraves, contrary to his contention that class discussion
evolution prohibited his and his children's free exercise of
anti-dogmatism policy provided that class distinctions of origins
emphasize that scientific explanations focus on "how," not "ultimate
and that any speculative statements concerning origins, both in
texts and in
classes, should be presented conditionally, not dogmatically. The
ruling also directed the Board of Education to widely disseminate
which in 1989 was expanded to cover all areas of science, not just
concerning issues of origins. (Segraves v. California, No. 278978
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
Constitution. The Arkansas statute required public schools to give
treatment to "creation-science" and "evolution-science." In a
gave a detailed definition of the term "science," the court declared
"creation science" is not in fact a science. The court also found
statute did not have a secular purpose, noting that the statute used
peculiar to creationist literature in emphasizing origins of life as
of the theory of evolution. While the subject of life's origins is
province of biology, the scientific community does not consider the
part of evolutionary theory, which assumes the existence of life and
directed to an explanation of how life evolved after it originated.
of evolution does not presuppose either the absence or the presence
creator. (McLean v. Arkansas Board of Education, 529 F. Supp. 1255,
U.S. Law Week 2412)
In 1987, in Edwards v. Aguillard, the U.S. Supreme Court held
Louisiana's "Creationism Act." This statute prohibited the teaching
evolution in public schools, except when it was accompanied by
"creation science." The Court found that, by advancing the religious
that a supernatural being created humankind, which is embraced by
creation science, the act impermissibly endorses religion. In
Court found that the provision of a comprehensive science education
undermined when it is forbidden to teach evolution except when
science is also taught. (Edwards v. Aguillard, 482, U.S. 578, 55
Law Week 4860, S. CT. 2573, 96 L. Ed. 2d510)
In 1990, in Webster v. New Lennox School District, the Seventh
of Appeals found that a school district may prohibit a teacher from
creation science in fulfilling its responsibility to ensure that the
Amendment's establishment clause is not violated, and religious
not injected into the public school curriculum. The court upheld a
court finding that the school district had not violated Webster's
rights when it prohibited him from teaching "creation science,"
since it is a
form of religious advocacy. (Webster v. New Lennox School District
F.2d 1004 (7th. Cir., 1990))
In 1994, in Peloza v. Capistrano Unified School District, the Ninth
Court of Appeals upheld a district court finding that a teacher's
Amendment right to free exercise of religion is not violated by a
district's requirement that evolution be taught in biology classes.
plaintiff Peloza's definition of a "religion" of "evolutionism," the
found that the district had simply and appropriately required a
teacher to teach a scientific theory in biology class. (Peloza v.
Unified School District, 37 F.3d 517 (9th Cir., 1994))
Matsumura, M., ed. 1995. Pp. 2-3 in Voices for Evolution. 2nd ed.
CA: National Center for Science Education.
Excerpt from "Religion in the Public Schools:
A Joint Statement of Current Law"1
Schools may teach about explanations of life on earth, including
(such as "creationism"), in comparative religion or social studies
science class, however, they may present only genuinely scientific
or evidence for, any explanation of life on earth, but not religious
(beliefs unverifiable by scientific methodology). Schools may not
teach evolutionary theory in order to avoid giving offense to
religion nor may
they circumvent these rules by labeling as science an article of
faith. Public schools must not teach as scientific fact or theory
doctrine, including "creationism," although any genuinely scientific
for or against any explanation of life may be taught. Just as they
advance nor inhibit any religious doctrine, teachers should not
example, a student's religious explanation for life on earth.
1. Excerpt from the brochure, "Religion in the Public Schools: A
Statement of Current Law." April 1995. Full copy available by
Religion in the Public Schools, 15 East 84th Street, Suite 501, New
10028 or by the World Wide Web at
Drafting Committee: American Jewish Congress, Chair; American Civil
Union; American Jewish Committee; American Muslim Council;
League; Baptist Joint Committee; Christian Legal Society; General
of Seventh-Day Adventists; National Association of Evangelicals;
Council of Churches; People for the American Way; Union of American
Congregations. Endorsing Organizations: American Ethical Union;
Humanist Association; Americans for Religious Liberty; Americans
Separation of Church and State; B'nai B'rith International;
Church; Church of the Brethren, Washington Office; Church of
International; Evangelical Lutheran Church in America, Lutheran
Governmental Affairs; Federation of Reconstructionist Congregations
Havurot; Friends Committee on National Legislation; Guru Gobind
Foundation; Hadassah, The Women's Zionist Organization of America;
Alliance; Interfaith Impact for Justice and Peace; National Council
Women; National Jewish Community Relations Advisory Council
Ministries, American Baptist Churches, USA; National Sikh Center;
American Council for Muslim Women; Presbyterian Church (USA);
Church of Jesus Christ of Latter Day Saints; Unitarian Universalist
Association of Congregations; United Church of Christ, Office for
Three Statements in Support of Teaching Evolution from
Science and Science Education Organizations
1. A NSTA (National Science Teachers Association) Position Statement
Teaching of Evolution1
Approved by the NSTA Board of Directors, July 1997
The National Science Teachers Association supports the position that
is a major unifying concept of science and should be included as
K—College science frameworks and curricula. NSTA recognizes that
not been emphasized in science curricula in a manner commensurate to
importance because of official policies, intimidation of science
general public's misunderstanding of evolutionary theory, and a
Furthermore, teachers are being pressured to introduce creationism,
"science," and other nonscientific views, which are intended to
eliminate the teaching of evolution.
Within this context, NSTA recommends that:
Science curricula and teachers should emphasize evolution in a
commensurate with its importance as a unifying concept in science
overall explanatory power.
Policy-makers and administrators should not mandate policies
teaching of creation science or related concepts such as so-called
"intelligent design," "abrupt appearance," and "arguments against
Science teachers should not advocate any religious view about
advocate the converse: that there is no possibility of supernatural
in bringing about the universe as we know it. Teachers should be
about the personal beliefs of students.
Administrators should provide support to teachers as they design and
curricula that emphasize evolution. This should include inservice
assist teachers to teach evolution in a comprehensive and
Administrators also should support teachers against pressure to
nonscientific views or to diminish or eliminate the study of
Parental and community involvement in establishing the goals of
education and the curriculum development process should be
nurtured in our democratic society. However, the professional
of science teachers and curriculum specialists to provide students
quality science education should not be bound by censorship,
inconsistencies, faulty scholarship, or unconstitutional mandates.
Science text books shall emphasize evolution as a unifying concept.
should not be required or volunteer to include disclaimers in
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
operates according to regularities and that through systematic
can understand these regularities. The methodology of science
logical testing of alternate explanations of natural phenomena
data. Because science is limited to explaining the natural world by
natural processes, it cannot use supernatural causation in its
Similarly, science is precluded from making statements about
because these are outside its provenance. Science has increased our
because of this insistence on the search for natural causes.
The most important scientific explanations are called "theories." In
speech, "theory" is often used to mean "guess," or "hunch," whereas
scientific terminology, a theory is a set of universal statements
the natural world. Theories are powerful tools. Scientists seek to
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
made. Theories and other explanations change. New theories emerge
theories are modified or discarded. Through-out this process,
formulated and tested on the basis of evidence, internal
consistency, and their
Evolution as a Unifying Concept
Evolution in the broadest sense can be defined as the idea that the
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
things today are different from what they were in the past:
planets, and life forms have evolved. Biological evolution refers to
scientific theory that living things share ancestors from which they
diverged: Darwin called it "descent with modification." There is
consistent evidence from astronomy, physics, biochemistry,
geology, biology, anthropology, and other sciences that evolution
As such, evolution is a unifying concept for science. The National
Education Standards recognizes that conceptual schemes such as
science disciplines and provide students with powerful ideas to help
understand the natural world," and recommends evolution as one such
addition, the Benchmarks for Science Literacy from the American
the Advancement of Science's Project 2061 and NSTA's Scope,
Coordination Project, as well as other national calls for science
name evolution as a unifying concept because of its importance
discipline of science. Scientific disciplines with a historical
as astronomy, geology, biology, and anthropology, cannot be taught
integrity if evolution is not emphasized.
There is no longer a debate among scientists over whether evolution
place. There is considerable debate about how evolution has taken
processes and mechanisms producing change, and what has happened
history of the universe. Scientists often disagree about their
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
The word "creationism" has many meanings. In its broadest meaning,
is the idea that a supernatural power or powers created. Thus to
Jews, and Muslims, God created; to the Navajo, the Hero Twins
created. In a
narrower sense, "creationism" has come to mean "special creation":
that the universe and all that is in it was created by God in
present form, at one time. The most common variety of special
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
Biblical Genesis. It is a specific, sectarian religious belief that
is not held
by all religious people. Many Christians and Jews believe that God
through the process of evolution. Pope John Paul II, for example,
statement in 1996 that reiterated the Catholic position that God
that the scientific evidence for evolution is strong.
"Creation science" is an effort to support special creationism
of science. Teachers are often pressured to include it or synonyms
"intelligent design theory," "abrupt appearance theory," "initial
theory," or "arguments against evolution" when they teach evolution.
creationist claims have been discredited by the available evidence.
They have no
power to explain the natural world and its diverse phenomena.
creationists seek out supposed anomalies among many existing
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,
religious values, mystical inspiration, superstition, or authority
personally useful and socially relevant, but they are not
science can only use natural explanations and not supernatural ones,
teachers should not advocate any religious view about creation, nor
converse: that there is no possibility of supernatural influence in
about the universe as we know it.
Several judicial rulings have clarified issues surrounding the
evolution and the imposition of mandates that creation science be
evolution is taught. The First Amendment of the Constitution
public institutions such as schools be religiously neutral; because
creation is a specific, sectarian religious view, it cannot be
"true," accurate scholarship in the public schools. When Arkansas
passed a law
requiring "equal time" for creationism and evolution, the law was
Federal District Court. Opponents of the bill included the religious
the United Methodist, Episcopalian, Roman Catholic, African
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
Supreme Court. In Edwards v. Aguillard 482 U.S. 578 (1987), the
that creationism was inherently a religious idea and to mandate or
in the public schools would be unconstitutional. Other court
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,
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
evolution through mandates that would require teachers to teach
"only a theory," or that require a textbook or lesson on evolution
preceded by a disclaimer. Regardless of the legal status of these
are bad educational policy. Such policies have the effect of
teachers, which may result in the de-emphasis or omission of
public will only be further confused about the special nature of
theories, and if less evolution is learned by students, science
American Association for the Advancement of Science (AAAS). 1993.
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,
Evidence. Boulder, CO: Westview Press.
McLean v. Arkansas Board of Education. 529 F. Supp. 1255 (D. Ark.
National Research Council (NRC). 1996. National Science Education
Washington, DC: National Academy Press.
National Science Teachers Association (NSTA). 1996. A Framework for
Science Education. Arlington, VA: National Science Teachers
NSTA. 1993. The Content Core: Vol. I. Rev. ed. Arlington, VA:
Peloza v. Capistrano Unified School District. 37 F.3d 517 (9th Cir.
Ruse, Michael. 1996. But Is It Science? The Philosophical Question
Creation/Evolution Controversy. Amherst, NY: Prometheus Books.
Webster v. New Lennox School District #122. 917 F.2d 1003 (7th Cir.
Task Force Members
Gerald Skoog, Chair, College of Education, Texas Tech University,
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,
2. National Association of Biology Teachers Statement on Teaching
As stated in The American Biology Teacher by the eminent scientist
Dobzhansky (1973), "Nothing in biology makes sense except in the
evolution."3 This often-quoted assertion accurately illuminates the
unifying role of evolution in nature, and therefore in biology.
in an effective and scientifically-honest manner requires classroom
and laboratory experiences on evolution.
Modern biologists constantly study, ponder and deliberate the
mechanisms and pace of evolution, but they do not debate evolution's
The fossil record and the diversity of extant organisms, combined
techniques of molecular biology, taxonomy and geology, provide
examples and powerful evidence for genetic variation, natural
speciation, extinction and other well-established components of
evolutionary theory. Scientific deliberations and modifications of
components clearly demonstrate the vitality and scientific integrity
evolution and the theory that explains it.
The same examination, pondering and possible revision have firmly
evolution as an important natural process explained by valid
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
"intelligent-design theory," "young-earth theory" or some other
creation beliefs have no place in the science classroom.
nonnaturalistic or supernatural events, whether or not explicit
made to a supernatural being, are outside the realm of science and
not part of a
valid science curriculum. Evolutionary theory, indeed all of
necessarily silent on religion and neither refutes nor supports the
a deity or deities.
Accordingly, the National Association of Biology Teachers, an
science teachers, endorses the following tenets of science,
The diversity of life on earth is the outcome of evolution: an
and natural process of temporal descent with genetic modification
affected by natural selection, chance, historical contingencies and
Evolutionary theory is significant in biology, among other reasons,
unifying properties and predictive features, the clear empirical
of its integral models, and the richness of new scientific research
The fossil record, which includes abundant transitional forms in
taxonomic groups, establishes extensive and comprehensive evidence
Natural selection, the primary mechanism for evolutionary changes,
demonstrated with numerous, convincing examples, both extant and
Natural selection—a differential, greater survival and reproduction
genetic variants within a population under an existing environmental
no specific direction or goal, including survival of a species.
Adaptations do not always provide an obvious selective advantage.
there is no indication that adaptations—molecular to organismal—must
perfect: adaptations providing a selective advantage must simply be
enough for survival and increased reproductive fitness.
The model of punctuated equilibrium provides another account of the
speciation in the fossil record of many lineages: it does not refute
overturn evolutionary theory, but instead adds to its scientific
Evolution does not violate the second law of thermodynamics:
from disorder is possible with the addition of energy, such as from
Although comprehending deep time is difficult, the earth is about
years old. Homo sapiens has occupied only a minuscule moment of that
duration of time.
When compared with earlier periods, the Cambrian explosion evident
fossil record reflects at least three phenomena: the evolution of
readily fossilized hard body parts; Cambrian environment
more conducive to preserving fossils; and the evolution from
forms of an increased diversity of body patterns in animals.
Radiometric and other dating techniques, when used properly, are
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
explanation developed from well-documented, reproducible sets of
experimentally-derived data from repeated observations of natural
The models and the subsequent outcomes of a scientific theory are
in advance, but can be, and often are, modified and improved as new
evidence is uncovered. Thus, science is a constantly self-correcting
to understand nature and natural phenomena.
Science is not teleological: the accepted processes do not start
conclusion, then refuse to change it, or acknowledge as valid only
that support an unyielding conclusion. Science does not base
theories on an
untestable collection of dogmatic proposals. Instead, the processes
are characterized by asking questions, proposing hypotheses, and
empirical models and conceptual frameworks for research about
Providing a rational, coherent and scientific account of the
and diversity of organisms requires inclusion of the mechanisms and
Similarly, effective teaching of cellular and molecular biology
inclusion of evolution.
Specific textbook chapters on evolution should be included in
curricula, and evolution should be a recurrent theme throughout
textbooks and courses.
Students can maintain their religious beliefs and learn the
foundations of evolution.
Teachers should respect diverse beliefs, but contrasting science
religion, such as belief in creationism, is not a role of science.
teachers can, and often do, hold devout religious beliefs, accept
a valid scientific theory, and teach the theory's mechanisms and
Science and religion differ in significant ways that make it
teach any of the different religious beliefs in the science
Opposition to teaching evolution reflects confusion about the nature
processes of science. Teachers can, and should, stand firm and teach
science with the acknowledged support of the courts. In Epperson v.
(1968), the U.S. Supreme Court struck down a 1928 Arkansas law
teaching of evolution in state schools. In McLean v. Arkansas
federal district court invalidated a state statute requiring equal
time for evolution and creationism.
Edwards v. Aguillard (1987) led to another Supreme Court ruling
so-called "balanced treatment" of creation science and evolution in
schools. In this landmark case, the Court called the Louisiana
statute "facially invalid as violative of the Establishment Clause
of the First
Amendment, because it lacks a clear secular purpose." This
restriction"—is now the controlling legal position on attempts to
teaching of creationism: the nation's highest court has said that
are unconstitutional. Subsequent district court decisions in
California have applied "the Edwards restriction" to teachers who
creation science, and to the right of a district to prohibit an
teacher from promoting creation science, in the classroom.
Courts have thus restricted school districts from requiring creation
the science curriculum and have restricted individual instructors
it. All teachers and administrators should be mindful of these court
remembering that the law, science and NABT support them as they
include the teaching of evolution in the science curriculum.
References and Suggested Reading
Clough, M. 1994. Diminish students' resistance to biological
Biology Teacher 56(Oct.):409-415. Futuyma, D. 1997. Evolutionary
ed. Sunderland, MA: Sinauer Associates, Inc.
Gillis, A. 1994. Keeping creationism out of the classroom.
Gould, S. 1994. The evolution of life on the earth. Scientific
Gould, S. 1977. Ever Since Darwin: Reflections in Natural History.
Mayr, E. 1991. One Long Argument: Charles Darwin and the Genesis of
Evolutionary Thought. Cambridge, MA: Harvard University Press.
McComas, W., ed. 1994. Investigating Evolutionary Biology in the
Reston, VA: National Association of Biology Teachers.
Moore, J. 1993. Science as a Way of Knowing: The Foundation of
Cambridge, MA: Harvard University Press.
National Center for Science Education, P.O. Box 9477, Berkeley, CA
Numerous publications such as Facts, faith and fairness: Scientific
clouds scientific literacy by S. Walsh and T. Demere.
Numbers, R. 1993. The Creationists: The Evolution of Scientific
Berkeley, CA: University of California Press.
Weiner, J. 1994. The Beak of the Finch: A Story of Evolution in Our
York: Alfred A. Knopf.
3. Resolution passed by the American Association for the Advancement
Commission on Science Education4
The Commission on Science Education of the American Association for
Advancement of Science, is vigorously opposed to attempts by some
education, and other groups, to require that religious accounts of
taught in science classes.
During the past century and a half, the earth's crust and the
in it have been intensively studied by geologists and
Biologists have intensively studied the origin, structure,
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
knowledge known as the biological theory of the origin and evolution
There is no currently acceptable alternative scientific theory to
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,
statements are not subject to study or verification by the
science. A scientific statement must be capable of test by
experiment. It is acceptable only if, after repeated testing, it is
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
to scientific explanations for the origin and evolution of life.
Resolution on Inclusion of the Theory of Creation in Science
WHEREAS some State Boards of Education and State Legislatures have
are considering requiring inclusion of the theory of creation as an
to evolutionary theory in discussions of origins of life, and
WHEREAS the requirement that the theory of creation be included in
an alternative to evolutionary theory represents a constraint upon
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
that reference to the theory of creation, which is neither
grounded nor capable of performing the roles required of scientific
not be required in textbooks and other classroom materials intended
for use in
Statement on Forced Teaching of Creationist Beliefs in Public School
WHEREAS it is the responsibility of the American Association for the
of Science to preserve the integrity of science, and
WHEREAS science is a systematic method of investigation based on
experimentation, observation, and measurement leading to evolving
of natural phenomena, explanations which are continuously open to
WHEREAS evolution fully satisfies these criteria, irrespective of
debates concerning its detailed mechanisms, and
WHEREAS the Association respects the right of people to hold diverse
about creation that do not come within the definitions of science,
WHEREAS creationist groups are imposing beliefs disguised as science
teachers and students to the detriment and distortion of public
education in the
THEREFORE be it resolved that because "creationist science" has no
validity it should not be taught as science, and further, that the
legislation requiring "creationist science" to be taught in public
schools as a
real and present threat to the integrity of education and the
Be it further resolved that the AAAS urges citizens, educational
and legislators to oppose the compulsory inclusion in science
curricula of beliefs that are not amenable to the process of
and revision that is indispensable to science.
Reprinted with permission from NSTA Publications, copyright 1997
Handbook, 1997-98, National Science Teachers Association, 1840
Boulevard, Arlington, VA 22201-3000.
Statement on Teaching Evolution, National Association of Biology
(NABT). Adopted by the NABT Board of Directors on March 15, 1995.
Dobzhansky, T. 1973. Nothing in biology makes sense except in the
evolution. American Biology Teacher 35:125-129.
American Association for the Advancement of Science (AAAS),
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
Council on January 7, 1982.
References for Further Reading and Other Resources
The following list of references represents a sampling of the vast
available on education, biology, and evolution. The reader is
explore the literature further as need and time allow.
Please visit our World Wide Web address at
for more extensive resource listings for these subjects.
Publications on Education
AAAS (American Association for the Advancement of Science). 1993.
Science Literacy. Project 2061. New York: Oxford University Press.
Bybee, R. 1997. Achieving Scientific Literacy: From Purposes to
Portsmouth, NH: Heinemann
Bybee, R. 1996. National Standards and the Science Curriculum:
Opportunities, and Recommendations. Dubuque, IA: Kendall/Hunt
NRC (National Research Council). 1996. National Science Education
Washington, DC: National Academy Press.
NSRC (National Science Resources Center). 1997. Science for All
Guide to Improving Elementary Science Education in Your School
Washington, DC: National Academy Press.
NSTA (National Science Teachers Association). 1996. A Framework for
Science Education. Arlington, VA: National Science Teachers
NSTA. 1993. Scope, Sequence, and Coordination of Secondary School
I. The Content Core: A Guide for Curriculum Designers. rev. ed.
National Science Teachers Association.
Publications on Biology and Other Sciences
Berg, P., and M. Singer. 1992. Dealing with Genes: The Language of
Mill Valley, CA: University Science Books.
BSCS (Biological Sciences Curriculum Study). 1998.
BSCS Biology: An Ecological Approach. 8th ed. Dubuque, IA:
BSCS. 1997. BSCS Biology: A Human Approach. Dubuque, IA:
BSCS. 1996. Biological Science: A Molecular Approach. 7th ed.
BSCS. 1993. Developing Biological Literacy: A Guide to Developing
Post-secondary Biology Curricula. Colorado Springs, CO: BSCS.
BSCS. 1983. Biological Science: Interaction of Experiments and
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:
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
Mayr, E. 1997. This Is Biology: The Science of the Living World.
Belknap Press of Harvard University Press.
Moore, J.A. 1993. Science as a Way of Knowing: The Foundations of
Biology. Cambridge, MA: Harvard University Press.
Oosterman, M., and M. Schmidt, eds. 1990. Earth Science
Alexandria, VA: American Geological Institute.
Raven, P.H., and G.B. Johnson. 1992. Biology. 3rd ed. St. Louis, MO:
Scientific American. 1994. Life in the universe: special issue.
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
Facts in the Evolution Debate. Stanford, CA: Stanford University
Clough, M. 1994. Diminish students' resistance to biological
Biology Teacher 56:409—415. Darwin, C. 1934. Charles Darwin's Diary
Voyage of H.M.S. Beagle, Nora Barlow, ed. Cambridge, UK: The
Darwin, C. 1859. On the Origin of Species by Means of Natural
Dawkins, R. 1996. Climbing Mount Improbable. New York: W.W. Norton.
Dawkins, R. 1986. The Blind Watchmaker: Why Evidence of Evolution
Universe Without Design. New York: W.W. Norton.
de Duve, C. 1995. Vital Dust: Life as a Cosmic Imperative. New York:
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
York: W.W. Norton.
Diamond, J. 1992. The Third Chimpanzee: The Evolution and Future of
Animal. New York: HarperCollins.
Diamond, J., and M.L. Cody, eds. 1975. Ecology and Evolution of
Cambridge, MA: Belknap Press of Harvard University Press.
Ewald, P. 1994. The Evolution of Infectious Disease. New York:
Futuyma, D. 1997. Evolutionary Biology. 3rd ed. Sunderland, MA:
Futuyma, D. 1995. Science on Trial: The Case for Evolution. 2nd ed.
MA.: Sinauer Associates, Inc.
Gillis, A. 1994. Keeping creationism out of the classroom.
Goldschmidt, T. 1996. Darwin's Dreampond: Drama in Lake Victoria.
Goldsmith, T. H. 1991. The Biological Roots of Human Nature: Forging
Between Evolution and Behavior. New York: Oxford University Press.
Gould, S.J. 1997. This view of life: Nonoverlapping magisteria.
Gould, S.J. 1994. The evolution of life on the earth. Scientific
Gould, S.J. 1989. Wonderful Life: The Burgess Shale and the Nature
New York: W.W. Norton.
Gould, S.J. 1980. The Panda's Thumb: More Reflections in Natural
York: W.W. Norton.
Gould, S.J. 1977. Ever Since Darwin: Reflections in Natural History.
Kitcher, P. 1982. Abusing Science: The Case Against Creationism.
Matsumura, M., ed. 1995. Voices for Evolution. 2nd ed. Berkeley, CA:
Center for Science Education.
Mayr, E. 1991. One Long Argument: Charles Darwin and the Genesis of
Evolutionary Thought. Cambridge, MA: Harvard University Press.
Mayr, E. 1972. The nature of the Darwinian revolution. Science
McComas, W., ed. 1994. Investigating Evolutionary Biology in the
Reston, VA: National Association of Biology Teachers.
McKinney, M.L. 1993. Evolution of Life: Processes, Patterns, and
Englewood Cliffs, NJ: Prentice Hall.
Moore, J.R. 1979. The Post-Darwinian Controversies: A Study of the
Struggle to Come to Terms with Darwin in Great Britain and America,
Cambridge, UK: Cambridge University Press.
Nesse, R., and G. Williams. 1995. Why We Get Sick: The New Science
Medicine. New York: Times Books.
Newell, N.D. 1982. Creation and Evolution: Myth or Reality? New
Numbers, R. 1993. The Creationists: The Evolution of Scientific
Berkeley, CA: University of California Press.
Quammen, D. 1996. The Song of the Dodo: Island Biogeography in an
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
Reading, MA: Addison-Wesley.
Ruse, M. 1979. The Darwinian Revolution: Science Red in Tooth and
University of Chicago Press.
Tiffin, L. 1994. Creationism's Upside-down Pyramid: How Science
Fundamentalism. Amherst, NY: Prometheus Books.
Walsh, S., and T. Demere. 1993. Facts, Faith and Fairness:
Creationism Clouds Scientific Literacy. Berkeley, CA: National
Weiner, J. 1994. The Beak of the Finch: A Story of Evolution in Our
York: Alfred A. Knopf.
Wills, C. 1989. The Wisdom of the Genes: New Pathways in Evolution.
Wilson, E. 1992. The Diversity of Life. Cambridge, MA: Harvard
Publications on the Nature of Science
Aicken, F. 1991. The Nature of Science. 2nd ed. Portsmouth, NH:
Bronowski, J. 1965. Science and Human Values. New York: Harper.
Chalmers, A. 1995. What Is This Thing Called Science? 2nd ed.
Chalmers, A. 1990. Science and Its Fabrication. Minneapolis, MN:
Daedalus. 1978. Limits of scientific inquiry. 107 (Spring).
Hull, D. 1988. Science as a Process: An Evolutionary Account of the
Conceptual Development of Science. Chicago: University of Chicago
Kuhn, T.S. 1970. The Structure of Scientific Revolutions. Chicago:
Laudan, Larry. 1996. Beyond Positivism and Relativism: Theory,
Evidence. Boulder, CO: Westview Press.
Popper, K. 1994. The Myth of the Framework: In Defense of Science
Rationality. London: Routledge.
Wolpert, L. 1992. The Unnatural Nature of Science. Cambridge, MA:
Woolgar, S. 1988. Science: The Very Idea. London: Routledge.
The Day the Universe Changed (episode #10, Worlds Without End).
Mills, MD: MPT-TV.
The Pleasure of Finding Things Out. 1982. Video interview with
New York: Time/Life video.
Darwin's Revolution in Thought. Talk given by Stephen Jay Gould (No.
Available from Into the Classroom Video, 351 Pleasant Street,
God, Darwin and the Dinosaurs. 1989. Boston: WGBH Educational
In the Beginning: The Creationist Controversy. 1994. Chicago: WTTW.
This report has been reviewed by individuals chosen for their
perspectives and technical expertise, in accordance with procedures
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
the study charge. The content of the review comments and draft
confidential to protect the integrity of the deliberative process.
We wish to
thank the following individuals for their participation in the
review of this
Evan Pugh Professor of Anthropology, Emeritus
Pennsylvania State University
Professor of Biology
Department of Embryology
Carnegie Institution of Washington
National Association of Biology Teachers
Essex High School
Essex Junction, Vermont
Adjunct Professor of Soil Physics
University of California at Berkeley
Professor of Physics
Carnegie Mellon University
Professor of Science
Professor of Biology
Providence, Rhode Island
Biology Teacher and
Science Department Chair
Ithaca High School
Ithaca, New York
National Institute of Environmental
Research Triangle Park, North Carolina
Professor of Biology, Emeritus
University of California at Santa Barbara
Santa Barbara, California
Helen DeVitt Jones Professor of Curriculum and Instruction
Texas Technology University
Department of Terrestrial Magnetism
Carnegie Institution of Washington
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.