Preadaptation

In evolutionary biology, preadaptation describes a situation where a species evolves to use a preexisting structure or trait inherited from an ancestor for a potentially unrelated function. One example of preadaptation is dinosaurs having used feathers for insulation and display before using them to fly, or sweat glands in mammals being transformed into mammary glands.

Another example is the hypothesis proposed by zoologist Jonathan Kingdon that before early humans became bipedal, they began engaging in squat feeding, i.e. turning over rocks and leaves to find insects, worms, snails and other food. Consequently, they adapted flatter feet than were necessary in their previous tree-dwelling ancestors, since that makes squatting much easier. Flatter feet are also extremely useful for bipedal animals, so they can be described as a preadaptation to bipedalism, even though (or rather because) the adaptation had nothing to do with bipedalism originally.

Arthropods provide the earliest identifiable fossils of land animals, from about in the Late Silurian, and terrestrial tracks from about  appear to have been made by arthropods. Arthropods were well pre-adapted to colonize land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.

Some biologists dislike the term 'preadaptation' as it could imply an intentional plan, which is contrary to the nature of evolution. Some alternative terms that have been suggested include "co-option" and exaptation, to avoid the implication of foresight.

Some phenomena could give the appearance of foresight, however, without actually involving it, being instead attributable to simple probability. For example, future environments (for example, hotter or drier ones), may resemble those already encountered by a population at one of its current spatial or temporal margins. This is not actual foresight, but rather the luck of having adapted to a climate which later becomes more prominent. Cryptic genetic variation may have the most strongly deleterious mutations purged from it, leaving an increased chance of useful adaptations, but this represents selection acting on current genomes with consequences for the future, rather than foresight.