Adaptation, Biological

Adaptation has a diversity of meanings, even within areas in which it is widely used, such as anthropology, biology, the humanities, and in common parlance.

The study of adaptations is a central activity in biology, where interpretations of the concept have received much scrutiny in recent years, for example, in the articles and monographs of Andrews, Brandon,Gould and Lewontin, Gould and Vrba, Rose and Lauder, Sober, and Williams. Gould and Verba pointed out the presence of two distinct adaptation concepts in the literature: one historical, emphasizing traits' origins and their past histories of selection, the other nonhistorical, emphasizing current functions of traits and their contributions to fitness. For example, some argue that to be regarded as an adaptation, a trait must have been produced by natural selection, and so must be genetically inherited. Gould and Lewontin distinguished adaptations from exaptions. The latter are preexisting traits that at some time in the past acquired beneficial effects without being selected for them at that time. However, exaptions may subsequently be modified by natural selection as a result of their new functions. The concept of exaptions seems to have little relevance to studies of extant species.

A meaning common in physiology and the social sciences is that an adaptation is a beneficial modification of an organism that adjusts it to changes in the environment. In many cases, these changes are homeostatic. For example, changes in the size of the pupil keep the light intensity at the retina within the optimal range for vision. If effects of such phenotypic adjustments lead to increased survival and reproductive success, that is, to greater fitness, they and the machinery of the body (nerves, muscles, and so on) that produce them are adaptations in the core sense described below.

Another common requirement for a trait to be considered adaptive is that its functional consequences must be consistent with a priori design specifications for accomplishing a specified task. For example, in order to transport oxygen from the lungs to the rest of the body, blood must include a component that binds to oxygen in the lungs yet subsequently releases it in tissues that are oxygen depleted. Hemoglobin has exactly this property, as revealed by its oxygen dissociation curve. Optimality models used in behavioral ecology and many other areas of functional biology [Page 12]are comparable to the engineering specifications used in functional anatomy. Both attempt to identify characteristics of functional designs for specific tasks. At the heart of the use of design specifications is the question: Under existing circumstances, how would a well-adapted organism of this species behave or be structured? Design specifications play an important role in the study of adaptations, at every level of biological organization.

At the core of these diverse interpretations is the idea that adaptations are traits that benefit the organism. In many cases, these benefits result from effects on vital processes, such as maintaining osmotic balance, obtaining food, avoiding predators, caring for offspring, and so forth. The ultimate criterion of benefit is that a trait's effects must be functional, in the sense of enhancing fitness. In some cases, adaptation refers to the processes that select for such traits. Adaptations may be at any level of organization: biochemical, physiological, anatomical, or behavioral.

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