Teaching Evolution & The Of Nature Science PAUL FARBER he famousgeneticistand co-founderof the modern theory of evolution, Theodosius Dobzhansky, publishedan articlein thisjournalin 1973 with the title "Nothingin BiologyMakesSense Exceptin the Lightof Evolution"(Dobzhansky, 1973). In that article he responded to religious attacks on the theory and defended its importance by noting how evolution explainsthe enormousdiversityof life,its unity,and the myriadempiricalfacts of biology. What does Dobzhansky's dictum mean today? Most biologists would say that Dobzhanskyhad it correct:The theory of evolution is the central organizing theoryof the life sciences.The theory explainsthe facts of biology,i.e., the theory tells us why the living world appearsas it does. It gives us the answerto a numberof interesting questions: Why do we observe so much diversityof life (750,000 namedinsects, 170,000 dicots, 12,000 nematodes,and 18,000 bony fish, for example)? PAUL FARBERis OregonState UniversityDistinguishedProfessorof History of Science and Chair of the History Department, Oregon State University,Corvallis, OR 97331. He holds a joint appointment with the Zoology Department. His e-mail address is pfarber@orst.edu. Giventhe vast amountof biodiversity,why are so many chemicalpathways(e.g., the KrebsCycle) the same in otherwise greatly different organisms?Why is DNA found so widely as the geneticmaterial?Evolutionprovides for us an understanding.It also addresses questions like, Why do organismshave highly specialized functions that permit them to live in hostile environments (hot springs), or in extraordinarilylimited environments(like the nematodePanagrellusredivivuswhich lives in Germanbeer coasters)?Biologyis a trulyamazing subject,and evolutionhelps explainwhy. The theory of evolution similarlyexplains biological relationships.Why do we observecomplex patterns of distribution among plants and animals?Why do some birdshave limitedrangeswhile others are cosmopolitan? The theory also relates bodies of scientific information. Subjects of study that utilize different methods, focus on differentorders of magnitude,conceive of naturein differenttime frames,or in different spatialcategoriesareunifiedby the theoryof evolution. Paleontology,biogeography,physiology, ecology, systematics,embryology,genetics, and cytologyare vastly differentdisciplines.Unlikethe physicalsciences,which arestill searchingfora plausibleunifyingtheory,the life sciences have a single unifyingtheory that synthesizes NATURE OFSCIENCE 347 them all and brings them into relationshipwith one another.Finally,the theoryof evolutionprovidesa powerfulguide to research.The questionsraisedby the theory havebeen extraordinarily productivein the past 140 years.Much of 20th centurygeneticsgrew out of scientists' attempts to reconcile Darwin's theory with the ideas on inheritancein his day.At presentthe theory of evolution provides directionin bringingknowledge in ecology to illuminatingproblems in paleontology;or knowledge in genetics to shedding light on issues in embryology,and suggestingnew paths of investigation. The Teaching of Evolution in the United States The theoryof evolution,then, is impressive,productive, and important.To become a serious biologist,one needs to havea graspof whatevolutionmeans,and to be an informedcitizen, one should have a generalunderstanding of what the theory claims. People in most of the modern,industrializedworldtakethatforgranted.It is terriblyironic,however,that the United States,where so much of the contemporarytheoryof evolutiondeveloped, has such an unsatisfactoryrecord of teachingit, both in K-12and in post-secondaryeducation. Consider the situation in K-12. The Thomas B. FordhamFoundationpublishedin January,2000, Good Science,Bad Science:TeachingEvolutionin the States (Lerner,2000). The reportis unsettling.It reviewsand evaluatesthe treatmentof evolutionby looking at state science standardsstate by state. Statestandardsdo not tell the full story,but by looking at them we can get a reasonablybroad stroke picture of the situation. The good news in the reportis that 31 of the states do an adequateto excellentjob; the bad news is that 19 states do a "weakto reprehensiblejob,"12 omit the word "evolution,"and fouromit teachingbiologicalevolutionaltogether. Since the report was issued there have been some changes, most notably, the Kansas Board of Educationhas reversedits scandalousdeletionof all references to biologicalevolutionin its standards.But the story is far from over, and the opposition to teaching evolution in the public schools continues to be a wellfinancedand powerfulforce. The author of the report, Professor Lawrence Lerner,explains that the opposition to the teachingof evolutiondoes not come fromreservationsin the scientific communityaboutwhetheror not evolutionoccurs. Yet,a significantnumberof Americansbelievethatboth Creationand evolution should be taught in the public schools, and the generalpublic is less than convincedin the validityof evolution.Some people worry about the moral effectsof teachingbiologicalevolution;and there is a diversecoalition of young Farthers,Biblicalliteralists, intelligentdesign proponents,new age spiritualists, MAY 2003 TEACHER, VOLUME 65,NO.5, 348 THE AMERICAN BIOLOGY and others,who add up to a potent forcethat can influence state boards of education,and, just as important, local school boards. What emergesfromthe Fordhamreportis a picture of a country that differsradicallyin its teachingof evolution from others in the modern industrializedworld. Thereis less of a commitmentfor instructionon evolution, and if Dobzhanskyis correct,manyof the K-12students in the United Statesarenot receivingan adequate preparationfor understandingthe life sciences. What about post-secondaryeducation?The problems differfrom K-12but carryimplicationsfor them. Colleges and universities,for the most part, teach evolution in biology courses. How they do it is another issue. I find two main problems(which also are reflected in most major and non-majorbiology textbooks) with the approachgenerallyused. The first problem with what I shall call the "standard treatmentof evolution"is that it is taught as one unit among a numberof others. In a typicalcourse we arelikelyto find a sequencesomethinglike this:the cell, genetics, evolution, animal/plant form and function, and ecology.A few programsuse evolutionas an organizing principle.For the most part,however,evolutionis just another topic. Studentshave to master the HardyWeinbergformulaand memorizedifferentformsof isolatingmechanisms;then they move on to anotherseemingly unrelated set of hurdles, like the nitrogen cycle and the characteristicsof the biomes. Evolution,which synthesizesthe disparatedisciplinesof the life sciences, rarelyemergesin biology courses or texts as the unifying threadthat makes sense of all the material. The second problem that concerns me with most college teachingof evolutionhas to do with the organizationand presentationof the subjectmatter.Manyprograms and texts present evolution in the following manner.The unit startswith some Pre-Darwinian ideas including:a brieflook at some earlynaturalistswho discussed change in time, such as the famous naturalists Georges-LouisLeclerc, Comte de Buffon, and JeanBaptistede Lamarck,followedby some great figuresof the past who, although they did not accept change in time, nonetheless contributedto the accumulationof knowledgethat made the "discovery"of evolutionpossible. CarlLinnaeusmakesa cameoappearancehere,as well as GeorgesCuvier,the greatcomparativeanatomist and paleontologist. Next a shortbiographicalsketchof CharlesDarwin, sometimes pointing out that he was not a particularly promising universitystudent (perhaps to suggest that students should not give up hope of becoming a useful citizen someday,even if they get a "C"in the course), and then, from Darwin'syouth to his voyage on the H.M.S. Beagle, focusing on the GalapagosIslands with their famous iguanas,tortoises,and finches.This is followed by a description of natural selection, the main force of evolutionarychange accordingto Darwin,and then an extensive treatmentof the "evidencefor evolution:"fossils, diversity,distribution,comparativeanatomy (adult and embryo),and finally,a case study in speciation-favoriteexamples include the Galapagosfinches, the Hawaiian Drosophila, or the cichlids of Lake Victoria.The evidence for evolutionis intended to provide a compellingargumentfor acceptingevolution,i.e, there is so much evidence for the theoryit must be correct, and, by implication,only an obscurantistor religious fanaticwould go againstall that evidence. Afterestablishingthe validityof evolution,programs elucidateevolutionarymechanisms:chiefly focusingon populationgenetics (explainingthe implicationsof the Hardy-Weinbergformula) and reproductivebarriers. Finally,studentsare given a concludingwiz throughthe recordof 700 millionyearsof life on Earth,with lots of interestingphotos and even moreLatinnames. What is wrong with this approach?There are two majorproblemsthat concernme: the impressionit conveys and the opportunities it misses. Going from Darwin,to a sketch of his centralidea of naturalselection, to a long list of evidencegives students the impression of a fortressmentality:Stakeout a claim,build an intellectualfortressof evidenceto defend againstall oncomers, and pour boiling oil over the rampartson the barbarianswho attack.As such, it can easilyappearnot only dauntinglydogmatic,but also static.And, if combined with a couple of throw-awaylines on religion,the approach can come off as threatening to students' deeply held values, even if they do not subscribeto any fundamentalistposition. More important, the typical method of presentingevolutionmisses a greatopportunity to discuss the natureof science. A caveatabout the natureof scienceis in orderhere. Like many others, I think it is important that those learningaboutevolutionbe given a broadersense of the subject.Likewise,it is necessarythatthose teachingevolution (or any science) have an adequateconception of the nature of science if they expect to teach their students effectively.It is pretty much a common sense notion-you should know the general nature of what you teach (Abd-El-Khalick,Bell, & Lederman, 1998; AmericanAssociationfor the Advancementof Science, 1990, 1993; National Academy of Sciences, 1998). Studentsand those preparingto be teachers,therefore, should be taughtabout the natureof science.The problem, of course,is that scholarshavebeen grapplingwith the subjectsince the earlypartof the 20th centurywithout achievingany consensus.It maynot be necessaryto achieve total consensus on the matter,but many feel that the currentlack of agreementundercutsattempts to teach the subject. Partof what makes uncoveringand explaining the natureof science difficultlies in the diversityof the scientific enterpriseitself. Consider the list in Table 1 of the sections of the American Association for the Advancementof Sciencewhich includes physics, chemistry, geology, geography,engineering, biology, statistics, psychology,politicalscience, and linguistics. What is it that holds togetherstatistics,the historicaldevelopment of the surfaceof Earth,classificationof plants, and experimentationon the behaviorof rats?Even the individual sciences (take biology, for instance), encompass a vast array of disciplines-as exemplified by the 78 member societies (and organizations)of the American Institute of Biological Sciences which include people who work at a rangeof activities:constructingcomputer models, givingnames to previouslyunknown lichens of the rain forest, testing the physiological consequences of alterationsin diet, and elucidating the life cycle of liverflukesin Africa. Table1. Sectionsof the AmericanAssociationforthe Advancement of Science 1. Agriculture, Resources Food,andRenewable 2. Anthropology 3. Astronomy 4. Atmospheric andHydrospheric Sciences 5. Biological Sciences 6. Chemistry 7. Dentistry 8. Education 9. Engineering inScienceandEngineering 10. General Interest 11. Geology andGeography 12. History andPhilosophy of Science 13. Industrial ScienceandTechnology 14. Information, andCommunication Computing, andLanguage 15. Linguistics Science 16. Mathematics Sciences 17. Medical 18. Neuroscience 19. Pharmaceutical Sciences 20. Physics 21. 22. 23. 24. Psychology Social, Economic, andPolitical Sciences Societal Impacts ofScience andEngineering Statistics NATURE OFSCIENCE 349 A Case Study Approach In spite of the perils of attemptingto construct a model that captures the nature of science, we can, nonetheless, make a number of useful observations aboutthe contemporarynatureof scienceand aboutscientific inquiry.Teachingthe theory of evolution is an invitationto do so. An effectiveway is to use a historical case study approach,one thatfocuseson scientificproblems. Instructorspresent biology as a set of questions that biologists are investigating,how those questions came into existence, and what are currentlyproposed answers.This perspectivestresses the lineages of questions that guide scientificresearch. Evolution as a case study illustrates this well. In such a treatment,one can startwith CharlesDarwinas a young man and surveywhat constitutedthe world of the life sciencesin his day.Studentslearnthat "biology" was not a term widely used, and that what lay at the heart of understandingthe living world was "natural history."Naturalhistory'sgoals were to describeand to classify the products of nature and to uncover the underlyingorderin nature(Farber,2000). The tradition in which Darwin worked had been establishedin the centurybeforeby CarlLinnaeus,who popularizedand pioneered a system of classifying and naming, and Georges-LouisLeclerc,Comte de Buffon,who stressed the importanceof extensiveobservationand arguedfor a complete survey of the living world (by which he meant external and internalcharacteristics,life stages, geographicaldistribution,geographicalvariation,and behavior). Naturalists believed that with sufficient empiricalobservationsthey would someday penetrate the veil of mysterythat shroudedthe living world, and discern the orderin nature. Between the founders of modern natural history and the generation of naturalists to which Darwin belonged, stood a majorrevisionof the naturalisttradition:the emergenceof comparativeanatomyas a serious science. Combiningthe carefulanatomicalresearchtradition on the human body with extensivenew studies on animals,a set of researchershoped to find the key to understandingthe orderin natureby examiningstructure.The most well-knownproponentof this new discipline was Georges Cuvier of the Paris Museum of NaturalHistory..He claimedthat his exhaustivedissections revealeda set of fundamentalbody plans. These basic plans could be furthersubdividedinto subgroups by using examinationof the structureof variousorgan systems. What was importantabout the new comparative anatomywas Cuvier'sclaimthat each species could be rigorouslydefined, and was morphologicallystable. They could not change because the individual units were so complex that any change would destroy their functionalintegrity. 65,NO. 5,MAY 2003 TEACHER, VOLUME AMERICAN BIOLOGY 350 THE When Darwin was a young man, comparative anatomywas the queen of the life sciences.Comparative anatomistswere uncoveringthe underlyingstructures of animal form, and they regarded these underlying structuresas the foundationfor classification.Systems of classification,it was hoped, would mirrorthe orderin nature.Enrichingthat picturewas the study of embryology. Of equal importance was the study of fossils which revealedthat the structuresof extinctanimalsfollowed the same generalpatternsas living ones. Thatis, the picturethat Cuvierconstructedapplied to living as well as to extinct forms. Darwin also lived at a time when the Industrial Revolutionin Europewas leadingEuropeansto actively colonize the globe. Nations sought new markets and new raw materials,and in the explorationand exploitation of the world, Europeansuncoveredthousands of new plants and animals.The materialcoming back to the museums and collectionsin Europerevealedinteresting geographicalpatterns. Largeareas of the globe seemed to have plants and animalsthat possessed family resemblances.Darwin's generation,therefore,was heir to the largest collections ever assembled.His colleagues differed qualitativelyfrom earlier naturalists because of the consequences of the Industrial Revolution.Increasedleisure time, lower cost of printing, innovationsin the reproductionof illustrations,and developmentof populareducationhad made it possible for more people to work on naturalhistory than ever before. This new, more professional,generation held high standards,and they were in communicationwith one another through such new genres as the monographand scientificarticle(Farber,1997). Darwin'scontemporariespuzzled overa numberof problems concerning their data. For one, naturalists were amazedby the diversitythat existed. Beetles!Why should God have createdso many beetles?You could find a thousand differentspeciesjust in the areawhere Darwin received his university education. Does God have an inordinatefondness forbeetles (as a laterbiologist would quip?).Giventhat therewere so many different species on the planet, naturalistsasked where they all had come from.One could imaginea Gardenof Eden with a few hundred species;or Noah's arkwith several thousandkinds, but naturalistswere uncoveringa staggeringnumberof life forms.Paleontologistscompounded the problemby showing that there were thousands, perhapsmillions,of extinctformsthat formerlyinhabited Earth.Where had all the new species come from? Why did all the old ones die off? What accounted for the similarities we notice among different species, among genera,etc.? On a practicallevel, what criteria should we use in classifyingthem? In additionto diversityand its origin(and the related issue of classification),the patterns of distribution called out for some explanation.Why should Australian animalstoday resembleAustraliananimalsof the past, rather than the animals of other continents? Why should plants on islands in the ocean resemblethose of the closest continent?Why should plants at high altitudes resembleplantslivingat greatdistancesaway,but at the same altitude? When Darwinbeganhis momentousvoyageon the H.M.S.Beagle,his contemporarieswereaskingthe above questions, and it his hardly surprisingthat he found himself asking the same ones on his journey. He discovered interesting fossils (e.g., armadillos) in South Americathatresembledanimalsstill livingin the region, and noted that similar animals and plants inhabited adjacent territories.He noticed that the life on the GalapagosresembledSouth Americanforms, but were separatespecies. AfterDarwinreturnedto Englandhe spent a number of years compiling his data and farmingit out to experts for them to describe and to name. One of his collaboratorsinformedhim that his collection of finches from the Galapagosconsisted of almost a dozen differentspecies. Anotherverifiedthat the fossil llama he found in South Americarepresenteda differentspecies than the ones found alivetheretoday.These novel findings reenforcedhis puzzlement over the leading questions in natural history.At some fairly early point in these years after his return, Darwin realized that IF species changed,if one species could give rise to another, then many of the centralproblemsthat he and other naturalists confronted could be answered. But, how could a species change?The comparativeanatomyof his day suggested it could not happen. So he spent years reading.He found earlierattemptsto establish change in natureunconvincing.Buffonand Lamarckwere too speculative.Darwinfelt that to be convincinghe had to discover the mechanismresponsible for the change of species. Why a mechanism?We do not know for sure, but it certainlyseems that he had absorbedthe scientific ethos of the generationof naturaliststo which he belonged.Theywanted the study of living organismsto be a sciencelike the study of chemistryor physics.Since the days of Newton, the physicalsciences had pursued a vision of nature as a vast, complex machine.During the 18th and 19th centuries the picture of nature became more and more that of machine, like the machinesdrivingthe IndustrialRevolutionthroughout Western Europe. While Darwin was working on his explanation of the origin of species, those studying physiologywere discoveringthe workingsof the animal and plant body-a series of physicaland chemicalmechanisms that explainednutrition,respiration,and other biological functions. So, looking for a mechanism to explain evolution was in keeping with the most advancedscience of his day, and is a traditionthat still characterizesa lot of what we do in the life sciences. Finally,as we know from Darwin'sdiaryand notebooks, on September28, 1838 he read a work by the Rev. Thomas Malthus on the consequences of overpopulation. In a flash he made a leap from the pressure that resulted from overpopulationto the notion that in nature,since there was so much destructionof life and so much variation, there must be a natural selection operating that in time could modify the descendants of a population (Browne, 1995, Mayr, 1972, Ruse, 1979). It was a huge leap. He went from thinking of a species as a blueprint-aunit rigorouslydefinedby comparativeanatomy-to a population of individuals. He realizedthat individualswith variationsthat gave them an adaptiveadvantagewere more likely to surviveand reproduce,and that in time the process would give rise to new species. Fossil forms resembled living forms because they were directly or indirectly related. Distributionpatternswere the traces of ancient movement of plants and animals.The resemblancesnoted in classificationwerenot the resultof an abstractplan,but the faintoutlines of common descent. Noticewhatis happeningherepedagogically.Those categoriesthat we usuallypresent as "evidencefor evolution" are presented as the problems that Darwin solved (fossils, diversity, distribution, comparative anatomy). These problems gave rise to the theory; rather than being the bricks and mortar of a fortress constructedafterthe theorywas conceived.By reorganizing the materialsthat are generallyused we can convey knowledgeof the theoryin a mannerthat avoidsthe static "fortressmentality" and better illustrates the natureof science. Darwin'seuphoriaof discoverydid not last long, for he realizedthat thereweremanyproblemswith his new theory. So much so that he doubted if he could ever convince anyone of it. This is not fortressscience;just the opposite (more like ecosystem development,if we need a differentmetaphor).Darwinworkedfor over 20 years before publishing his theory,for he knew that it would be controversialand he knew that he had not solved many of the problems that the theory faced. In this regard,Darwinwas like other proponents of new, revolutionarytheories. What were some of the majorscientificproblems? Firstoff, of course,was the issue raisedby comparative anatomy-thatspecieswereso complexit was difficultto fathom how they could change. Next, if the species inhabiting Earth were the descendants of previous species, then there would have to have been a vast amount of variationin those earlierpopulationsto permit naturalselection to create new forms. Was there? Wheredid it come from?Werethereforcesoperatingon the productionof new variations?Closelyrelatedto the issue of variationwas the concern about whether new NATURE OFSCIENCE 351 variationcould be inherited.The study of inheritancein Darwin'sday also posed the troublingobservationthat characteristicsseemed to blend when they were inherited-a red floweringplant crossedwith a white flowering plant often gaverise to a pink floweringplant. Or,if the characteristicsdid not blend, then they seemed to mix-the childrenin a family typicallyseemed to have features resembling the father, some the mother. Blendinginheritancesuggestedthat advantageousvariation, so importantfor Darwin,might get swamped in successivegenerationsand thereforenot be acted upon by naturalselection. The problemsthat Darwinfacedwere real and serious. Not surprisingly,therefore,many scientists had serious reservationsabout his theory when he published it in 1859, and a livelydebateensued.The debate, however, stimulated a vast amount of significant research.The researchdone on heredity,for example, turned out to be of criticalimportancein biology,for it led ultimately to the rediscoveryof "Mendel'sLaws" (which had been completelyignored in Mendel'sday) and to the developmentof modern genetics. So, one set of problems,the ones Darwingrappled with, led to a solution, Darwin's theory of evolution based on naturalselection,which led to another set of problems.The work on those problemsled to new solutions in genetics,and other subjects,and eventuallyto a new theory of evolution (the Modern Synthesis that was formulated by Theodosius Dobzhansky, Ernst Mayr,GeorgeGaylordSimpson,JulianHuxley,Ledyard Stebbins,and others beginningin the late 1930s). And, of course,the new theoryraiseda host of new and exciting issues and questions.The lineage of problemscontinues, and when we teach this way students see how fundamentalquestions are to science. Religion What about religion? Religion resides under the surface in any discussion of evolution. Here, again, I think an historicalapproachhelps greatly.In Darwin's day many scientists and most educated people in the English-speakingworld had grown up with the view that the wonders of nature served as testimony to the wisdom and power of God, the Creator.Darwin,however,had come to the view thatnaturalhistorywould be more productiveif it severed its explicit ties with religion. That is, if scientific explanationdid not include any referenceto the supernaturalabout which we have no way of resolving disputes. Such was the case in physics,chemistry,and recently,in Darwin'sday,geology. Not everyoneagreedwith that position. We most often, however, read caricaturesof the story: for example, accounts of the famous HuxleyWilberforcedebate at the 1860 BritishAssociationfor MAY 2003 VOLUME 65,NO.5, TEACHER, AMERICAN BIOLOGY 352 THE the Advancementof Science meeting where Darwin's bulldog allegedlydefeatedand put down "SoapySam," the Bishopof Oxford.Whatis left out of this renditionis the very serious considerationgiven to Darwin'sideas by the religiousminded (cf. Livingstone,1987, Moore, 1979). Therewerea numberof theologianswho considered Darwin's theory a threat to received opinion. CharlesHodge, for example,arguedthat by stressinga deterministicuniverse,Darwinwas slipping into atheism. But other theologiansarguedthat Darwin'stheory John Fiske, opened the path for a renewedChristianity. a leadingpopularizerof evolutionin America,made that a centralargumentin his writings.Similarly,Asa Gray, Darwin'schief scientificsupporterin the United States, wrotea set of articlesshowinghow Darwincould be reconciled with traditionalChristianbeliefs (Gray,1963). To Gray,Darwinhad shown howthe Creatoroperated. Instead of what Gray took to be the commonly held naive and crude belief that God had createdeverynew species individually (think of all those beetles!), Darwin'sOriginof Speciesgave a dynamicdimensionto Creationand removedthe awkwardnessthe fossil and geologicalrecordposed to men of faith.Fiske and Gray werenot alone.BenjaminWarfield,foremostdefenderof the theologicallyconservativedoctrineof the inerrancy of the Bible was an evolutionist. One enthusiastic Americanwritereven produceda book titled,TheGospel to Darwinthat claimedthe Originwas the fifth According Gospel.It is not an overstatementto say that the majority of the Americanbiologistswho acceptedevolutionin the late 19th centurydid not believeit posed any threat to religion,but, quite the contrary,felt their religious beliefswere strengthenedby it. Now, the point is not to teachthe historyof religion in biology classes,but ratherto brieflyconveythat there aremanyways to interpretDarwin,and there are many ways to reconcileevolutionwith religion.My experience has been that once students see that, they realize that evolutionis not the flame-breathing dragonof atheism, but a theory that explains biologicalphenomena, that relatesbodies of information,and that guides research, and like other aspectsof science,is open to manyphilosophical and religious interpretations.They have to work out for themselveshow they want to view it. Butit is not an either/or situation:science or religion.There are a number of models of negotiatedrelationships(cf. Miller,1999). Nature of Science So given this case-studyapproachto the theory of evolution,which presents science as a lineage of questions, which sees Darwin'stheoryas a response to a set of problemsin his day,andwhich sees Darwin'ssolution as one thatraisednew questionswhich ultimatelyresulted in a new theory, how can we use it to exemplify aspects of the nature of science?What generalizations aboutthe natureof sciencemightwe be able to drawout of our story? One obvious generalizationis the dynamicnature of science. Science changes through time. In Darwin's case we see the basic concepts of biology redefined (species, distribution,etc.) We also see a new interpretation of how to explain the facts of biology come into being. The changing dimension of science needs to be conveyedto studentswho too often aregiven a simplistic view that science consists of the "Truth"about nature.This is not to say that we need get entangledin a post-modernfree-for-all, but merelyto make the point that in science new questions emerge,new interpretations come into being, opinions, often long held, can be revised. Anotherimportantdimensionto understandingscience is its levels of generality.Althoughwe discuss facts, hypotheses, laws, and theories, we often neglect to point out in our science teachingat what level the material under considerationfalls, and the result can generate a lot of confusion.How, afterall, can we evaluatea scientificclaim if we do not know the intended level of generalization?Is the claim a fact (a well-confirmed observationor empiricalstatement),a hypothesis(a calculated guess open to investigation),a law (a well-confirmed regularity,or, a definitionusually of a relationship thought to be invariableand universal),or a theory (a complex explanation,based on assumptionsand definitions, that relates observations and bodies of knowledge and guides research)?All have different appropriatestrategiesfor evaluation.One uses observation for most facts and experimentsfor most hypotheses. Theories are evaluatedon how well they explain and relate, and therefore are not "proved"or "disproved."They are inherently open-ended and always have "problems"to be solved,which is a strength,not a weakness. The theory of evolution has to be presented as a theory,and this is an invitationto discuss the levels of generalityin science. It also permits us to circumvent pseudo-arguments,like the alleged "scientific problems" of the theory which turn out to be areas of research.The theory of evolution has problems. Yes; they make the theory more interesting!Lookingat levels of generalitygives students a more sophisticated frameworkwith which to judge claims, and permits them to see how a theory like evolutioncan be considered "powerful"while being "unproved"and raising serious questions. Relatedto the issue of the level of generalityis the level of certaintyin science. Sciencemakes some statements about the world that have a high degree of cer- tainty,but science is not a monolithicenterprise.Some of the claimsin sciencewe hold with a greatdeal of certainty:Mendel'slaws, the gas laws, for example.They arehighlyconfirmed,and some of us would literallybet our lives on them. Other claimsin science are less confidently held. The Big Bang theory explains a lot, but one could imagine another theory replacing it given new observationsor new discoveriesin physics. What about evolution?Thatlife has changedon Earthis well established and one could say with great confidence that evolutionhas occurred.Some of the broaderclaims of the theory, however,are held with less confidence, and otherparts,for example,the evolutionaryhistoryof individualgroups, such as Homosapiens,rest on slight evidenceand are constantlybeing rethoughtin light of new discoveries. Scientificmethod?Thereis not, of course, a single scientificmethod. Scientistsuse a number of methods depending upon what questions or problems they are tackling.In the case of evolution,biologistsuse different methods to examinedifferentissues. Some of the work is experimental,as in the investigationof the peppered moths in Britain;some of the work is observationalor comparative,as in paleontology,that continuallybroadens our understandingof past flora and fauna; and finally,some of it consists of creativeintellectualconstructions,like Darwin making an analogy to human populationgrowth to formulatethe concept of natural selection.By paying attentionto the methods scientists employ,we obtain a sense of the wide range of issues that they can tackle. Finally,science is alwaysdone in a specificcultural context. Darwin's work took place in industrial England. The vast amount of material pouring into Europeanmuseums raised specific scientificproblems. Competition was a fact of life, and the metaphors Darwinemployedfit his day.Had he lived at a different place or period he may well have framedhis ideas differently. Dobzhanskypassionatelybelieved that the process of evolution was fundamentalto an understandingof biology.Today,his contentionis as valid as it was when he made it more than a quarter of a century ago. Moreover,the theoryof evolutioncan serveas an exemplar of how scientists examine the natural world Biologyis not a body of facts to memorizebut a quest towardsunderstanding,one that is ever changing and one that has roots not only in the phenomena that we observe,but in the human world that shapes our concerns and questions.If we can move the study of biology towardwhat excites biologists and away from what makes students' eyes glaze over,we shall have accomplished an importantand valuabletask. NATURE OFSCIENCE 353 Acknowledgments This articleis a writtenversionof a plenarysession talkdeliveredat the 2001 AnnualMeetingof the National AssociationforResearchin ScienceTeaching.Comments by MarkLargent,Norm Lederman,MikeMix, and Ben Mutschlerwerehelpfulin preparingthis article. References Farber, P. (1997). Discovering Birds: The Emergence of Ornithologyas a ScientificDiscipline, 1760-1850. Baltimore: Johns HopkinsUniversityPress. Gray,A. (1963). Darwiniana: Essays and Reviews Pertaining to Darwinism.Cambridge,MA:HarvardUniversityPress. Lerner, L.S. (2000). Good Science, Bad Science: Teaching Evolution in the States. http://www.edexcellence.netlibrary/lerner/gsbsteits.html. Livingstone,D. N. (1987). Darwin's ForgottenDefenders: The F., Bell, R.L.,& Lederman,N.G. (1998). The Abd-El-Khalick, nature of science and instructionalpractice:making the unnatural natural. Science Education,82(4), 417-436. AmericanAssociationfor the Advancementof Science(1990). Science for All Americans. 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Baltimore: Johns HopkinsUniversityPress. National Academy of Sciences (1998). Teaching about Evolution and the Nature of Science. Washington, DC: NationalAcademyPress. Ruse, M., (1979). The Darwinian Revolution: Science Red in Toothand Claw. Chicago:Universityof ChicagoPress. WORLD INTHEREAL SCIENCE Actio MicrGbesiin Bring the world of microbes to your classroom! *ClassroomActivities *MicrobiologyTechniques *TroubleshootingTips *Workshops ,Articles & News about Microorganisms Visit our web site for more information: http://www.umsl.edu/-microbes/ 2003 65, NO.5,MAY VOLUME TEACHER, AMERICAN BIOLOGY 354 THE