Teaching About Evolution And The Nature Of Science National Academy of Sciences pot

Library of Congress Cataloging-in-Publication Data Teaching about evolution and the nature of science / [Working Group on Teaching Evolution, National Academy of Sciences].. Teaching Abo

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Teaching about evolution and the nature of science / [Working Group on Teaching Evolution, National Academy of Sciences].

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Printed in the United States of America Copyright 1998 by the National Academy of Sciences All rights reserved.

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The report is also available online at www.nap.edu/readingroom/books/evolution98

Donald Kennedy (Chairman)

Bing Professor of Environmental StudiesStanford University

Stanford, California

Bruce Alberts

PresidentNational Academy of Sciences Washington, DC

Danine Ezell

Science Department Bell Junior High SchoolSan Diego, California

Tim Goldsmith

Department of BiologyYale UniversityNew Haven, Connecticut

Robert Hazen

Staff Scientist, Geophysical LaboratoryCarnegie Institution of Washington Washington, DC

Norman Lederman

Professor, College of ScienceScience and Mathematics EducationOregon State University

Corvallis, Oregon

Joseph McInerney

DirectorBiological Sciences Curriculum StudyColorado Springs, Colorado

John Moore

Professor Emeritus of BiologyUniversity of CaliforniaRiverside, California

Eugenie Scott

Executive DirectorNational Center for Science Education

El Cerrito, California

Maxine Singer

PresidentCarnegie Institution of WashingtonWashington, DC

Mike Smith

Associate Professor of Medical EducationMercer University School of MedicineMacon, Georgia

Dover, Delaware

S TAFF OF THE C ENTER FOR S CIENCE , M ATHEMATICS , AND E NGINEERING E DUCATION :

Rodger Bybee, Executive Director Peggy Gill, Research Assistant Jay Hackett, Visiting Fellow

Patrice Legro, Division Director Steve Olson, Consultant Writer

Any opinions, findings, conclusions, or recommendations expressed in this publicationare those of the authors and do not necessarily reflect the view of the organizations that

provided financial support for this project

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The National Academy of Sciences gratefully acknowledges contributions from:

Howard Hughes Medical InstituteThe Esther A and Joseph Klingenstein Fund, Inc

The Council of the National Academy of SciencesThe 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 Supervisorsand teachers who participated in focus groups and provided guidance

on the development of this document

• viii • Preface

• 1 •

CHAPTER1

Why Teach Evolution?

Dialogue: The Challenge to Teachers 7

• 11 •

CHAPTER2

Major Themes in Evolution

Dialogue: Teaching About the Nature of Science 22

• 27 •

CHAPTER3

Evolution and the Nature of Science

Dialogue: Teaching Evolution Through Inquiry 44

A Six Significant Court Decisions Regarding Evolution and Creationism Issues 121

B Excerpt from “Religion in the Public Schools: A Joint Statement of Current Law,” 123

C Three Statements in Support of Teaching Evolution from Science and Science Education Organizations 124

D References for Further Reading and Other Resources 130

E Reviewers 133

• 135 • Index

Contents

In a 1786 letter to a friend, ThomasJefferson called for “the diffusion of knowl-edge among the people No other sure foun-dation can be devised for the preservation offreedom and happiness.”1 Jefferson saw clear-

ly what has become increasingly evident sincethen: the fortunes of a nation rest on theability of its citizens to understand and useinformation about the world around them

We are about to enter a century in whichthe United States will be even more depen-dent on science and technology than it hasbeen in the past Such a future demands acitizenry able to use many of the same skillsthat scientists use in their work—close obser-vation, careful reasoning, and creative think-ing based on what is known about the world

The ability to use scientific knowledge andways of thinking depends to a considerableextent on the education that people receivefrom kindergarten through high school Yetthe teaching of science in the nation’s publicschools often is marred by a serious omission

Many students receive little or no exposure tothe most important concept in modern biolo-

gy, a concept essential to understanding keyaspects of living things—biological evolution

People and groups opposed to the teaching ofevolution in the public schools have pressedteachers and administrators to present ideasthat conflict with evolution or to teach evolu-tion as a “theory, not a fact.” They have per-suaded some textbook publishers to downplay

or eliminate treatments of evolution and havechampioned legislation and policies at thestate and local levels meant to discourage theteaching of evolution

These pressures have contributed towidespread misconceptions about the state ofbiological understanding and about what isand is not science Fewer than one-half ofAmerican adults believe that humans evolvedfrom earlier species.2 More than one half ofAmericans say that they would like to havecreationism taught in public school class-rooms3—even though the Supreme Courthas ruled that “creation science” is a religiousidea and that its teaching cannot be mandat-

ed in the public schools.4

The widespread misunderstandings aboutevolution and the conviction that creationism

should be taught in science classes are ofgreat concern to the National Academy ofSciences, a private nonpartisan group of1,800 scientists dedicated to the use of sci-ence and technology for the general welfare.The Academy and its affiliated institutions—the National Academy of Engineering, theInstitute of Medicine, and the NationalResearch Council—have all sought tocounter misinformation about evolutionbecause of the enormous body of data sup-porting evolution and because of the impor-tance of evolution as a central concept inunderstanding our planet

The document that you are about to read

is addressed to several groups at the center ofthe ongoing debate over evolution: theteachers, other educators, and policy makerswho design, deliver, and oversee classroominstruction in biology It summarizes theoverwhelming observational evidence for evo-lution and suggests effective ways of teachingthe subject It explains the nature of scienceand describes how science differs from otherhuman endeavors It provides answers to fre-quently asked questions about evolution andthe nature of science and offers guidance onhow to analyze and select teaching materials.This publication does not attempt specifi-cally to refute the ideas proffered by thosewho oppose the teaching of evolution in pub-

lic schools A related document, Science and Creationism: A View from the National Academy of Sciences, discusses evolution and

“creation science” in detail.5 This publicationinstead provides information and resourcesthat teachers and administrators can use toinform themselves, their students, parents,and others about evolution and the role ofscience in human affairs

One source of resistance to the teaching

of evolution is the belief that evolution flicts with religious principles But acceptingevolution as an accurate description of thehistory of life on earth does not mean reject-ing religion On the contrary, most religiouscommunities do not hold that the concept ofevolution is at odds with their descriptions ofcreation and human origins

con-Nevertheless, religious faith and scientificknowledge, which are both useful and impor-

Preface

tant, are different This publication isdesigned to help ensure that students receive

an education in the sciences that reflects thisdistinction

The book is divided into seven chaptersand five appendices, plus three interspersed

“dialogues” in which several fictional teachersdiscuss the implications of the ideas discussed

in the book

• Chapter 1, “Why Teach Evolution,”

introduces the basic concepts of evolutionarytheory and provides scientific definitions ofseveral common terms, such as “theory” and

“fact,” used throughout the book

• The first dialogue, “The Challenge toTeachers,” follows the conversation of threeteachers as they discuss some of the prob-lems that can arise in teaching evolution andthe nature of science

• Chapter 2, “Major Themes in Evolution,”

provides a general overview of evolutionaryprocesses, describes the evidence supportingevolution, and shows how evolutionary theory

is related to other areas of biology

• The second dialogue, “Teaching Aboutthe Nature of Science,” follows the threeteachers as they engage in a teaching exercisedesigned to demonstrate several prominentfeatures of science

• Chapter 3, “Evolution and the Nature

of Science,” uses several scientific theories,including evolution, to highlight importantcharacteristics of scientific endeavors

• The third dialogue, “Teaching EvolutionThrough Inquiry,” presents a teacher using anexercise designed to interest and educate herstudents in fossils and the mechanisms ofevolution

• Chapter 4, “Evolution and the National Science Education Standards,” begins by

describing the recent efforts to specify whatstudents should know and be able to do as aresult of their education in the sciences Itthen reproduces sections from the 1996

National Science Education Standards

released by the National Research Councilthat relate to biological evolution and thenature and history of science

• Chapter 5, “Frequently Asked QuestionsAbout Evolution and the Nature of Science,”

gives short answers to some of the questions

asked most frequently by students, parents,educators, and others

• Chapter 6, “Activities for TeachingAbout Evolution and the Nature of Science,”

provides eight sample activities that teacherscan use to develop students’ understanding ofevolution and scientific inquiry

• Chapter 7, “Selecting InstructionalMaterials,” lays out criteria that can be used

to evaluate school science programs and thecontent and design of instructional materials

• The appendices summarize significantcourt decisions regarding evolution and cre-ationism issues, reproduce statements from anumber of organizations regarding the teach-ing of evolution, provide references for fur-ther reading and other resources, and con-clude with a list of reviewers

Teaching About Evolution and the Nature

of Science was produced by the Working

Group on Teaching Evolution under theCouncil of the National Academy of Sciences

The Working Group consists of 13 scientistsand educators who have been extensivelyinvolved in research and education on evolu-tion and related scientific subjects The groupworked closely with teachers, school adminis-trators, state officials, and others in preparingthis publication, soliciting suggestions for whatwould be most useful, and responding to com-ments on draft materials We welcome addi-tional input and guidance from readers that

we can incorporate into future versions of thispublication Please visit our World Wide Web

site at www4.nas.edu/opus/evolve.nsf for

additional information

1 Thomas Jefferson, To George Wythe, “Crusade

Against Ignorance” in Thomas Jefferson on

Education, ed Gordon C Lee 1961 New York:

Teachers College Press, pp 99-100.

2 National Science Board 1996 Science and

Engineering Indicators—1996 Washington, DC:

U.S Government Printing Office.

3 Gallup Poll, News Release, May 24, 1996.

4 In the 1987 case Edwards v Aguillard, the U.S.

Supreme Court reaffirmed the 1982 decision of a federal district court that the teaching of “creation science” in public schools violates the First Amendment of the U.S Constitution.

5 National Academy of Sciences (in press) Science

and Creationism: A View from the National Academy of Sciences Washington, DC: National

Academy Press (See www.nap.edu)

Why is it so important

to teach evolution?

After all, many tions in biology can be answeredwithout mentioning evolution: How

ques-do birds fly? How can certain plantsgrow in the desert? Why do children resem-ble their parents? Each of these questionshas an immediate answer involving aerody-namics, the storage and use of water byplants, or the mechanisms of heredity

Students ask about such things all the time

The answers to these questions oftenraise deeper questions that are sometimesasked by students: How did things come to

be that way? What is the advantage to birds

of flying? How did desert plants come todiffer from others? How did an individualorganism come to have its particular geneticendowment? Answering questions like theserequires a historical context—a framework

of understanding that recognizes changethrough time

People who study nature closely havealways asked these kinds of questions Overtime, two observations have proved to beespecially perplexing The older of thesehas to do with the diversity of life: Why are there so many different kinds of plantsand animals? The more we explore theworld, the more impressed we are with themultiplicity of kinds of organisms In themid-nineteenth century, when Charles

Darwin was writing On the Origin of Species, naturalists recognized several tens

of thousands of different plant and animalspecies By the middle of the twentiethcentury, biologists had paid more attention

to less conspicuous forms of life,from insects to microorganisms,and the estimate was up to 1 or

2 million Since then, tions in tropical rain forests—thecenter of much of the world’s biologicaldiversity—have multiplied those estimates atleast tenfold What process has created thisextraordinary variety of life?

investiga-The second question involves the inverse

of life’s diversity How can the similaritiesamong organisms be explained? Humanshave always noticed the similarities amongclosely related species, but it graduallybecame apparent that even distantly relatedspecies share many anatomical and functionalcharacteristics The bones in a whale’s frontflippers are arranged in much the same way

as the bones in our own arms As organismsgrow from fertilized egg cells into embryos,they pass through many similar developmen-tal stages Furthermore, as paleontologistsstudied the fossil record, they discoveredcountless extinct species that are clearlyrelated in various ways to organisms livingtoday

This question has emerged with evengreater force as modern experimental biolo-

gy has focused on processes at the cellularand molecular level From bacteria to yeast

to mice to humans, all living things use thesame biochemical machinery to carry outthe basic processes of life Many of theproteins that make up cells and catalyzechemical reactions in the body are virtuallyidentical across species Certain humangenes that code for proteins differ littlefrom the corresponding genes in fruit flies,Why Teach Evolution?

1

mice, and primates All living things usethe same biochemical system to pass genet-

ic information from one generation toanother

From a scientific standpoint, there isone compelling answer to questions aboutlife’s commonalities Different kinds oforganisms share so many characteristics ofstructure and function because they arerelated to one another But how?

Solving the Puzzle

The concept of biological evolutionaddresses both of these fundamental ques-tions It accounts for the relatednessamong organisms by explaining that themillions of different species of plants, ani-mals, and microorganisms that live on earthtoday are related by descent from commonancestors—like distant cousins Organisms

in nature typically produce more offspringthan can survive and reproduce given theconstraints of food, space, and otherresources in the environment These off-spring often differ from one another in waysthat are heritable—that is, they can pass onthe differences genetically to their own off-spring If competing offspring have traitsthat are advantageous in a given environ-ment, they will survive and pass on thosetraits As differences continue to accumu-late over generations, populations of organ-isms diverge from their ancestors

This straightforward process, which is anatural consequence of biologically repro-ducing organisms competing for limitedresources, is responsible for one of the mostmagnificent chronicles known to science.Over billions of years, it has led the earliestorganisms on earth to diversify into all ofthe plants, animals, and microorganismsthat exist today Though humans, fish, andbacteria would seem to be so different as todefy comparison, they all share some of thecharacteristics of their common ancestors.Evolution also explains the great diversity

of modern species Populations of organismsInvestigations of forest ecosystems have helped reveal

the incredible diversity of earth's living things.

with characteristics enabling them to occupyecological niches not occupied by similarorganisms have a greater chance of surviving.

Over time—as the next chapter discusses inmore detail—species have diversified andhave occupied more and more ecologicalniches to take advantage of new resources

Evolution explains something else aswell During the billions of years that lifehas been on earth, it has played an increas-ingly important role in altering the planet’sphysical environment For example, thecomposition of our atmosphere is partly aconsequence of living systems During pho-tosynthesis, which is a product of evolution,green plants absorb carbon dioxide andwater, produce organic compounds, andrelease oxygen This process has createdand continues to maintain an atmosphererich in oxygen Living communities alsoprofoundly affect weather and the move-ment of water among the oceans, atmos-phere, and land Much of the rainfall in theforests of the western Amazon basin consists

of water that has already made one or morerecent trips through a living plant In addi-tion, plants and soil microorganisms exertimportant controls over global temperature

by absorbing or emitting “greenhouse gases”

(such as carbon dioxide and methane) thatincrease the earth’s capacity to retain heat

In short, biological evolution accountsfor three of the most fundamental features

of the world around us: the similaritiesamong living things, the diversity of life, andmany features of the physical world weinhabit Explanations of these phenomena

in terms of evolution draw on results fromphysics, chemistry, geology, many areas ofbiology, and other sciences Thus, evolution

is the central organizing principle that gists use to understand the world To teachbiology without explaining evolutiondeprives students of a powerful concept thatbrings great order and coherence to ourunderstanding of life

biolo-The teaching of evolution also has greatpractical value for students Directly orindirectly, evolutionary biology has mademany contributions to society Evolutionexplains why many human pathogens havebeen developing resistance to formerlyeffective drugs and suggests ways of con-fronting this increasingly serious problem(this issue is discussed in greater detail inChapter 2) Evolutionary biology has also

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.

Fossil fish image not available in this format

contributed to many important agriculturaladvances by explaining the relationshipsamong wild and domesticated plants andanimals and their natural enemies Anunderstanding of evolution has been essen-tial in finding and using natural resources,such as fossil fuels, and it will be indispens-able as human societies strive to establishsustainable relationships with the naturalenvironment.

Such examples can be multiplied manytimes Evolutionary research is one of themost active fields of biology today, and dis-coveries with important practical applica-tions occur on a regular basis

Those who oppose the teaching of lution in public schools sometimes ask thatteachers present “the evidence against evo-lution.” However, there is no debate withinthe scientific community over whether evo-lution occurred, and there is no evidencethat evolution has not occurred Some ofthe details of how evolution occurs are stillbeing investigated But scientists continue

evo-to debate only the particular mechanismsthat result in evolution, not the overallaccuracy of evolution as the explanation oflife’s history

Evolution and the Nature of Science

Teaching about evolution has anotherimportant function Because some peoplesee evolution as conflicting with widely heldbeliefs, the teaching of evolution offers edu-cators a superb opportunity to illuminatethe nature of science and to differentiatescience from other forms of human endeav-

or and understanding

Chapter 3 describes the nature of ence in detail However, it is importantfrom the outset to understand how themeanings of certain key words in sciencediffer from the way that those words areused in everyday life

sci-Think, for example, of how people usuallyuse the word “theory.” Someone might refer

to an idea and then add, “But that’s only atheory.” Or someone might preface a remark

by saying, “My theory is ” In commonusage, theory often means “guess” or “hunch.”

In science, the word “theory” meanssomething quite different It refers to anoverarching explanation that has been wellsubstantiated Science has many other pow-erful theories besides evolution Cell theorysays that all living things are composed of

20

10

Start of rapid O 2 accumulation (Fe 2+

in oceans used up)

Oxygen Levels in Atmosphere (%)

Time (Billions of Years)

Formation

of the earth

First vertebrates

Present day Formation of

oceans and continents

First living cells

First photosynthetic cells

First water-splitting photosynthesis releases O2

Aerobic respiration becomes widespread

Origin of eucaryotic photosynthetic cells

First multicellular plants and animals

Living things have

altered the earth's

oceans, land surfaces,

and atmosphere For

example,

photosyn-thetic organisms are

responsible for the

oxygen that makes up

about a fifth of the

earth's atmosphere.

The rapid

accumula-tion of atmospheric

oxygen about 2 billion

years ago led to the

evolution of more

structured eucaryotic

cells, which in turn

gave rise to

multicellu-lar plants and animals.

cells The heliocentric theory says that theearth revolves around the sun rather thanvice versa Such concepts are supported bysuch abundant observational and experi-mental evidence that they are no longerquestioned in science.

Sometimes scientists themselves use theword “theory” loosely and apply it to tenta-tive explanations that lack well-establishedevidence But it is important to distinguishthese casual uses of the word “theory” withits use to describe concepts such as evolu-tion that are supported by overwhelmingevidence Scientists might wish that theyhad a word other than “theory” to apply tosuch enduring explanations of the naturalworld, but the term is too deeply engrained

in science to be discarded

As with all scientific knowledge, a

theo-ry can be refined or even replaced by an

alternative theory in light of new and pelling evidence For example, Chapter 3describes how the geocentric theory thatthe sun revolves around the earth wasreplaced by the heliocentric theory of theearth’s rotation on its axis and revolutionaround the sun However, ideas are notreferred to as “theories” in science unlessthey are supported by bodies of evidencethat make their subsequent abandonmentvery unlikely When a theory is supported

com-by as much evidence as evolution, it is heldwith a very high degree of confidence

In science, the word “hypothesis” veys the tentativeness inherent in the com-mon use of the word “theory.” A hypothesis

con-is a testable statement about the naturalworld Through experiment and observa-tion, hypotheses can be supported or reject-

ed As the earliest level of understanding,hypotheses can be used to construct morecomplex inferences and explanations

Like “theory,” the word “fact” has a ferent meaning in science than it does incommon usage A scientific fact is anobservation that has been confirmed overand over However, observations are gath-ered by our senses, which can never betrusted entirely Observations also canchange with better technologies or withbetter ways of looking at data For exam-ple, it was held as a scientific fact for manyyears that human cells have 24 pairs ofchromosomes, until improved techniques ofmicroscopy revealed that they actually have

dif-23 Ironically, facts in science often aremore susceptible to change than theories—

which is one reason why the word “fact” isnot much used in science

Finally, “laws” in science are typicallydescriptions of how the physical worldbehaves under certain circumstances

For example, the laws of motion describehow objects move when subjected to cer-tain forces These laws can be very useful

in supporting hypotheses and theories, but like all elements of science they can

be altered with new information andobservations

Glossary of Terms Used in Teaching About the Nature

of Science

Fact: In science, an observation that

has been repeatedly confirmed

Law: A descriptive generalization

about how some aspect of the natural world behaves under statedcircumstances

Hypothesis: A testable statement

about the natural world that can

be used to build more complexinferences and explanations

Theory: In science, a

well-substanti-ated explanation of some aspect

of the natural world that can porate facts, laws, inferences, andtested hypotheses

incor-Those who oppose the teaching of lution often say that evolution should betaught as a “theory, not as a fact.” Thisstatement confuses the common use ofthese words with the scientific use In science, theories do not turn into factsthrough the accumulation of evidence.

evo-Rather, theories are the end points of science They are understandings thatdevelop from extensive observation, experimentation, and creative reflection

They incorporate a large body of scientificfacts, laws, tested hypotheses, and logicalinferences In this sense, evolution is one

of the strongest and most useful scientifictheories we have

Evolution and Everyday Life

The concept of evolution has an tance in education that goes beyond itspower as a scientific explanation All of uslive in a world where the pace of change isaccelerating Today’s children will facemore new experiences and different condi-tions than their parents or teachers havehad to face in their lives

impor-The story of evolution is one chapter—perhaps the most important one—in a sci-entific revolution that has occupied much ofthe past four centuries The central feature

of this revolution has been the ment of one notion about stability afteranother: that the earth was the center ofthe universe, that the world’s living thingsare unchangeable, that the continents of theearth are held rigidly in place, and so on.Fluidity and change have become central toour understanding of the world around us

abandon-To accept the probability of change—and tosee change as an agent of opportunityrather than as a threat—is a silent messageand challenge in the lesson of evolution.The following dialogue dramatizes some

of the problems educators encounter inteaching evolution and demonstrates ways

of overcoming these obstacles Chapter 2returns to the basic themes that character-ize evolutionary theory, and Chapter 3 takes

a closer look at the nature of science

Scientists examining the

head of Chasmosaurus

mariscalensis hone their

understanding of nature

by comparing it against

observations of the world.

Clockwise from upper

right: 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.

Teaching evolution presents special challenges to science teachers Sources of support upon which teachers can draw include high-quality curricula, adequate preparation, exposure to information useful

in documenting the evidence for evolution, and resources and contacts provided by professional associations.

One important source of support for teachers is to share problems and explore solutions with other teachers The following vignette illustrates how a group of teach- ers—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 CentralHigh School As the bell rings for thirdperiod, Karen, the newest teacher on thefaculty, walks into the teachers’ lounge Shegreets her colleagues, Barbara and Doug

“How are your first few days going?”

asks Doug

“Fine,” Karen replies “The period Biology I class is full, but it’ll beokay By the way, Barbara, thanks for let-ting me see your syllabus for Bio I But

second-I wanted to ask you about teaching tion—I didn’t see it there.”

evolu-“You didn’t see it on my syllabusbecause it’s not a separate topic,” Barbarasays “I use evolution as a theme to tie thecourse together, so it comes into just aboutevery unit You’ll see a section called

‘History of Life’ on the second page, andthere’s a section called ‘Natural Selection.’

But I don’t treat evolution separatelybecause it is related to almost every othertopic in biology.”1

“Wait a minute, Barbara,” Doug says

“Is that good advice for a new teacher?

I mean, evolution is a controversial subject,and a lot of us just don’t get around toteaching it I don’t You do, but you’rebraver than most of us.”

“It’s not a matter of bravery, Doug,”

Barbara replies “It’s a matter of whatneeds to be taught if we want students tounderstand biology Teaching biology with-out evolution would be like teaching civicsand never mentioning the United StatesConstitution.”

“But how can you be sure that evolution

is all that important Aren’t there a lot ofscientists who don’t believe in evolution?

Say it’s too improbable?”

“The debate in science is over some ofthe details of how evolution occurred, notwhether evolution happened or not A lot

of science and science education tions have made statements about why it isimportant to teach evolution ”2

organiza-“I saw a news report when I was a dent,” Karen interjects, “about a school dis-trict or state that put a disclaimer againstevolution in all their biology textbooks Itsaid that students didn’t need to believe inevolution because it wasn’t a fact, only a the-ory The argument was that no one reallyknows how life began or how it evolvedbecause no one was there to see it happen.”3

stu-“If I taught evolution, I’d sure teach it as

a theory—not a fact,” says Doug

“Just like gravity,” Barbara says

“Now, Barbara, gravity is a fact, not atheory.”

“Not in scientific terms The fact is thatthings fall The explanation for why thingsfall is the theory of gravitation Our problem

is definitions You’re using ‘fact’ and ‘theory’

the way we use them in everyday life, but weneed to use them as scientists use them Inscience, a ‘fact’ is an observation that has

D i a l o g u e

T HE C HALLENGE TO T EACHERS

been made so many times that it’s assumed

to be okay How facts are explained iswhere theories come in: theories are expla-nations of what we observe One placewhere students get confused about evolu-tion is that they think of ‘theory’ as meaning

‘guess’ or ‘hunch.’ But evolution isn’t ahunch It’s a scientific explanation, and avery good one.”

“But how good a theory is it?” asksDoug “We don’t know everything aboutevolution.”

“That’s true,” says Karen “A student inone of my classes at the university told methat there are big gaps in the fossil record

Do you know anything about that?”

“Well, there’s Archaeopteryx,” says

Doug “It’s a fossil that has feathers like abird but the skeleton of a small dinosaur

It’s one of those missing links that’s notmissing any more.”

“In fact, there are good transitional sils between primitive fish and amphibiansand between reptiles and mammals,”

fos-Barbara says “Our knowledge of fossil

intermediates is actually pretty good.4 And,Doug, it sounds like you know more aboutevolution than you’re letting on Why don’tyou teach it?”

“I don’t want any trouble Every time Iteach evolution, I have a student announcethat ‘evolution is against his religion.’”

“But most of the major religious inations have taken official positions thataccept evolution,” says Barbara “Onesemester a friend of mine in the middleschool started out her Life Science unit byhaving her students interview their minis-ters or priests or rabbis about their reli-gion’s views on evolution She said thatmost of her students came back really sur-prised ‘Hey,’ they said, ‘evolution is okay.’

denom-It defused the controversy in her class.”

“She didn’t have Stanley in her class,”says Doug

“Who’s Stanley?” asks Karen

“The son of a school board member.Given his family’s religious views, I’m sure

he would not come back saying evolutionwas okay.”

“That can be a hard situation,” saysBarbara “But even if Stanley came back toclass saying that his religion does not acceptevolution, it could help a teacher show thatthere are many different religious viewsabout evolution That’s the point: religiouspeople can still accept evolution.”

“Stanley will never believe in evolution.”

“We talk about ‘believing’ in evolution,but that’s not necessarily the right word Weaccept evolution as the best scientific expla-nation for a lot of observations—about fossilsand biochemistry and evolutionary changes

we can actually see, like how bacteria becomeresistant to certain medicines That’s whypeople accepted the idea that the earth goesaround the sun—because it accounted formany different observations that we make

In science, when a better explanation comesaround, it replaces earlier ones.”

“Does that mean that evolution will bereplaced by a better theory some day?” asksKaren

“It’s not likely Not all old theories are

A fossil of Archaeopteryx,

a bird that lived about

150 million years ago

and had many reptilian

characteristics, was

dis-covered in 1861 and

helped support the

hypothesis of evolution

proposed by Charles

Darwin in The Origin of

Species two years earlier.

replaced, and evolution has been tested andhas a lot of evidence to support it Thepoint is that doing science requires beingwilling to refine our theories to be consis-tent with new information.”

“But there’s still Stanley,” says Doug

“He doesn’t even want to hear about tion.”

evolu-“I had Stanley’s sister in AP biology oneyear,” Barbara replies “She raised a fussabout evolution, and I told her that I wasn’tgoing to grade her on her opinion of evolu-tion but on her knowledge of the facts andconcepts She seemed satisfied with thatand actually got an A in the class.”

“I still think that if you teach evolution,it’s only fair to teach both.”

“What do you mean by both?” asksBarbara “If you mean both evolution andcreationism, what kind of creationism do youwant to teach? Will you teach evolution andthe Bible? What about other religions likeBuddhism or the views of Native Americans?

It’s hard to argue for ‘both’ when there are awhole lot more than two options.”

“I can’t teach a whole bunch of creationstories in my Bio class,” says Doug

“That’s the point We can’t add subjects

to the science curriculum to be fair togroups that hold certain beliefs Teachingecology isn’t fair to the polluter, either

Biology is a science class, and what should

be taught is science.”

“But isn’t there something called ation science’?” asks Karen “Can creation-ism be made scientific?”

‘cre-“That’s an interesting story ‘Creationscience’ is the idea that scientific evidencecan support a literal interpretation ofGenesis—that the whole universe was cre-ated all at once about 10,000 years ago.”

“It doesn’t sound very likely.”

“It’s not Scientists have looked at thearguments and have found they are not sup-ported by verifiable data Still, back in theearly 1980s, some states passed laws requir-ing that ‘creation science’ be taught when-ever evolution was taught But theSupreme Court threw out ‘equal time’ laws,

saying that because creationism was ently a religious and not a scientific idea, itcouldn’t be presented as ‘truth’ in scienceclasses in the public schools.”5

inher-“Well, I’m willing to teach evolution,”

says Karen, “and I’d like to try it your way,Barbara, as a theme that ties biology togeth-

er But I really don’t know enough aboutevolution to do it Do you have any sugges-tions about where I can get information?”

“Sure, I’d be glad to share what I have

But an important part of teaching evolutionhas to do with explaining the nature of sci-ence I’m trying out a demonstration afterschool today that I’m going to use with myBio I class tomorrow Why don’t you bothcome by and we can try it out?”

“Okay,” say Karen and Doug “We’ll seeyou then.”

Barbara, Doug, and Karen’s discussion

of evolution and the nature of scienceresumes following Chapter 2

1 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)

2 Appendix C contains statements from science and science education organizations that support the need to teach evolution.

3 In 1995, the Alabama board of education ordered that all biology textbooks in public schools carry inserts that read, in part, as follows: “This text- book discusses evolution, a controversial theory some scientists present as a scientific explanation for the origin of living things, such as plants, ani- mals, 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.

4 The book From So Simple a Beginning: The Book

of Evolutionby Philip Whitfield (New York:

Macmillan, 1993) presents a well-illustrated

overview of evolutionary history Evolution by

Monroe W Strickberger (Boston: Jones and Bartlett, 2nd edition, 1995) is a thorough text writ- ten at the undergraduate level.

5 In the 1987 case Edwards v Aguillard, the U.S.

Supreme Court reaffirmed the 1982 decision of a federal district court that the teaching of “creation science” in public schools violates the First Amendment of the U.S Constitution.

The world around us

changes This ple fact is obviouseverywhere we look

sim-Streams wash dirt andstones from higher places to lower places

Untended gardens fill with weeds

Other changes are more gradual butmuch more dramatic when viewed overlong time scales Powerful telescopesreveal new stars coalescing from galacticdust, just as our sun did more than 4.5 bil-lion years ago The earth itself formedshortly thereafter, when rock, dust, and gascircling the sun condensed into the planets

of our solar system Fossils of primitivemicroorganisms show that life had emerged

on earth by about 3.8 billion years ago

Similarly, the fossil record reveals found changes in the kinds of living thingsthat have inhabited our planet over its longhistory Trilobites that populated the seashundreds of millions of years ago no longercrawl about Mammals now live in a worldthat was once dominated by reptilian giants

pro-such as Tyrannosaurus rex More than 99

percent of the species that have ever lived

on the earth are now extinct, either becauseall of the members of the species died, thespecies evolved into a new species, or itsplit into two or more new species

Many kinds of cumulative changethrough time have been described by theterm “evolution,” and the term is used inastronomy, geology, biology, anthropology,and other sciences This document focuses

on the changes in living things during thelong history of life on earth—on what is

called biological evolution.The ancient Greeks werealready speculating aboutthe origins of life andchanges in species overtime More than 2,500 years ago, theGreek philosopher Anaximander thoughtthat a gradual evolution had created theworld’s organic coherence from a formlesscondition, and he had a fairly modern view

of the transformation of aquatic species intoterrestrial ones Following the rise ofChristianity, Westerners generally acceptedthe explanation provided in Genesis, thefirst book of the Judeo-Christian-MuslimBible, that God created everything in itspresent form over the course of six days.However, other explanations existed eventhen Among Christian theologians, forexample, Saint Thomas Aquinas (1225 to1274) stated that the earth had received thepower to produce organisms and criticizedthe idea that species had originated inaccordance with the timetables in Genesis.1

During the early 1800s, many naturalistsspeculated about changes in organisms,especially as geological investigationsrevealed the rich story laid out in the fos-silized remains of extinct creatures Butalthough ideas about evolution were pro-posed, they never gained wide acceptancebecause no one was able to propose a plau-sible mechanism for how the form of anorganism might change from one genera-tion to another Then, in 1858, two Englishnaturalists—Charles Darwin and AlfredRussel Wallace—simultaneously issuedpapers proposing such a mechanism Both

Major Themes in

Evolution

2

The Hubble Space Telescope has revealed many astronomical phenomena that ground-based telescopes cannot see.

The images at right show disks of matter around young stars that could give rise to planets In the image below, stars are forming in the tendrils of gas and dust extending from a gigantic nebula.

men observed that the individual members

of a particular species are not identical butcan differ in many ways For example,some will be able to run a little faster, have

a different color, or respond to the same cumstance in different ways (Humans—

cir-including any class of high school dents—have many such differences.) Bothmen further observed that many of thesedifferences are inherited and can be passed

stu-on to offspring This cstu-onclusistu-on was dent from the experiences of plant and ani-mal breeders

evi-Darwin and Wallace were both deeplyinfluenced by the realization that, eventhough most species produce an abundance

of offspring, the size of the overall populationusually remains about the same Thus, anoak tree might produce many thousands ofacorns each year, but few, if any, will survive

to become full-grown trees

Darwin—who conceived of his ideas

in the 1830s but did not publish them untilWallace came to similar conclusions—

presented the case for evolution in detail

in his 1859 book On the Origin of Species

by Natural Selection Darwin proposed

that there will be differences between spring that survive and reproduce and thosethat do not In particular, individuals thathave heritable characteristics making themmore likely to survive and reproduce intheir particular environment will, on aver-age, have a better chance of passing thosecharacteristics on to their own offspring Inthis way, as many generations pass, naturewould select those individuals best suited toparticular environments, a process Darwincalled natural selection Over very longtimes, Darwin argued, natural selection act-ing on varying individuals within a popula-tion of organisms could account for all ofthe great variety of organisms we see today,

off-as well off-as for the species found off-as fossils

If the central requirement of naturalselection is variation within populations,what is the ultimate source of this variation?

This problem plagued Darwin, and he never

From left, Charles Darwin (1809-1882), Alfred Russel Wallace (1823-1913), and Gregor Mendel (1822- 1884) laid the founda- tions of modern evolu- tionary theory.

Glossary of Terms Used in Teaching About Evolution

Evolution: Change in the hereditary

character-istics of groups of organisms over the course ofgenerations (Darwin referred to this process as

“descent with modification.”)

Species: In general, a group of organisms that

can potentially breed with each other to duce fertile offspring and cannot breed with the members of other such groups

pro-Variation: Genetically determined differences

in the characteristics of members of the samespecies

Natural selection: Greater reproductive success

among particular members of a species arisingfrom genetically determined characteristics thatconfer an advantage in a particular environment

found the answer, although he proposedsome hypotheses Darwin did not know that

a contemporary, Gregor Mendel, had

provid-ed an important part of the solution In hisclassic 1865 paper describing crossbreeding

of varieties of peas, Mendel demonstratedthat organisms acquire traits through dis-crete units of heredity which later came to

be known as genes The variation producedthrough these inherited traits is the rawmaterial on which natural selection acts

Mendel’s paper was all but forgottenuntil 1890, when it was rediscovered andcontributed to a growing wave of interestand research in genetics But it was notimmediately clear how to reconcile newfindings about the mechanisms of inheri-tance with evolution through natural selec-tion Then, in the 1930s, a group of biolo-gists demonstrated how the results ofgenetics research could both buttress andextend evolutionary theory They showedthat all variations, both slight and dramatic,arose through changes, or mutations, ingenes If a mutation enabled an organism

to survive or reproduce more effectively,that mutation would tend to be preservedand spread in a population through naturalselection Evolution was thus seen todepend both on genetic mutations and onnatural selection Mutations providedabundant genetic variation, and naturalselection sorted out the useful changesfrom the deleterious ones

Selection by natural processes offavored variants explained many observa-tions on the geography of species differ-ences—why, for example, members of thesame bird species might be larger anddarker in the northern part of their range,and smaller and paler in the southern part

In this case, differences might be explained

by the advantages of large size and darkcoloration in forested, cold regions And, ifthe species occupied the entire range con-tinuously, genes favoring light color andsmall size would be able to flow into thenorthern population, and vice versa—pro-hibiting their separation into distinct

species that are reproductively isolatedfrom one another

How new species are formed was a tery that eluded biologists until informationabout genetics and the geographical distrib-ution of animals and plants could be puttogether As a result, it became clear thatthe most important source of new species isthe process of geographical isolation—through which barriers to gene flow can becreated In the earlier example, the inter-position of a major mountain barrier, or theorigin of an intermediate desert, might cre-ate the needed isolation

mys-Other situations also encourage the mation of new species Consider fish in ariver that, over time, changes course so as

for-to isolate a tributary Or think of a set ofoceanic islands, distant from the mainlandand just far enough from one another thatinterchange among their populations is rare.These are ideal circumstances for creatingreproductive barriers and allowing popula-tions of the same species to diverge fromone another under the influence of naturalselection After a time, the species becomesufficiently different that even when reunit-

ed they remain reproductively isolated.They have become so different that they areunable to interbreed

In the 1950s, the study of evolutionentered a new phase Biologists began to

be able to determine the exact molecularstructure of the proteins in living things—that is, the actual sequences of the aminoacids that make up each protein Almostimmediately, it became clear that certainproteins that serve the same function in dif-ferent species have very similar amino acidsequences The protein evidence was com-pletely consistent with the idea of a com-mon evolutionary history for the planet’s liv-ing things Even more important, thisknowledge provided important clues aboutthe history of evolution that could not beobtained through the fossil record

The discovery of the structure of DNA

by Francis Crick and James Watson in 1953extended the study of evolution to the most

fundamental level The sequence of thechemical bases in DNA both specifies theorder of amino acids in proteins and deter-mines which proteins are synthesized inwhich cells In this way, DNA is the ulti-mate source of both change and continuity

in evolution The modification of DNAthrough occasional changes or rearrange-ments in the base sequences underlies theemergence of new traits, and thus of newspecies, in evolution At the same time, allorganisms use the same molecular codes totranslate DNA base sequences into proteinamino acid sequences This uniformity inthe genetic code is powerful evidence for

the interrelatedness of living things, gesting that all organisms presently aliveshare a common ancestor that can be tracedback to the origins of life on earth

sug-One common misconception among dents is that individual organisms changetheir characteristics in response to the envi-ronment In other words, students oftenthink that the environment acts on individ-ual organisms to generate physical charac-teristics that can then be passed on geneti-cally to offspring But selection can workonly on the genetic variation that already ispresent in any new generation, and geneticvariation occurs randomly, not in response

stu-Discovery of a Missing Link

As a zoologist I have discovered many nomena that can be rationally explained only as products of evolution, but none so striking as the ancestor of the ants Prior to

phe-1967 the fossil record had yielded no

speci-mens of wasps or other Hymenopterous

insects that might be interpreted as the ancestors of the ants This hypothetical form was a missing link of major impor- tance in the study of evolution We did have many fossils of ants dating back 50 million years These were different species from those existing today, but their bodies still possessed the basic body form of mod- ern ants The missing link of ant evolution was often cited by creationists as evidence against evolution Other ant specialists and

I were convinced that the linking fossils would be found, and that most likely they would be associated with the late Mesozoic era, a time when many dinosaur and other vertebrate bones were fossilized but few insects And that is exactly what happened.

In 1967 I had the pleasure of studying two specimens collected in amber (fossilized resin) from New Jersey, and dating to the late Mesozoic about 90 million years ago.

They were nearly exact intermediates between solitary wasps and the highly

social modern ants, and so I gave them the

scientific name Sphecomyrma, meaning

“wasp ant.” Since that time many more

Sphecomyrma specimens of similar age have

been found in the United States, Canada, and Siberia, but none belonging to the modern type With each passing year, such fossils and other kinds of evidence tighten our conception of the evolutionary origin of this important group of insects.

—Edward O Wilson

to the needs of a population or organism.

In this sense, as Francois Jacob has written,evolution is a “tinkerer, not an engineer.”2

Evolution does not design new organisms;

rather, new organisms emerge from theinherent genetic variation that occurs inorganisms

Genetic variation is random, but naturalselection is not Natural selection tests thecombinations of genes represented in themembers of a species and allows to proliferatethose that confer the greatest ability to surviveand reproduce In this sense, evolution is notthe simple product of random chance

The booklet Science and Creationism: A View from the National Academy of

Sciences3summarizes several compellinglines of evidence that demonstrate beyondany reasonable doubt that evolutionoccurred as a historical process and contin-ues today In brief:

• Fossils found in rocks of increasingage attest to the interrelated lineage of liv-ing things, from the single-celled organisms

that lived billions of years ago to Homo sapiens The most recent fossils closely

resemble the organisms alive today, whereasincreasingly older fossils are progressivelydifferent, providing compelling evidence ofchange through time

• Even a casual look at different kinds oforganisms reveals striking similaritiesamong species, and anatomists have discov-ered that these similarities are more thanskin deep All vertebrates, for example,from fish to humans, have a common bodyplan characterized by a segmented bodyand a hollow main nerve cord along theback The best available scientific explana-tion for these common structures is that allvertebrates are descended from a commonancestor species and that they havediverged through evolution

• In the past, evolutionary relationshipscould be studied only by examining the con-sequences of genetic information, such as

the anatomy, physiology, and embryology ofliving organisms But the advent of molec-ular biology has made it possible to read thehistory of evolution that is written in everyorganism’s DNA This information hasallowed organisms to be placed into a com-mon evolutionary family tree in a muchmore detailed way than possible from previ-ous evidence For example, as described inChapter 3, comparisons of the differences

in DNA sequences among organisms vides evidence for many evolutionary eventsthat cannot be found in the fossil record.Evolution is the only plausible scientificexplanation that accounts for the extensivearray of observations summarized above.The concept of evolution through randomgenetic variation and natural selectionmakes sense of what would otherwise be ahuge body of unconnected observations It

pro-is no longer possible to sustain scientificallythe view that the living things we see todaydid not evolve from earlier forms or that thehuman species was not produced by thesame evolutionary mechanisms that apply tothe rest of the living world

The following two sections of this ter examine two important themes in evolu-tionary theory The first concerns theoccurrence of evolution in “real time”—how changes come about and result in newkinds of species The second is the ecologi-cal framework that underlies evolution,which is needed to understand the expan-sion of biological diversity

chap-Evolution as a Contemporary Process

Evolution by natural selection is notonly a historical process—it still operatestoday For example, the continual evolution

of human pathogens has come to pose one

of the most serious public health problemsnow facing human societies Many strains

of bacteria have become increasingly tant to once-effective antibiotics as naturalselection has amplified resistant strains thatarose through naturally occurring geneticvariation The microorganisms that causemalaria, gonorrhea, tuberculosis, and manyother diseases have demonstrated greatlyincreased resistance to the antibiotics andother drugs used to treat them in the past

resis-The continued use and overuse of otics has had the effect of selecting forresistant populations because the antibioticsgive these strains an advantage over nonre-sistant strains.4

antibi-Similar episodes of rapid evolution areoccurring in many different organisms

Rats have developed resistance to the poisonwarfarin Many hundreds of insect speciesand other agricultural pests have evolvedresistance to the pesticides used to combatthem—and even to chemical defenses genet-ically engineered into plants Species ofplants have evolved tolerance to toxic metalsand have reduced their interbreeding withnearby nontolerant plants—an initial step

in the formation of separate species Newspecies of plants have arisen through thecrossbreeding of native plants with plantsintroduced from elsewhere in the world

The creation of a new species from apre-existing species generally requires

Deciduous Woodland

Coniferous Woodland

Jan Feb Mar Apr May June

Breeding Periods

July Aug Sept Oct Nov Dec.

Chrysoperla carnea Chrysoperla downesi

The North American lacewing

species Chrysoperla carnea and

Chrysoperla downesi separated

from a common ancestor species recently in evolutionary time and are very similar But they are different in color, reflecting their different habitats, and they breed

at different times of the year.

thousands of years, so over a lifetime a gle human usually can witness only a tinypart of the speciation process Yet eventhat glimpse of evolution at work powerfullyconfirms our ideas about the history andmechanisms of evolution For example,many closely related species have beenidentified that split from a common ances-tor very recently in evolutionary terms Anexample is provided by the North American

sin-lacewings Chrysoperla carnea and Chrysoperla downesi The former lives in

deciduous woodlands and is pale green insummer and brown in winter The latterlives among evergreen conifers and is darkgreen all year round The two species aregenetically and morphologically very similar.Yet they occupy different habitats andbreed at different times of the year and soare reproductively isolated from each other.The fossil record also sheds light on spe-ciation A particularly dramatic examplecomes from recently discovered fossil evi-dence documenting the evolution of whalesand dolphins The fossil record shows thatthese cetaceans evolved from a primitivegroup of hoofed mammals called

Mesonychids Some of these mammals

crushed and ate turtles, as evidenced by theshape of their teeth This mammal gaverise to a species with front forelimbs andpowerful hind legs with large feet that wereadapted for paddling This animal, known

as Ambulocetus, could have moved between

sea and land Its fossilized vertebrae alsoshow that this animal could move its back

in a strong up and down motion, which isthe method modern cetaceans use to swimand dive A later fossil in the series fromPakistan shows an animal with smaller functional hind limbs and even greater back

flexibility This species, Rodhocetus,

proba-bly did not venture onto land very often, if

at all Finally, Basilosaurus fossils from

Egypt and the United States present a ognizable whale, with front flippers forsteering and a completely flexible back-bone But this animal still has hind limbs(thought to have been nonfunctional),

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

Mesonychid

Ambulocetus

Rodhocetus

Basilosaurus

which have become further reduced inmodern whales.5

Another focus of research has been the evolution of ancient apelike creaturesthrough many intermediate forms into

modern humans Homo sapiens, one of 185

known living species in the primate order, is

a member of the hominoids, a category thatincludes orangutans, gorillas, and chim-panzees The succession of species thatwould give rise to humans seems to haveseparated from the succession that wouldlead to the apes about 5 to 8 million yearsago The first members of our genus,

Homo, had evolved by about 1.5 million

years ago According to recent evidence—

based on the sequencing of DNA found in

a part of human cells known as dria—it has been proposed that a smallgroup of modern humans evolved in Africaabout 150,000 years ago and spreadthroughout the world, replacing archaic

mitochon-populations of Homo sapiens.

Evolution and Ecology

Animals and plants do not live in tion, nor do they evolve in isolation Indeed,much of the pressure toward diversificationcomes not only from physical factors in theenvironment but from the presence of otherspecies Any animal is a potential host forparasites or prey for a carnivore A plant hasother plants as competitors for space andlight, can be a host for parasites, and pro-vides food for herbivores The interactionswithin the complex communities, or ecosys-tems, in which organisms live can generatepowerful evolutionary forces

isola-Evolution in natural communities arisesfrom both constraints and opportunities

The constraints come from competitors,primarily among the same species Thereare only so many nest holes for bluebirdsand so much food for mice Genetically dif-ferent individuals that are able to move to adifferent resource—a new food supply, forexample, or a hitherto uninhabited area—

Ongoing Evolution Among Darwin’s Finches

A particularly interesting example of temporary evolution involves the 13 species

con-of finches studied by Darwin on the Galapagos Islands, now known as Darwin’s finches A research group led by Peter and Rosemary Grant of Princeton University has shown that a single year of drought on the islands can drive evolutionary changes in the finches 6 Drought diminishes supplies

of easily cracked nuts but permits the vival of plants that produce larger, tougher nuts Drought thus favors birds with strong, wide beaks that can break these tougher seeds, producing populations of birds with these traits The Grants have estimated that if droughts occur about

sur-once every 10 years on the islands, a new species of finch might arise in only about

200 years 7

are able to exploit that resource free ofcompetition As a result, the trait thatopened up the new opportunity will befavored by natural selection because theindividuals possessing it are able to surviveand reproduce better than other members

of their species in the new environment

An ecologist would say that the varianthad occupied a new niche—a term thatdefines the “job description” of an organism.(For example, a bluebird would have theniche of insect- and fruit-eater, inhabitant offorest edges and meadows, tree-hole nester,and so on.) One often finds closely relatedspecies in the same place and occupying whatlook like identical niches However, if theniches were truly identical, one of the speciesshould have a competitive advantage over theother and eventually drive the less fit species

to extinction or to a different niche Thatleads to a tentative hypothesis: where wefind such a situation, careful observationshould reveal subtle niche specialization ofthe apparently competing species

This hypothesis has been tested by manybiologists For example, in the 1960sRobert MacArthur carefully studied threeNorth American warblers of the same genusthat were regularly seen feeding on insects

in coniferous trees in the same areas—indeed, often in the same trees MacArthur’spainstaking observations revealed that thethree were actually specialists: one fed oninsects on the major branches near thetrunk; another occupied the mid-regions ofbranches and ate from different parts of thefoliage; and the third fed on insects occupy-ing the finest needles near the periphery ofthe tree Although the three warblersoccurred together, they were in fact notcompetitors for the same food resources.Often, species that are evolving together

in the same ecosystem do so through ahighly interactive process For example,natural selection will favor organisms withdefenses against predation; in turn, preda-tors experience selection for traits that over-come those defenses Such coevolutionarycompetitions are common in nature Many

Early hominids had

smaller brains and

larger faces than

White parts of the

skulls are

recon-structions, and the

skulls are not all on

the same scale.

plants manufacture and store chemicals thatdeter herbivorous insects; but usually one

or more insect species will have evolvedbiochemical mechanisms for inactivatingthe deterrent, providing them with a plantthey can eat relatively free of competitors

Another classic example of coevolutioninvolves the introduction of rabbits and themyxomatosis virus into Australia After rab-bits were brought to Australia, they multi-plied rapidly and threatened the wool indus-try because they grazed on the same plants

as sheep To control the rabbit population, avirulent pathogen of rabbits, the myxomato-sis virus, also was introduced into Australia

Within a decade, rabbits had become moreresistant to the virus, and the virus hadevolved into a less virulent form, allowingboth the host and pathogen to coexist.9

Conclusion

As the examples in this chapter strate, evolutionary biology provides anextremely active and rich source of newinsights into the world By exploring thehistory of life on earth and shedding light

demon-on how evolutidemon-on works, evolutidemon-onary

biolo-gy is linking fundamental scientific research

to knowledge needed to meet importantsocietal needs, including the preservation

of our environment Few other ideas in science have had such a far-reaching impact

on our thinking about ourselves and how werelate to the world

1 Biological Sciences Curriculum Study 1978.

Biology Teachers’ Handbook 3rd ed William V.

Mayer, ed New York: John Wiley and Sons.

2 Francois Jacob June 10, 1977 Evolution and

tinkering Science 196:1161-1166.

3 National Academy of Sciences (in press) Science

and Creationism: A View from the National Academy of Sciences Washington, DC: National

Academy Press (See www.nap.edu)

4 P Ewald 1994 The Evolution of Infectious

Disease New York: Oxford University Press.

5 “Evolution, Science, and Society: A White Paper

on Behalf of the Field of Evolutionary Biology,”

Draft, June 4, 1997.

6 Jonathan Weiner 1994 The Beak of the Finch.

New York: Alfred A Knopf.

7 Peter R Grant 1991 Natural selection and

Darwin’s finches Scientific American, October,

pp 82-87.

8 James H Tumlinson, W Joe Lewis, and Louise E.

M Vet 1993 How parasitic wasps find their

hosts Scientific American, March, pp 100-106.

9 F Fenner and F.N Ratcliffe 1965 Myxomatosis.

Cambridge: Cambridge University Press

A Chemical Distress Signal

J H Tumlinson and colleagues at the U.S.

Department of Agriculture’s Research Service Laboratories in Gainesville, Florida, have explored a fascinating case that illustrates the intricacy of many ecological relation- ships Cotton plants, like many other crops, are attacked by caterpillars One destructive cotton pest, the army worm, produces a com- plex series of reactions when it feeds on the plant—a reaction that involves the caterpillar itself, the tissues of the plant, and a third participant, a wasp that preys on the cater- pillar When the caterpillar chews on the cotton plant leaf, a reaction occurs that caus-

es the plant to synthesize and release a class

of volatile chemicals that escape into the air and travel rapidly downwind The chemicals are detected by wasps, who follow the scent

and are able to find the caterpillars and deposit eggs within them The eggs hatch, and the wasp larvae destroy the caterpillar 8

This complex case of “chemical ecology”

required a series of linked coevolutionary events: the response of the plant to a special signal from its predator, and the response of the wasp to a special signal from the host of its prey.

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 noon,” Barbara says “To begin thisdemonstration I first need to ask you whatyou think science is.”

after-“Oh, I had that in college,” says Karen

“The scientific method is to identify a tion, gather information about it, develop ahypothesis that answers the question, andthen do an experiment that either proves ordisproves the hypothesis.”

ques-“But that was one of my points aboutevolution,” Doug says “No one was therewhen evolution happened and we can’t doany experiments about what happened inthe past So by your definition, Karen, evo-lution isn’t science.”

“Science is a lot more than just ing or rejecting hypotheses,” Barbarareplies “It also involves observation, cre-ativity, and judgment Here’s an activity

support-I use to teach the nature of science.”

Barbara takes a cardboard mailing tubeabout one foot long that has the ends offour ropes extending from it

As Barbara tugs on the various ropesone at a time, she has Doug and Karenmake observations of what happens Afterthree or four pulls, she asks Karen andDoug to predict what will happen whenshe pulls on one of the ropes Both are

able to predict that if Barbara pulls rope A,rope B will move Barbara then asks ifthere are additional manipulations theywould like to see, and she follows theirrequests

Barbara then asks Doug and Karen tosketch a model of what is inside the tubethat could explain their observations.When Karen and Doug show theirsketches to each other, they realize thatthey have come up with different models.Barbara asks them if they want to make anychanges to their sketches based on thecomparison, and both of them make modi-fications, although their final models arestill different

Barbara then gives them their owncardboard tubes and some string and asksthem to build the model they proposed.When their models are built, Barbara holds

up her tube and asks Doug and Karen tofollow her actions with their own models,

to see if the two models behave in thesame way as Barbara’s tube But whenBarbara pulls string A in her tube, Karen’smodel does not work the same way Karenasks if she can make some changes in hermodel, and once she does her new modelseems to work the same way as Barbara’s.Doug’s model consistently behaves thesame way as Barbara’s tube

“Now wait a minute,” Karen says

“What do ropes and tubes have to do withscience and evolution?”

“You might not know it, but what wejust did is much of what science is about.You observed what happened when

D i a l o g u e

T EACHING A BOUT THE N ATURE

OF S CIENCE

I pulled these ropes Then, based on yourinitial observations, you made a predictionabout what would happen if we manipulat-

ed the system in a specific way How rate was your prediction?”

accu-“We were right,” Doug responds

“And why were you able to predict whatwould happen before I pulled the rope?”

“I used what I observed in the first fewpulls to help me predict what would hap-pen later.”

“Basically what each of you did was tospeculate about how my tube was working

on the basis of some limited observations

Scientists do that type of thing all the time

They make observations and try to explainwhat’s going on, or sometimes they recog-nize that more than one explanation fitstheir data Then they try out their pro-posed explanations by making predictionsthat they test At first I had you draw apicture of how you thought my tubeworked and had you each explain your pic-ture You got to hear each other’s view onhow the system worked Doug, did youchange your ideas at all based on what youheard from Karen?”

“Well, yes I first thought that ropes Aand C were the two ends of the same ropeand B and D were two ends of anotherrope Karen had A and B as ends of thesame rope and C and D as ends of anotherrope, and her explanation seemed to fitbetter than mine.”

“Right Communication about tions and interpretations is very importantamong scientists because different scien-tists may interpret data in different ways

observa-Hearing someone else’s views can help ascientist revise his or her interpretation

In essence that was what you were doingwhen you shared your diagrams Karen,when your model didn’t work, what didyou do?”

“All I did was adjust the length of onerope, and then it worked fine.”

“So as a result of your formal testing ofthe predictions from your model, yourevised your explanation of the system

Your understanding improved In scientificterms, you revised your model to make itmore consistent with your further observa-tions In science, the validity of any expla-nation is determined by its coherence withobservations in the natural world and by itsability to predict further observations.”

“But we still have different models,”

Karen observes “How do we know whichone is right?”

Doug says: “You told us that, didn’tyou, Barbara There can be two possibleexplanations for the same observation.”

“So it’s possible for scientists to disagreesometimes,” says Karen “But does thatmean that we don’t understand evolutionbecause scientists disagree about how evo-lution takes place?”

“Not at all,” Barbara answers, “you bothcreated different models of my tube, butboth of your models are fairly accurate

And don’t forget there were constraints on

B A

B A

Doug’s initial model

Karen’s initial model

the possible models you could create thatwould be consistent with the data Just anyexplanation would not be acceptable Inevolution, there are some things we knowcould not have happened, just as we areconfident that some things have happened.”

“And if different scientists can have ferent explanations, like Karen and I did,then I guess science also has to involvejudgment to some extent,” Doug says

dif-“But I thought scientists were supposed

to be totally objective,” says Karen

“Good science always attempts to beobjective, but it also relies on the individualinsights of scientists And the questionsthey choose to ask as well as the methodsthey choose to use, not to mention theinterpretations they may have, can be col-ored by their individual interests and back-grounds But scientific explanations arereviewed by other scientists and must beconsistent with the natural world andfuture experiments, so there are checks onsubjectivity What we read in sciencebooks is a combination of observations andinferred explanations of those observationsthat can change with new research.”

“Still, I’m wondering,” says Karen, “howcan we find out which model is right?”

“Let’s just open up Barbara’s tube,” saysDoug

“We could do that,” Barbara says “Butlet’s assume in this analogy that opening thetube is not possible Sometimes scientistsfigure out how to open up the natural worldand look inside, but sometimes they can’t

And not opening up the tube is a goodmetaphor for how science often works

Science involves coming up with tions that are based on evidence With time,additional evidence might require changingthe explanations, so that at any time what wehave is the best explanation possible for howthings work In the future, with additionaldata, we may change our original explana-tion—just like you did, Karen

explana-“Remember when we were talking thismorning about evolution being fact or the-ory? That conversation is very relevant towhat we have been doing with the tubes

As scientists started to notice patterns innature, they began to speculate about someexplanations for these patterns Theseexplanations are analogous to your initialideas about how my tube worked In theterms of science, these initial ideas arecalled hypotheses You noticed some pat-terns in how the ropes were related to eachother, and you used these patterns todevelop a model to explain the patterns.The model you created is analogous to thebeginning of a scientific theory Except inscience, theories are only formalized aftermany years of testing the predictions thatcome from the model

“Because of our human limitations incollecting complete data, theories necessar-ily contain some judgments about what isimportant Judgments aren’t a weakness ofscientific theory They are a basic part ofhow science works.”

“I always thought of science as a bunch

of absolute facts,” says Doug “I neverthought about how knowledge is developed

by scientists.”

“Creativity and insight are what helpmake science such a powerful way ofunderstanding the natural world

“There’s another important thing that Itry to teach my students with this activity,”Barbara continues “It’s important forthem to be able to distinguish questionsthat can be answered by science from thosethat cannot be answered by science Here’s

a list of questions that I use to get themtalking I ask them if a question can beanswered by science, cannot be answered

by science, or has some parts that belong toscience and others that do not Then I askthe group to select a couple of questionsand discuss how they would go aboutanswering them.”

Barbara hands Doug and Karen the lowing list of questions:

fol-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 otherquestions if something is happening in thenews or if it’s related to an earlier lesson

And sometimes I include moral or religiousquestions to make it clear that they lie out-side science.”

“I can see that these would get studentsthinking,” says Karen “I guess understand-ing the nature of science really is relevant

to real life.”

“That’s what this exercise is about.”

Science is a particular

way of knowing aboutthe world In science,explanations are restricted tothose that can be inferredfrom confirmable data—theresults obtained through observations andexperiments that can be substantiated byother scientists Anything that can beobserved or measured is amenable to sci-entific investigation Explanations that can-not be based on empirical evidence are not

a part of science

The history of life on earth is a ing subject that can be studied throughobservations made today, and these obser-vations have led to compelling accounts ofhow organisms have changed over time

fascinat-The best available evidence suggests thatlife on earth began more than three and ahalf billion years ago For more than twobillion years after that, life was housed inthe bodies of many kinds of tiny, single-celled organisms, some of which producedthe oxygen that now makes up more than afifth of the earth’s atmosphere Less than abillion years ago, much more complexorganisms appeared By about half a billionyears ago, evolution had resulted in a widevariety of multicellular animals and plantsliving in the sea that are the clear ancestors

of many of the major types of organismsthat continue to live to this day Somewhatmore than 400 million years ago, somemarine plants and animals began one of thegreatest of all innovations in evolution—

they invaded dry land For our own lum, the Chordata, this move away from the

phy-nurturing sea led to theappearance of amphibians,reptiles, birds, and mam-mals—the latter including,

of course, our own species,

Homo sapiens.

This chapter looks at how science works

in the context of our overall understanding

of how biological evolution occurred Itbegins, however, by discussing another scientific development that challengedlong-held understandings and beliefs: thediscovery of heliocentricism

Heliocentricism and the Nature

sur-Archaeological evidence and earlyrecords make it clear that our ancestors real-ized that not only does the sun appear torise and set, but so do the moon and stars.The movements of the moon and stars, how-ever, are not precisely synchronized with

Evolution and the Nature of Science

3

Clockwise from top left, Nicolaus Copernicus (1473- 1543), Johannes Kepler (1571-1630), Galileo Galilei (1564-1642), and Isaac Newton (1642-1727) led the way to a new understanding

of the relationship between the earth and the sun and initiated an age of scientific progress that continues today.

Illustration from the 18th century depicts the Ptolemaic system in the

upper left corner and the Copernican system in other corners and

center.

those of the sun The moon is slower byabout one hour per day The stars remainalmost the same on successive nights, butslowly it becomes obvious that they, too, areslowed in their movements compared to thesun Thus, the stars of summer are differ-ent from those visible in the winter In fact,

it takes a full year for the stars to return totheir previous position, an interval of timethat defines our year

The ancient observers realized that notall stars move in unison Although mostmove in majestic unity, a few others are

“wanderers”—appearing now with onegroup of stars and a week later somewhereelse The majority were called “fixed stars,”

the wanderers were called “planets.”

During the late Middle Ages, and cially in the Renaissance, beautiful brassmodels known as orreries were made toshow the relative positions and movements

espe-of the sun, planets, and moon as they cled the earth As the center of the uni-verse, the earth was a sphere in the center

cir-of the orrery The other celestial bodieswere positioned on rings of metal, eachmoving by clockwork at its own rate Thefixed stars required a simple solution—theycould be considered stuck in an outermostshell, also moved by clockwork

The problem with orreries—and withthe theories of the cosmos then prevailing—

was that they had to become successivelymore complex as more became known

Careful observations of the movements ofthe stars and planets greatly complicated thehypotheses used to account for those move-ments This growing complexity stimulatedsome of the leading astronomers of the 16thand 17th centuries, including Copernicus,Kepler, and Galileo, to make even more pre-cise observations of the movements of theheavenly bodies Astronomers used thesemeasurements to demonstrate that the age-old human explanations of the heavens wereincomplete In the process, they replaced acomplex and confusing explanation with asimple one: the sun, rather than the earth,

is at the center of a “solar system,” and the

earth revolves around it That simplestep—a bold departure from past thinkingdue mainly to the insights of Copernicus(1473 to 1543)—dramatically changed thepicture of the then known universe

This concept of heliocentricism initiallyran counter to the positions of religiousauthorities The view of Christianity overmost of its history, based on a literal inter-pretation of the Bible, was that the earth isthe center of the universe around which the celestial bodies revolve Copernicus dedicated his book describing the theory of

heliocentricism, De revolutionibus orbium coelestium, to Pope Paul III and promptly

died That saved him the troubles thatwere to beset Galileo (1564 to 1642), whoseastronomical observations confirmed theviews of Copernicus Galileo was told toabandon his beliefs, and he later was tried

by the Inquisition and sentenced to theequivalent of house arrest The Churchheld that his views were dangerous to faith

Continued study and ever more carefulmeasurements of the movements of theplanets and sun continued to support theheliocentric hypothesis Then, in the latterhalf of the 17th century, Isaac Newton(1642 to 1727) showed that the force ofgravity—as measured on the earth—couldaccount for the movements of the planetsgiven the laws of motion that Newtonderived As a result of the steady accumu-lation of evidence, the theological interpre-tation of celestial movements gave way tothe naturalistic explanation, and it is nowaccepted that night and day are the conse-quences of the rotation of the earth on itsaxis Today, we can see for ourselves therotation of the earth from satellites orbitingthe planet

Like biological evolution, the theory ofheliocentricism brought order and newunderstanding to an otherwise chaotic andconfusing aspect of nature It also had greatpractical applications, in that the exploration

of the world by European seafarers used themore accurate understanding of celestialmechanics to assist in navigation

Looking at the night sky remains a erful experience But that experience isnow informed not only by the beauty andmajesty of the heavens, but by a deeperunderstanding of nature and by an appreci-ation of the power of the human intellect.

pow-This triumph of the human mind says agreat deal about the nature of science

First, science is not the same as commonsense Common sense indicates that thesun does rise and set Nevertheless, therecan be other explanations of that phenome-non, and one of them, the rotation of theearth on its axis, is responsible for day andnight A concept based on observationproved to need extensive modification asnew observations accumulated

Second, the statements of scienceshould never be accepted as “final truth.”

Instead, over time they generally form asequence of increasingly more accuratestatements Nevertheless, in the case ofheliocentricism as in evolution, the data are so convincing that the accuracy of thetheory is no longer questioned in science

Third, scientific progress depends onindividuals, but the contributions of oneindividual could be made by others IfCopernicus had kept his ideas to himself,the discovery of heliocentricism would havebeen postponed, but it would not have beenblocked, since other astronomers eventuallywould have come to the same conclusion

Similarly, had Darwin and Wallace not lished their hypotheses, the concept of bio-logical evolution would nevertheless haveemerged as the accepted explanation for thehistory of life on earth The same cannot besaid in other areas of human endeavor; forexample, had Shakespeare never published,

pub-we would most assuredly never have had hisplays The publications of scientists, unlikethose of playwrights, are a means to anend—they are not the end itself

Science Requires Careful Description

What are the scientific methods thathave led to our current understanding ofthe history of life over vast eons of time?They begin with careful descriptions of thematerial being studied

The material for the study of biologicalevolution is life itself One basic aspect oflife is that individuals can be grouped assimilar kinds, or species Another impor-tant observation is that many species seem

to be closely related to each other The entific classification of species and theirarrangement into groups began with the

sci-publication in 1758 of Systema Naturae, or

system of nature, by the Swedish naturalistCarolus Linnaeus (1707 to 1778) Forexample, Linnaeus knew seven dog-likespecies, and he gave each a double name.Subsequently many more species were dis-covered and some of the names werechanged—and continue to be changed asmore information is obtained The domestic

dog is Canis familiaris; the coyote of North America is Canis latrans; the Australian dingo is Canis dingo; and the wolf of the northern hemisphere is Canis lupus Thus Canis is the name of the genus of dog-like

animals, and the distinctive second name isthe species name

Generations of scientists have ered new species, described them, and

discov-Biologists have used

construction cranes to

study the many newly

discovered species that

live in the canopies of

tropical forests, as in

this research project in

Panama.