States of organismic phenotypes (an item of behavior, physiological process, or anatomical property) shaped by natural selection to perform a specific role. The evolutionary process of natural selection acting to shape, maintain, or modify such properties is also known as adaptation. The theory of adaptation is the evolutionary biological explanation for the design apparent in nature, whereby organisms appear to display a close fit to their environments. Adaptation is the central focus of Darwin’s original formulation of evolutionary theory and of most modern formulations of the evolutionary process.
Much remains to be learned about the process of adaptation. On the one hand, theorists since Darwin have argued that selection should constantly improve the quality of adaptations or modify adaptations to keep pace with changing environments. According to this view of adaptation, constant, gradual change should be the norm. On the other hand, many species remain stable in most of their characteristics for long periods of their history (the phenomenon of stasis), and thus it is assumed that natural selection lends stability and conserves adaptations for large portions of a species’ history. According to this “punctuational” view, adaptive change is relatively rare in evolution, is relatively rapid when it occurs, and is most often associated with speciation.
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