in biology, process by which an animal or plant becomes fitted to its environment; it is the result of natural selection acting upon heritable variation. Even the simpler organisms must be adapted in a great variety of ways: in their structure, physiology, and genetics; in their locomotion or dispersal; in their means of defense and attack; in their reproduction and development; and in other respects.

Accurate adaptations may involve migration to, or survival in, favourable conditions of, for example, temperature. Alternatively, organisms may partly manufacture their own environment, as do the mammals, for example, which produce their own optimum body temperature. To be useful, adaptations must often occur simultaneously in a number of different parts of the body. A change from a more carnivorous to a more vegetarian diet necessitates alterations not only of the teeth, digestive juices, and length of the digestive tract but also in habit and defense mechanisms.

Some of the most fundamental biological adaptations are chemical and genetic. The conditions in which the cells of the body can live are restricted and have changed very little since life first arose in the sea. Exploitation of fresh water and of land was possible only through the evolution of adaptive mechanisms capable of maintaining at least a semblance of the original constitution of body fluids. Thus, even in humans and other mammals, the blood is chemically quite closely related to seawater.