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1. adjustment of the pupil to light, constricting with increased light intensity, dilating with decreased intensity.
2. any anatomical, physiological, developmental or behavioral adjustment to the environment of an organism which enhances its chances of leaving descendants. The ability of animals to adapt to a limited supply of drinking water and to high or low environmental temperatures is an important aspect of animal husbandry. The selection of animals which are capable of a high level of such adaptation has made it possible to improve the productivity of herds and flocks in some countries. See also general adaptation syndrome.
3. the process by which organisms are modified so as to improve their chances of survival in an environment.
adaptation of the eye to vision in the dark or in reduced illumination.
adaptation of the eye to vision in sunlight or in bright illumination (photopia), with reduction in the concentration of the photosensitive pigments of the eye.
the rate at which afferent sensory receptors discharge into their afferent axons. The rates differ between different types of receptors. For example, there are slow adaptors which signal the more persistent changes such as steady pressure. See also receptor adaptation (below).
sensory receptors vary in their individual response to stimuli, the response declining after an initial period of rapid response. The rate at which different kinds of receptors change these responses is the adaptation rate (see above).