Paleontological Society Exaptation-A Missing Term in the Science of Form Author(s): Stephen Jay Gould and Elisabeth S. Vrba Reviewed work(s): Source: Paleobiology, Vol. 8, No. 1 (Winter, 1982), pp. 4-15 Published by: Paleontological Society Stable URL: http://www.jstor.org/stable/2400563 . Accessed: 27/08/2012 17:43 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Paleontological Society is collaborating with JSTOR to digitize, preserve and extend access to Paleobiology. http://www.jstor.org Paleobiology,8(1), 1982, pp. 4-15 Exaptation-a missing term in the science of form StephenJay Gould and Elisabeth S. Vrba* Abstract.-Adaptationhas been definedand recognizedby two differentcriteria: historical genesis (fea- turesbuilt by naturalselection for their present role) and currentutility (features now enhancingfitness no matterhow theyarose). Biologistshave oftenfailed to recognizethe potentialconfusion between these differentdefinitions because we have tendedto view naturalselection as so dominantamong evolutionary mechanismsthat historical process and currentproduct become one. Yet if manyfeatures of organisms are non-adapted,but available foruseful cooptation in descendants,then an importantconcept has no name in our lexicon (and unnamed ideas generallyremain unconsidered):features that now enhance fitnessbut were not built by naturalselection for their current role. We propose that such featuresbe called exaptationsand that adaptationbe restricted,as Darwin suggested,to featuresbuilt by selection fortheir current role. We presentseveral examples of exaptation,indicating where a failureto concep- tualizesuch an idea limitedthe range of hypotheses previously available. We exploreseveral consequences of exaptationand proposea terminologicalsolution to the problemof preadaptation. StephenJ. Gould. Museum of ComparativeZoology, Harvard University,Cambridge, Massachusetts 02138 Elisabeth S. Vrba. TransvaalMuseum, Paul KrugerStreet, P.O. Box 413, Pretoria,South Africa Accepted: October 15, 1981 I. Introduction (Foucault's interpretationhas been challenged We wish to proposea termfor a missingitem by Britishhistorian of scienceRoy Porter,MS). in the taxonomyof evolutionarymorphology. In other systemsof thought,what seems pe- Termsin themselvesare trivial,but taxonomies ripheralto us becomescentral, and distinctions revisedfor a differentordering of thoughtare essentialto us do not matter(whether idleness not withoutinterest. Taxonomies are not neu- is internallyinevitable, as in insanity,or exter- tralor arbitraryhat-racks for a set of unvarying nally imposed,as in unemployment). concepts;they reflect (or even create) different II. Two Meanings of Adaptation theoriesabout the structureof the world. As Michel Foucault has shown in several elegant In the vernacular,and in sciencesother than books (1965 and 1970, forexample), when you evolutionarybiology, the word adaptation has know whypeople classifyin a certainway, you several meanings all consistentwith the ety- understandhow theythink. mologyof ad + aptus,or towardsa fit(for a par- Successivetaxonomies are the fossiltraces of ticularrole). When we adapt a tool for a new substantialchanges in human culture. In the role, we changeits designconsciously so thatit mid 17thcentury, madmen were confined in in- will workwell in its appointedtask. When cre- stitutionsalong with the indigentand unem- ationistsbefore Darwin spoke of adaptation- ployed,thus endinga long traditionof exile or for the term long precedes evolutionary tolerationfor the insane. But what is the com- thought-theyreferred to God's intelligentac- mon groundfor a taxonomythat mixes the mad tion in designingorganisms for definiteroles. with the unemployed-an arrangementthat When physiologistsclaim that larger lungs of strikesus as absurd. The "key character"for Andean mountainpeoples are adapted to local the "highertaxon," Foucault argues, was idle- climates,they specify directed change for better ness, the cardinal sin and danger in an age on function.In short,all these meaningsrefer to the brinkof universalcommerce and industry historicalprocesses of change or creation for definitefunctions. The "adaptation"is designed * An equal timeproduction; order of authorshipwas de- specificallyfor the task it performs. terminedby a transoceaniccoin flip. In evolutionarybiology, however, we en- (?) 1982 The PaleontologicalSociety. All rightsreserved. 0094-83 73/82/0801-0002/$1.00 EXAPTATION 5 TABLE 1. A taxonomyof fitness. Process Character Usage Natural selectionshapes the character adaptation function fora currentuse-adaptation A character,previously shaped by naturalselection fora particularfunction (an adaptation),is coopted aptation fora new use-cooptation exaptation effect A characterwhose origincannot be ascribedto the directaction of naturalselection (a nonaptation),is coopted fora currentuse-cooptation countertwo differentmeanings-and a possible useful characternot built by selectionfor its conflationof concepts-for featurescalled ad- currentrole-and we shall followthis restric- aptations.The firstis consistentwith the ver- tion here (Table 1). Williams also recognizes nacular usages cited above: a featureis an ad- that much hagglingabout adaptationhas been aptationonly if it was builtby naturalselection "encouragedby imperfectionsof terminology" for the functionit now performs.The second (1966, p. 8), a situationthat we hope to alleviate definesadaptation in a static,or immediateway slightly. as any featurethat enhances current fitness, re- Bock, on the otherhand, championsthe sec- gardless of its historicalorigin. (As a further ond, or broader, meaning in the other most confusion,adaptation refersboth to a process widely-citedanalysis of adaptation from the and a state of being. We are only discussing 1960's(Bock and von Wahlert1965; Bock 1967, state of being here-that is, featurescontribut- 1979, 1980). "An adaptationis, thus, a feature ing to fitness.We includesome comments about of the organism,which interactsoperationally thisfurther problem in sectionVIE.) withsome factorof its environmentso that the Williams,in his classic book on adaptation, individual survives and reproduces"(1979, p. recognizedthis dilemma and restrictedthe term 39). to its first,or narrower,meaning. We should The dilemmaof subsumingdifferent criteria speak of adaptation,he argues, only when we of historicalgenesis and currentutility under a can "attributethe originand perfectionof this singleterm may be illustratedwith a neglected design to a long period of selectionfor effec- example froma famoussource. In his chapter tivenessin this particularrole" (1966, p. 6). In devotedto "difficultieson theory,"Darwin wrote his terminology,"function" refers only to the (1859, p. 197): operationof adaptations. Williams furtherar- The suturesin the skulls of youngmammals gues that we must distinguishadaptations and have been advanced as a beautifuladaptation theirfunctions from fortuitous effects. He uses foraiding parturition,and no doubt theyfa- "effect" in its vernacular sense-something cilitate,or may be indispensablefor this act; caused or produced, a resultor consequence. but as suturesoccur in the skulls of young Williams'concept of "effect"may be applied to birdsand reptiles,which have onlyto escape a character,or to its usage, or to a potential(or froma broken egg, we may inferthat this process), arisingas a consequenceof true ad- structurehas arisenfrom the laws of growth, aptation. Fortuitouseffect always connotes a and has been taken advantage of in the par- consequencefollowing "accidentally," and not turitionof the higheranimals. arisingdirectly from construction by naturalse- lection.Others have adopted variousaspects of Darwin assertsthe utility,indeed the necessity, this terminologyfor "effects"sensu Williams of unfusedsutures but explicitlydeclines to la- (Paterson 1981; Vrba 1980; Lambert, MS). bel theman adaptationbecause theywere not However, Williams and othersusually invoke builtby selectionto functionas theynow do in the term'effect' to designatethe operationof a mammals.Williams follows Darwin and would 6 STEPHEN JAY GOULD & ELISABETH S. VRBA decline to call this featurean adaptation; he towardsfitness. They owe theirfitness to fea- would designateits role in aiding the survival tures presentfor otherreasons, and are there- of mammals as a fortuitouseffect. But Bock forefit (aptus) by reason of (ex) theirform, or would call the suturesand the timingof their ex aptus. Mammaliansutures are an exaptation fusionan adaptation,and a vital one at that. forparturition. Adaptations have functions;ex- As an example of unrecognizedconfusion, aptationshave effects.The general,static phe- considerthis definition of adaptationfrom a bi- nomenonof beingfit should be called aptation, ologicaldictionary (Abercrombie et al. 1951, p. not adaptation.(The set of aptationsexisting at 10): "Any characteristicof living organisms any one time consistsof two partiallyoverlap- which, in the environmentthey inhabit, im- ping subsets:the subsetof adaptationsand the provestheir chances of survivaland ultimately