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Home , Adaptationism, Brian Goodwin, Richard Levins, Spandrel (biology)

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Peirce, Charles Sanders(1931), Colkcted Papers.Edited by Thagard, Paul (1978), "The Best Explanation: Criteria for C. Hartshorn and P. Weiss. Cambridge, MA: Harvard Theory Choice," Jourool of Philosophy75: 76-92. University Press. Putnam. Hilary (1978), Meaning and the Moral Sciences. See also Bayesianism; Confirmation Theory; Induc- London: Hutchinson. tion, Problem of; Instrumentalism; Realism

ADAPTATION AND ADAPTATIONISM

In evolutionary , a phenotypic trait is said that were wholly untrue and that God to be an adaptation if the trait's existence, or its created the universe in sevendays, many phenotypic prevalence in a given population, is the result of traits would still qualify as adaptive in this sense,for . So for example, the opposable they undeniably benefit their current possessors.By thumb is almost certainly an adaptation: Modem contrast. to describe a trait as an adaptation is lo primates possess opposable thumbs because of say something about evolutionary history, namely the selective advantage that such thumbs conferred that natural selection is responsible for the trait's on their ancestors, which led to the retention and . If Darwinism turned out to be false, gradual modification of the trait in the lineage it would follow that the opposable thumb is not leading to modern primates. Usually, biologists an adaptation for grasping branches, though it will describe a trait not as ..an adaptation per se would still be adaptive for primates in their current but rather as an adaptation for a given task, environment. So tbe adaptive/adaptation distinc- where the task .refers to the environmental "prob- tion corresponds to the distinction between a trait's lem" that the trait helps the organism to solve. current utility and its selective history. Thus the opposable thwnb is an adaptation for In general, most traits that are are gi-asping branches; the ability of cacti to store also adaptive and vice versa. But the two concepts water is an adaptation for living in arid deserts; do not always coincide. The gastrointestinal the brightly adorned tail of the peacock is an adap- appendix is not adaptive for contemporary human tation for attracting mates; and so on. Each of beings-which is why it can be removed without these statements implies that the trait in question loss of physiological . But the appendix is was favored by natural selection because it con- nonethe1essan adaptation. for it evolved to help its ferred on its bearer the ability to perform the bearers break down cellulose in their diet. The fact task. In general, if a trait T is an adaptation for that the appendix no longer servesthis function in task X, this means that T evolved because it en- contemporary does not alter the (pre- abled its bearers to perform X, which enhanced sumed) fact that this is why it originally evolved. their Darwinian fitness. This can also be expressed In general. when a speciesis subject to rapid envi- by saying that the function of the trait T is to ronmental change, traits that it evolved in response perform X. Thus there is a close link between the to previous environmental demands, which thus concepts of adaptation and evolutionary function count as adaptations, may ceaseto be adaptive in (Sterelny and Griffiths 1999;Ariew, Cummins, and the new environment. Given sufficient time, evolu- Perlman 2002: Buller 1999). tion may eventually lead such traits to disappear, Many authors have emphasized the distinction but until this happens these traits are examples of between a trait that is an adaptation and a trait adaptations that are not currently adaptive. that is adaptive. To describe a trait as adaptive is It is also possible for a trait co be adaptive with- to say that it is currently beneficial to the organisms out being an adaptation, though examples falling that possessit, in their current environment. This is into this category tend to be controversial. Some a statement solely about the present-it says noth- linguists and biologists believe that the capacity of ing about evolutionary history. If it turned out humans to use language was not directly selected

3 ADAPTATION AND ADAPT A TIONISM

for, but emergedas a sideeffect of natural selection resemblance between stick insects and the foliage for larger brains. According to this theory, there they inhabit. It seemsmost unlikely that this resem- was a direct selectiveadvantage to having a large blance is a coincidence or the result of purely chance brain, and the emergenceof languagewas simply processes(Dawkins 1986, 1996). Much more plau- an incidentalby-product of the resultingincrease in sibly, the resemblance is the result of many rounds brain size among proto-humans.{f this theory is of natural selection, continually favoring those correct, then human linguistic ability does not insectswho most closely resembledtheir host plants, qualify as an adaptation and has no evolutionary thus gradually bringing about the insect/plant function; thus it would be a mistake to look for a match. It is obvious why insects would have bene~ specific environmental demand to which it is an fited from resembling their host plants--they would evolved response.But the ability to use language have been less visible to predators-so it seemssafe is presumablyadaptive for humansin their current to conclude that the resemblance is an adaptation environment, so this would be an exampleof an for reducing visibility to predators. Biologists re- adaptive trait that is not an adaptation. It should peatedly employ this type of reasoning to infer a be noted. however,that many biologists and lin- trait's evolutionary function. guistsare highly suspiciousof the idea that human Second, if a phenotypic trait is highly complex, linguistic capacitywas not directly shapedby natu- then many biologists believe it is safe to infer that it ral selection. (See Pinker and Bloom 1990 and is an adaptation, even if the trait's evolutionary Fodor 2000for opposingviews on this issue.) function is not initially known. Bodily organs It sometimeshappens that a trait evolvesto per- such as eyes, kidneys, , and livers are exam- form one function and is later co-opted by evolu- ples of complex traits: Each involves a large num- tion for a quite different task. For example,it is ber of component parts working together in a thought that birds originally evolvedfeathers as a coordinated way, resulting in a mechanism as intri- way of staying warm, and only later usedthem to cate as the most sophisticated man-made device. assistwith flight. This is an interestingevolutionary The inference from complexity to adaptation rests phenomenon,but it createsa potential ambiguity. On the assumption that natural selection is the only Shouldbirds' feathersbe regardedas an adaptation serious scientific explanation for how organic com- for thennoreguJationor for efficient flight? Or per~ plexity can evolve. (Appealing to an intelligent haps for both? There is no simple answer to this deity, though intellectually respectable in pre-Dar- question, particularly since feathers underwent winian days, no longer counts as a serious explana- considerableevolutionary modification after they tion.) Again, inferences of this sort do not strictly first began to be usedas a flying aid. Gould and amount to proof, but in practice biologists routine- Vrba (1982)coined the term "" to help ly assume that complex organismic traits are ad- resolvethis ambiguity. An exaptation is any trait aptations and thus have evolutionary functions that originally evolves for one use (or arisesfor waiting to be discovered. nonadaptive reasons) and is later co~opted by The defU1ition of an adaptation given above-- evolution for a different use. any trait that has evolved by natural selection-is How is it possibleto tell which traits are adapta- standard in contemporary discussions. In this tions and which are not? And if a particular trait is sense, all biologists would agree that every extant thought to be an adaptation, how is it possibleto organism possessescountless adaptations. Howev- discover what the trait is an adaptationfor. that er, the term has sometimes b~en understood slight- is, its evolutionary function? These are pressing ly differently. R. A. Fisher, one of the founders of questionsbecause evolutionary history is obviously modem Darwinism, wrote that an organism not directly observable.so can be known only via inference. Broadly speaking, there are two main "is regardedas adapted to a particular situation. . . only typesof evidencefor a trait's being ~n adaptation, in 50 far as we can imaginP an assemblageof slightly both of which were identified by Darwin (1859)in different situations, or environmen~, to which the ani- .First, if a trait contributes ma! would on the whole be less well adapted; and in an obvious way to the "fit" betweenorganism equally only in so far as we can im;:)glnean assemblage of slightly different organic forms, which would be less and environment,this is a prima facie reason for well adapted to that environment" {193O, 41) thinking it hasbeen fashioned by natural selection. The organism/environmentfit refersto the fact that It is easyto seetbat Fisher'snotion of adaptationis organismsoften possessa suite of traits that seem more demandingthan the notion employedabove. specifically tailored for in the environments Fisher requiresa very high degreeof fit between they inhabit. Considerfor examplethe astonishing organism and environment before the concept of

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~ ADAPT A nON AND ADAPT A naNISM adaptation applies, such that any small modifica- organism would qualify as optimally adapted. tion of either the organism or the environment With sufficient imagination. it is always possible would lead to a reduction in fitness. In modern to think of phenotypic changesthat would boost parlance, this would nonnally be expressed by an organism's fitness-for exampLe,doubling its saying that the organism is optimally adapted to fecundity while leavingeverything else unchanged. its environment. (As wrote, Hit is clearly im- It is quite possible for an organism to possess possibleto say what is the "best" phenotypeunless many adaptations in the above sense, i.e.. traits one knows the range of possibilities" [Maynard that are the result of natural selection, without Smith J978, 32]). So to avoid trivia1izingthe con- being optimally adapted in the way Fisher des- cept of optimality altogether, some restriction must cribes. There are a number of reasons why this is be placed on the class of possible modifications so. First, natural selection is a gradual process: whose effects on organismic fitness are relevant to Many generations are required in order to produce judging how well adapted the organism is in its a close adaptive fit between organism and environ- current state. Spelling out the necessaryrestriction ment. Suboptimality may result simply becausese- will lead to a concept similar to Fisher's, with its lection has yet to nm its full course. Second, unless attendant vagueness. there is considerable environmental constancy over The constraints on optimality noted in the earlier time, it is unJikely that organisms win evolve traits discussion of suboptimality show that natural selec- that adapt them optimally to any particular envi- tion may fail to produce organisms that are optimal~ ronment, given the number of generations required. ly adapted. But how important theseconstraints are So suboptimality may result from insufficient envi- in practice is a matter of considerable controversy. ronmental constancy. Third, there may be evolu- Some biologists think it is reasonableto assumethat tionary trade-offs. For example, the long necks of most extant organisms are optimally or nearly opti- giraffes enable them to graze on high foliage, but the mally adapted to their current environment. On this price of a long neck might be too high a center of view, any phenotypic trait of an organism can be gravity and thus a suboptimal degree of stability. studied on the assumption that selection has fine- Evolution (;anIlul always modify an organism's tuned the trail very pnx:i~ely, so that then: is au phenotypic traits independently of each other: evolutionary reason for the character being exactly Adjusting one trait to its optimal state may in- the way it is. Other biologists have lessconfidence in evitably bring suboptimality elsewhere. Finally, as the power of natural selection. While not denying Lewontin (1985) and others have stressed, natural that selection has shaped extant , they selection can drive a speciesfrom one point in phe- see the constraints on optimality as sufficiently im- notypic space to another only if each intermediate PQrtant to invalidate the assumption that what has .ftage is fitness enhancing. So, suboptimality may actually evolved is optimal in Fisher's sense.They result becausethe optimal phenotypic state cannot would not seek adaptive significance in every last be accessedfrom the actual state by a series of detail of an organism's . (See Maynard incremental changes, each of which increases fit- Smith 1978 and Maynard Smith et al. 1985 for a ness. For all these reasons, it is an open question good discussion ofthis issue.) whether natural selection will produce optimally The optimality question is just one aspect of an adapted organisms. important and sometimes heated debate concern- It is worth noting that the Fisherian concept of ing the legitimacy of what is called "adapta- optimal adaptation employed above is not totally tionism" in (Sober and precise. and probably could not be made so, for it Orzack 2001; Dupre 1987). Adaptationism encom- hinges on the idea of a small or slight modification passes both an empirical thesis about the world to either the organism or the environment. leading and a methodology for doing evolutionary research to a reduction in fitness. But "small" and "slight" (Godfrey-Smith 2001). Empirically, the main claim are vague terms. How large can a modification be is that natural selection has been by far the most before it counts as too big to be relevant to as- important detenninant of organismic phenotypes sessing whether a given organism is optimally in evolutionary history-all or most traits have adapted? Questions such as this do not have prin- been directly fashioned by natural selection. Typi- cipled answers. However, any workable concept cally, adaptationists will also show some sympathy of optimality is likely to face a similar problem. It for the view that extant organisms are optimally is unacceptable to say that an organism is opti- adapted to their environments, in at least certain mally adapted if there is no possible modification respects. Methodologically, adaptationists believe that would raise its fitness. for by that token no that the best way to study the living world is to

5 ADAPT A nON AND ADAPT A naNISM

search for the evolutionary function of organisms' trait has adaptivesignificance of its own. phenotypic traits. Thus, for example, if an adapta- is an architectural tenD that refers to the roughly tionist observesan unusual pattern of behavior in a triangular spacebetween two adjacentarches and speciesof insect, the adaptationist will immediately the horizontal abovethem; they are necessaryby- assume that the behavior has an evolutionary func- products of placing a (or a flat roof) on tion and will devote effort to trying to discover that arches.The spandrelsbeneath the great dome of function. Opponents of adaptationism reject both S1.Mark's Cathedralin Veniceare decoratedwith the empirical thesis and the methodological strate- elaboratemosaics of the four evangelists.Gould gy. They emphasize the constraints on optimality and Lewontin's point is that despite their ornate noted above, as well as others; additionally, they design,the spandrelsare obviously not the raison point out that natural selection is not the only d'etre of the whole construction: Rather, they are cause of evolutionary change and that organisms inevitable by~productsof the architectural design. possesscertain features that are nonadaptive and Similarly, they suggest.certain anatomjcal and even maJadaptive. Thus, it is a mistake to view the morphologicaltraits of modern organismsmay be living world through an exclusively adaptationist inevitableby~products of their overall design,rath- lens, they argue. er than directly shapedby selection.If so, such The basic contours of the adaptationism debate traits would not be adaptations,and it would be have been in place for a long time, and indeed trace inappropriateto searchfor their evolutionaryfunc- right back to Darwin. But the modern debate was tion. The humanchin is a commonlycited example instigated by and Richard of a spandrel. Lewontin's famous article "The Spandrels of San Gould and Lewontin's attack on adaptationism Marco and the Panglossian Paradigm" (Gould and provoked an array of different reactions.Some of Lewontin 1979). These authors launched a forth- their opponentsaccused them of caricaturingadap- right attack on what they saw as the extreme adap- tationism and thus attacking a strawman, on the tationism prevalent in many evolutionary circles. grounds that no evolutionist had ever claimed They accused adaptationists of (a) uncritically as~ every phenotypic trait of every organism to be suming that every organismic trait must have an adaptation,less still an optimal adaptation.There evolutionary function, (b) failing to accord a proper is certainly an element of truth to this charge. role to forces other than natural selection in evolu- Nonetheless,Gould and Lewontin were writing at tion, and (c) paying insufficient heed to the con- the height of the controversyover human sociobi- straining factors that limit selection's power to oJogy;and it is also true that some of the early modify phenotypes at will. Unusually for a scientific proponentsof that disciplineadvanced highJy spec- article, "Spandrels" contains two striking literary ulative hypothesesabout the supposedevolution- allusions. Firstly, adaptationists are compared to ary function of various behavioral patterns in Dr. Pang)oss, a protagonist in Voltaire's satirical humans,often on the basisof flimsy and anecdotal novel Candide; who despite suffering terrible mis- evidence.(This was not true of the best work in fortunes continues to believe that he inhabits the human .)Gould and Lewontin's cri~ "best of all possible worlds." GouJd and Lewontin's tique, evenif overstated,was a usefulcorrective to suggestion is that adaptationists commit a similar this sort of naiveadaptationism and led to a greater absurdity by viewing every aspect of an organism's degree of methodological self-awarenessamong phenotype as optimized by selection. Secondly, evolutionary biologists. . adaptationists are accused of inventing "Just So With hindsight,it seemsthat Gould and Lewon- Stories" in their relentless search for evolutionarv tin's article has tempered,but not altogethereIi~ functions, that is, devising speculativehypotheses minated, the enthusiasm felt by evolutionary about traits' adaptivesignificance that owe more to biologists for adaptationism (cr. Walsh, "Span- their ingenuity than to empirical evidence.The ref- drels," forthcoming). Many biologistscontinue to erencehere is to Rudyard Kipling's famouscolJec- believe that cumulative natural seleclion over a tion of children's stories,which include "How the large number of generationsis the most plausible Leopard Got Its Spots" and "How the CamelGot way of explaining complex adaptive traits, and His Hump." such traits are abundant in nature. And despite The title of Gould and Lewontin's paper illus- the potential methodological pitfalls that the trates what is perhaps their central complaint "Spandrels"paper warns against, the adaptationist against adaptationist logic: the assumption, in researchprogram continues to be highly fruitful, advanceof spedfic empirical evidence,that every yielding rich insights into how nature works, and

6 ADAPTATION AND ADAPT A TIONISM it has no seriousrivals. Moreover, it is possibleto References test hypothesesabout the adaptive significanceof Ariew, Andre, Robert Cummins, and Mark Perlman (cds.) particular traits, in a variety of different ways.The (2002), Functions: N~ E5Saysin the Philo~'ophyof P.ry- comparative method, which involves comparing chologyand Biolugy. Oxford: Oxford University Press. closelyrelated species and trying to correlatephe- Bejerano, Gill, Michael Pheasant. Igor Makunin, Stuart notypic differencesamong them with ecological Stephen, W. James Kent, John S. Mattick, and David Haussler (2004), "Ultraconserved Elements in the differencesamong their habitats, is one of the Human Genome," Science304: 1321-1325. most common (cf. Harvey and Pagel 1991);it was Butter, David J. (ed.) (1999), Fllnction, Selectionund Design. employedby Darwin himself in his discussionof Albany: State University of Press. the Galapagosfinches' beaks. Experimentally alter- Darwin, Charles (1859), On Ihe Origin t>l Spedt!.~by Mean.r ing a trait, e.g.,painting the plumageof a bird, and of Natural Selection.London: John Murray. Dawkins, Richard (1986" The Blind Watchmaker. New then carefully observing the effect on the - York: W. W. Norton. ism's survival and reproductivesuccess is another (1996), Climbing Moul1Ilmp/'ob(lble.New York: W. way of learning about a trait's adaptive signifi- W. Norton. cance.The most sophisticatedwork in evolutionary Dennett, Daniel (1995), Dant'in 's DangerOlLtIdea. New biology routinely usesthese and other teststo ad- York: Simon and Schuster. Dupre, John (ed.) (1987), The Latt'st on the Best: Er,wly.tOil judicate hypothesesabout evolutionary function, Evolution and Opt/nut/ity. Cambridge, MA: MIT Press. and they bear little relation to the crudestoryteUing Endler, John A. (1986), Natural SelecTionin the Wild. Prin- that Gould and Lewontin criticize. (See Endler ceton, NJ: Princeton University Pres.<;. 1986for a good discussionof thesetests.) Fisher, Ronald A. (1930), The Gmcrical Theory Qf Ncilural On the other hand, there is a grain of truth to Select/on.Oxford: Clarendon Press. Fodor, Jerry (2000), The Mind Doe.tn't Work That Way. Gould and Lewontin's chargethat when a particu- Cambridge MA: MIT Press. lar hypothesisabout a trait's adaptive function is Godfrey-Smith, Peter (2001), "Three Kinds of Adaptation- falsified, biologists will nonnal1y invent another ism." In ElJioLt Sober and Steven Hecht Orzack (eds.), adaptationisthypothesis rather than concludethat AdaptatiottiSII1unci Opt/maUty. Cambridge: Cambridge the trait is not an adaptation at all. However,not University Press.335-357. Gould, Stephen Jay, and Elizabeth Vrba (1982), "Exapta- everyoneagrees that reasoningin this way is meth- lion: A Missing Term in the Scienceof Form," Paleobi- odologically suspect. agreesthat o/~g)18:4-15. . adaptationists like himself offer "purely theory- Gould, Stephen Jay, and (1979), "The driven explanations,argued a priori from the as- Spandrelsof San Marco and the PanglossianParddigm: sumption that natural selection tells the true A Critique of the Adaptationist Programme," Pruc('ed- ingsofthe RoyaISoderyofLont/on. SeriesB 205:581-598. story-some true story or other-about everycurl. Harvey, Paul, and Mark Pagel (1991), The Compa1'(lti~'e ous featureof the biosphere,"but he regardsthis as Method in Evolutionary Biowgy. Oxford: Oxford Univer- perfectly reasonable,given the overall successof sity Press. Darwinian theory (1995,245). It is doubtful wheth- Lewontin, Richard (1985),"Adaptation."In Richard Levins er what Dennett says is literally true, however. and Richard Lewontin (eds.), Tilt: Diulet'til'al Biologist. Cambridge MA: Press,65-84. Thereare many "curious features"of the biosphere Maynard Smith., John (1978), "Optimization Theory in for which it is not known whether there is an Evolution," Annual Rt",iew of and Sy.ftematic.t adaptationist story to be told or not. Take for 9: 31-56. cxampk the prevalem.:t:of repeatsequences of non- Maynard Smith, John, Richard Burian, Stuart. Kaufman, coding "junk" DNA in the eukaryotic genome. Pen: Alberch, JamesCampbell, Brian Goodwin, Russell Lande, David Raup, and (1985),"Devel- This certainly qualifies as a curious feature-it opmental Constraints and Evolution," Quartcrly Rev;cw took molecular biologists greatly by surprise ofB;ology 60: 265-287. when it was first discoveredin the 19708.But junk Pinker, Steven,and Paul Bloom (1990),"Natural Language DNA has no known function-hence its name- and Natural Selection," Behuvloraland Brain Sciences and many peoplesuspect that it hasno function at 13: 707-784. Sober, ElJiott, and Steven Hecht Orzack (eds.) (2001), all (though the current evidenceon this point is Adaptation/sm cl1ldOptimality. Cambridge: Cambridge equivocal; see Bejerano et a1. 2004 for a recent University Press. assessment).So although Dennett is right that Sterelny,Kim, and Paul Griffiths (l999). Sex and : An there is a generalpresumption in favor of adapta- Introduction to Philosophyof Biology. Chicago: Chicago tionist explanationsamong biologists, it is not true University Press. Walsh, Denis (ed.) (forthcoming), Spandrelsof Sun Mal'co: that every trait is automaticallyassumed to be an 25 YearsLater. Cambridge: Cambridge University Press. adaptation. SAMJR OKASHA See also Evolution; Fitness~ Natural Selection