Exaptation

Exaptation is the evolutionary process by which a trait that was either adapted for one function, or arose as a non-adaptive byproduct, is subsequently coopted for a new and different function. The term was introduced by Stephen Jay Gould and Elisabeth Vrba in 1982 as a corrective to the assumption that every currently functional trait was selected for that function. Feathers are the paradigm case: they appear to have originated as adaptations for thermoregulation in theropod dinosaurs, then were exapted for flight in the lineage leading to birds. The immune system exapted ancient genomic defense mechanisms for adaptive immunity. Language, on influential accounts, exapted neural machinery evolved for motor planning and social cognition.

Exaptation dissolves the false dichotomy between adapted for and not adaptive: evolutionary history is opportunistic, coopting whatever variation is available for whatever function selection currently favors. The concept belongs beside Adaptation as a permanent corrective: understanding what a trait is for now does not reveal what it was selected for originally, and conflating these is the adaptationist error Gould spent his career naming.