Copyright © 1999 Massachusetts Institute of Technology
In current usage a biological adaptation is a trait whose form can be explained by natural selection. The blink reflex, for example, exists because organisms with the reflex were fitter than organisms without this adaptation to protect the eyes. Biological adaptation must be distinguished from physiological adaptation. The fact that human beings can form calluses when their skin is subjected to friction is probably a biological adaptation, but the particular callus caused by my hedge-trimmers is not. The location and form of this particular callus cannot be explained by the differential reproduction of heritable variants, that is, by natural selection. Adaptation is still used in its nonevolutionary sense in disciplines such as exercise physiology. An adaptive trait is one that currently contributes to the fitness of an organism. The ability to read is highly adaptive, but is unlikely to be an adaptation. Reading is probably a side effect of other, more ancient cognitive abilities. There are also adaptations that are no longer adaptive. The human appendix is a vestigial trait -- a trace of an earlier process of adaptation.
Some authors distinguish adaptations from exaptations (Gould and Vrba 1982). A trait is an exaptation if it is an adaptation for one purpose but is now used for a different purpose. It is unlikely that feathers evolved from scales because they helped the ancestors of birds to fly better. It is thought that they evolved as insulation and were only later exapted for flight. Other authors doubt the value of the exaptation concept (Griffiths 1992; Reeve and Sherman 1993). The importance of the concept of adaptation in biology is that it explains many traits of the organisms we see around us. It explains not only how traits first arose but also why they persisted and why they are still here. If we want to understand why there are so many feathers in the world, their later use in flight is as relevant as their earlier use in thermoregulation.
The adaptation concept underwrites the continuing use of teleology in biology, something that distinguishes life sciences from the physical sciences (Allen, Bekoff, and Lauder 1997). Adaptations have biological purposes or functions -- the tasks for which they are adaptations. Hemoglobin is meant to carry oxygen to the tissues. It is not meant to stain the carpet at murder scenes, although it does this just as reliably. Some authors use the term teleonomy to distinguish adaptive purposes from earlier concepts of natural purpose. The fact that the adaptation concept can create naturalistic distinctions between function and malfunction or normal and abnormal has made it of the first interest to cognitive science. Several authors have used the adaptation concept in analyses of INTENTIONALITY.
To identify an adaptation it is necessary to determine the selective forces responsible for the origins and/or maintenance of a trait. This requires understanding the relationship between organism and environment, something more onerous than is typically recognized (Brandon 1990). Some biologists think this task so onerous that we will frequently be unable to determine whether traits are adaptations and for what (Gould and Lewontin 1978; Reeve and Sherman 1993). Others argue that we can successfully engage in both reverse engineering -- inferring the adaptive origins of observed traits -- and adaptive thinking -- inferring what adaptations will be produced in a particular environment (Dennett 1995; Dawkins 1996). Many advocates of EVOLUTIONARY PSYCHOLOGY believe that adaptive thinking about the human mind has heuristic value for those who wish to know how the mind is structured (Cosmides, Tooby, and Barkow 1992).
Adaptationism is the name given by critics to what they see as the misuse of the adaptation concept. Steve Orzack and Elliot Sober (1994) distinguish three views about adaptation: first, that adaptation is ubiquitous, meaning that most traits are subject to natural selection; second, that adaptation is important: a "censored" model that deliberately left out the effects of natural selection would make seriously mistaken predictions about evolution; and third, that adaptation is optimal: a model censored of all evolutionary mechanisms except natural selection could predict evolution accurately. Most biologists accept that natural selection is ubiquitous and important. In Orzack and Sober's view the distinctive feature of adaptationism is the thesis that organisms are frequently optimal. They argue that the adaptationist thesis should be empirically tested rather than assumed. Other authors, however, argue that adaptationism is not an empirical thesis, but a methodological one. Optimality concepts provide a well defined goal which it is equally illuminating to see organisms reach or to fall short of. Building models that yield the observed phenotype as an optimum is the best way to identify all sorts of factors acting in the evolutionary process (Maynard-Smith 1978).
There are several strands to antiadaptationism. One is the claim that many adaptive explanations have been accepted on insufficient evidence. Adaptationists claim that the complexity and functionality of traits is sufficient to establish both that they are adaptations and what they are adaptations for (Williams 1966). Antiadaptationists argue that adaptive scenarios do not receive confirmation merely from being qualitatively consistent with the observed trait. Some are also unsatisfied with quantitative fit between an adaptive model and the observed trait when the variables used to obtain this fit cannot be independently tested (Gray 1987). Many antiadaptationists stress the need to use quantitative comparative tests. Independently derived evolutionary trees can be used to test whether the distribution of a trait in a group of species or populations is consistent with the adaptive hypothesis (Brooks and McLennan 1991; Harvey and Pagel 1991). Other strands of antiadaptationism are concerned with broader questions about what biology should be trying to explain. Biology might focus on explaining why selection is offered a certain range of alternatives rather than explaining why a particular alternative is chosen. This would require greater attention to developmental biology (Smith 1992; Amundson 1994; Goodwin 1994). Another antiadaptationist theme is the importance of history. The outcome of an episode of selection reflects the resources the organism brings with it from the past, as well as the "problem" posed by the environment (Schank and Wimsatt 1986; Griffiths 1996). Finally, antiadaptationists have questioned whether the environment contains adaptive problems that can be characterized independently of the organisms that confront them (Lewontin 1982; Lewontin 1983).
Allen, C., M. Bekoff, and G. V. Lauder, Eds. (1997). Nature's Purposes: Analyses of Function and Design in Biology. Cambridge, MA: MIT Press.
Amundson, R. (1994). Two concepts of constraint: adaptationism and the challenge from developmental biology. Philosophy of Science 61(4):556-578.
Brandon, R. (1990). Adaptation and Environment. Princeton: Princeton University Press.
Brooks, D. R., and D. A. McLennan. (1991). Phylogeny, Ecology and Behavior. Chicago: University of Chicago Press.
Cosmides, L., J. Tooby, and J. H. Barkow. (1992). Introduction: evolutionary psychology and conceptual integration. In J. H. Barkow, L. Cosmides, and J. Tooby, Eds., The Adapted Mind: Evolutionary Psychology and the Generation of Culture. Oxford: Oxford University Press, pp. 3-15.
Dawkins, R. (1996). Climbing Mount Improbable. London: Viking.
Dennett, D. C. (1995). Darwin's Dangerous Idea. New York: Simon and Schuster.
Goodwin, B. C. (1994). How the Leopard Changed its Spots: The Evolution of Complexity. New York: Charles Scribner and Sons.
Gould, J. A., and E. S. Vrba. (1982). Exaptation -- a missing term in science of form. Paleobiology 8:4-15.
Gould, S. J., and R. Lewontin. (1978). The Spandrels of San Marco and the Panglossian Paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London 205:581-598.
Gray, R. D. (1987). Faith and foraging: a critique of the "paradigm argument from design." In A. C. Kamil, J. R. Krebs and H. R. Pulliam, Eds., Foraging Behavior. New York: Plenum Press, pp. 69-140.
Griffiths, P. E. (1992). Adaptive explanation and the concept of a vestige. In P. E. Griffiths, Ed., Essays on Philosophy of Biology. Dordrecht: Kluwer, pp. 111-131.
Griffiths, P. E. (1996). The historical turn in the study of adaptation. British Journal for the Philosophy of Science 47(4):511-532.
Harvey, P. H., and M. D. Pagel. (1991). The Comparative Method in Evolutionary Biology. Oxford: Oxford University Press.
Lewontin, R. C. (1982). Organism and environment. In H. Plotkin, Ed., Learning, Development, Culture. New York: Wiley, pp. 151-170.
Lewontin, R. C. (1983). The organism as the subject and object of evolution. Scientia 118:65-82.
Maynard Smith, J. (1978). Optimisation theory in evolution. Annual Review of Ecology and Systematics 9:31-56.
Orzack, S. E., and E. Sober. (1994). Optimality models and the test of adaptationism. American Naturalist 143:361-380.
Reeve, H. K., and P. W. Sherman. (1993). Adaptation and the goals of evolutionary research. Quarterly Review of Biology 68(1):1-32.
Schank, J. C., and W. C. Wimsatt. (1986). Generative entrenchment and evolution. Proceedings of the Philosophy of Science Association.Vol 2:33-60.
Smith, K. C. (1992). Neo-rationalism versus neo-Darwinism: integrating development and evolution. Biology and Philosophy 7:431-452.
Williams, G. C. (1966). Adaptation and Natural Selection. Prince ton: Princeton University Press.
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