Melanism, Population

the preferential spread of the dark-colored individuals of a species.

The black, cinnamon-brown, or gray-brown color of the integuments of animals, which is determined by melanins (pigments), has a variety of genetic origins and may be picked up in natural selection if the dark forms have an adaptive advantage over the light ones. Dark coloration may have a protective function, as in industrial melanism—the displacement of light-colored butterflies and other insects by dark forms in areas with considerable industrial pollution (for example, in the peppered moth Biston betularia in Great Britain). In some cases, the distribution of melanistic individuals is unrelated to the adaptive value of the dark color but has some correlation with physiological characteristics of the organisms or reflects random genetic population shifts (for example, the spread of the melanistic form of the hamster Cricetus cricetus in the Ukraine and Bashkiria).