Exaptation

Exaptation is an evolutionary process in which a trait that evolved for one purpose becomes useful for a different function. The term was introduced by Stephen Jay Gould and Elizabeth Vrba in 1982 to emphasize that features can acquire new roles through change in context, environment, or selective pressures, rather than being directly evolved for their current use. Exaptation is distinct from adaptation, which refers to traits that have been shaped by natural selection for their present function.

Common examples illustrate the concept. Feathers likely originated for insulation, camouflage, or display and were later co-opted for flight in birds. The panda’s pseudo-thumb, a broadened radial sesamoid bone, helps grasp bamboo and represents an existing structure repurposed for a new task. In mammals, the jaw bones of ancestral species were repurposed over time to become the middle ear bones (malleus and incus), enabling more efficient hearing. Forelimbs that evolved for one set of tasks can also be exapted for flight or other functions in different lineages, illustrating how a structure’s role can shift.

Exaptation is often discussed alongside the related idea of spandrels, which are traits that arise as byproducts of other evolutionary changes rather than through direct selection for their current use. Recognizing exaptation helps explain why complex traits may persist and later acquire new functions, highlighting the nonlinear path of evolutionary innovation.