ADAPTATION AND ADAPTATIONISM 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 biology, a phenotypic trait is said that Darwinism 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 natural selection. 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 evolution. 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 adaptations 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 human 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 function. 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 humans 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, hearts, 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 "exaptation" 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- On the Origin of Species.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 life in the environments Fisher requiresa very high degreeof fit between they inhabit. Considerfor examplethe astonishing organism and environment before the concept of '4 ~ 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.