industrial melanism


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industrial melanism

a phenomenon found in several groups, notably moths, in which a heavily pigmented variant (a MORPH) becomes the most frequent type in an area with heavy, man-made atmospheric pollution. Melanism is an excellent example of MICROEVOLUTION, in that rapid evolutionary change has been brought about by strong forces of natural selection acting in favour of a dominant allele for melanism. The agents of selection against moths (such as the peppered moth Biston betularia) are birds, which predate more heavily those types resting against a colour-contrasting background: poorly pigmented forms resting on sooty tree trunks, for example. The proportion of melanics to nonmelanics in an area is correlated with the level of pollution, although neither type appears to be completely absent in any environment, creating a GENETIC POLYMORPHISM. See also KETTLEWELL.
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