Exaptation

Birds initially developed wings and feathers as a means of heat regulation. The use of wings for flight is an example of exaptation.

Source: wiktionary

I believe that Stephen Gould and Elizabeth Vrba were correct in proposing exaptation as a missing term in evolutionary biology. I want to make a distinction between adaptation and exaptation here. Again it is about mechanistic differences. An exaptation can be viewed as the acquisition of a new and useful function once the novelty has spread and once the environment has changed.

Source: wiktionary

These presumed exaptations can be assigned to major categories associated with theories of movement (Nathan et al. 2008). In table 1.2, we list five categories of exaptations that we believe make resident birds possessing them preadapted for migration.

Source: wiktionary

An interesting consequence of this way of characterizing exaptations is that whether or not a trait counts as an exaptation depends upon how we describe it. Human legs are adaptations for locomotion, but exaptations for walking.

Source: wiktionary