The term adaptation refers to changes in an organism's structure, function, or behavior that increase its ability to live in a particular environment. As such, adaptation is a central term in the life sciences. The many known examples of animals and plants adapting to their environment were the basis for the theories of evolution formulated by Charles Darwin (1809–1882) and Jean–Baptiste de Lamarck (1744–1829). Adaptation in the Darwinian sense describes a process of evolutionary change by natural selection. In this process the average performance of the individuals in a population with respect to survival and reproduction is improved.

The term adaptation is also used to describe the result of the process of evolutionary change (the state of being adapted) or to describe the "solution" to a problem that is set by the environment. The word is used this way in the adaptationist program, which has been criticized for explaining traits post hoc as having evolved to serve certain functions. Because the environment of any organism is continuously changing, the degree of adaptation is never optimal, and adaptation is, therefore, a never-ending process.

Not all traits in an organism or features of an organism's appearance are necessarily the result of adaptation; they may be by-products of selection acting on other traits. For example, the increased brain size in humans is considered to be a side effect of selection favoring increased body size. Specific traits can also be the result of adaptations for other functions that have since changed. For example, feathers in birds originally evolved to provide insulation, and only later were they used for flying. Physiological adaptations are plastic responses to the physical environment that occur within a lifetime and are not inherited by the next generation. Such adaptations can be of short duration and reversible, such as the adaptation of the eye to light and dark, or they may be long-lasting, such as the increased number of red blood cells in humans who live at high altitudes.

See also EVOLUTION; FITNESS; LIFE SCIENCES; SELECTION, LEVELS OF

VOLKER LOESCHCKE