Adaptation

In evolutionary biology, an adaptation is a heritable feature that arose and spread through natural selection because it improved survival or reproduction in a particular environment. Adaptations can be structural, like a platypus's webbed feet or a snow leopard's thick fur; chemical or physiological, like a protein that works at body temperature or a plant toxin that holds back rival plants; or behavioral, like a bat's use of echolocation to hunt insects. The word also names the evolutionary process that produces such traits.

Whether a trait is adaptive depends on the environment. Large leaves may help a plant in a wet climate but become a burden if the climate dries, when smaller leaves that lose less water are favored instead. Adaptations are therefore never perfect or final, and natural selection can only build on variation that already exists.

Not every useful-looking feature is an adaptation. Changes an individual undergoes during its own life, such as a suntan or muscle built by training, are not inherited, so they are not evolutionary adaptations. Vestigial structures, such as the reduced eyes of some cave fish, are leftovers from ancestors rather than adaptations to present conditions. In physiology, the word also has a separate, short-term meaning, as when the eye adjusts to darkness.