exapted

Exapted is used in biology to describe a trait that originated for one function but was later repurposed for a different function. The noun form is exaptation, a concept introduced by Stephen Jay Gould and Elisabeth Vrba in 1982 to emphasize that evolutionary change can involve reusing existing structures rather than creating new ones from scratch. An exapted trait is not necessarily an adaptation for its current use; it is a historical reconfiguration that proved advantageous in a new context.

In practice, exaptation refers to a shift in function of a structure over evolutionary time. It is distinct from straightforward adaptation, which denotes a feature that evolved because of selective pressure for its present role. Exaptation does not imply intentional design; it reflects historical contingencies and gradual modification.

Common examples include feathers in birds: originally evolved for insulation or display, later co-opted for flight. The mammalian middle ear bones, such as the malleus and incus, are often cited as exaptations because these bones were once part of the jaw joint in ancestral species and were repurposed to enhance hearing in mammals. Another frequently discussed case is the panda’s enlarged radial sesamoid, or “pseudo-thumb,” which is used to grasp bamboo and illustrates a structural repurposing rather than a trait selected for its current use.

In contemporary discussions, exaptation helps explain why some traits seem ill-suited to their present function when viewed in isolation and highlights the complexity of evolutionary history. Some critics caution that the term can be over-applied or imprecise, and alternatives such as co-option or functional shift are used for greater clarity.