Evolution by Anthropogenic Sources — Industrial Melanism

Industrial melanism in the peppered moth, Biston betularia, is the classic real-world demonstration of natural selection in action. Before industrialisation, tree bark and building walls in England were pale-coloured, so the light-coloured form of the moth was well camouflaged and the rarer dark (melanic) form stood out and was more easily caught by predators; both colour forms nonetheless persisted in the population. After industrialisation, soot and smoke from factories darkened tree trunks and walls across industrial areas. This reversed the camouflage advantage: the dark moths now blended into the sooty bark while the light moths became conspicuous and were preferentially eaten by predatory birds. Over successive generations the proportion of dark-coloured moths in the population rose sharply relative to the light form, because nature was now applying positive selection pressure to the dark phenotype. The case demonstrates, in a directly observable and historically documented setting, that when an environmental change alters which phenotype survives best, …