An exaptation is a biological adaptation where the biological function currently performed by the adaptation was not the function performed while the adaptation evolved under earlier pressures of natural selection.
The origin of the term exaptation is attributed to the evolutionary biologists Stephen J. Gould and Elizabeth Vrba "Exaptation - a missing term in the science of form," Paleobiology 8 (1982): 4-15 - an explanation of how complex physical traits might evolve from simpler structures.
The function defining an exaptation is often the primary function performed by the anatomical structure, physiological process or behavioural trait. Furthermore, this defining function is usually postulated as replacing a defunct function of the adaptation that gave an advantage to the organism under earlier pressures of natural selection. Thus, the postulated, defunct function and its earlier, evolutionary advantages are often a subject of contention - in detail if not in fact.
Controversies range in degrees, between: there are no exaptations to all complex organisms are exaptations (of the constituent simpler organism’s adaptations).
One problem that had been troubling Darwinian evolution was the question of how complex structures could evolve gradually if their earlier incarnations would not have a survival advantage. As one of Darwin's critics, Mivart, pointed out, a bird's wing, for example, would not be of much use at 5% of what is needed for flying. If there were no adaptive value to the earlier structures, it would seem unlikely that the trait would survive long enough to become of use to the organism.
Gould and Vrba, using earlier theories on preadaptation, coined the term exaptation to explain how such traits may evolve. Complex physical traits, they hypothesized, evolved from earlier traits that had a different adaptive value. 5% of a wing, especially if covered in air-trapping feathers, could have been a very efficient thermoregulator: a cover to duck under when too cold and to lift up when too warm. Organisms with such an adaptation would thus be fitter than those without, leading to the spread of this trait.
The thermoregulator would grow larger over time, and become covered in thinner feathers, as this would be an improvement over smaller, less well covered versions. Eventually, the thermoregulator would have grown so large that some of these animals would learn to glide with it. At this point the structure would have become an entirely new trait. Organisms with wings could glide, greatly improving their mobility. Now gradual evolution could take place on the wing instead of the thermoregulator, and the structure and organisms themselves would become entirely adapted to flying.
The evolution of numerous other physical traits have been ascribed to exaptation: The bones of vertebrates, which appear very early in the evolution of our aquatic ancestors, may have originally been formed as protective armor or as a method of storing calcium phosphate. Later, when vertebrates migrated to the land, these structures found a new adaptive function in support.
The term has spread within literature on evolutionary biology. Hence: exapted used as a verb, where a biological function exapted an earlier function of an adaptation.