Industrial melanism

Odontopera bidentata, the scalloped hazel moth, in typical and melanic forms. The dark form became common in polluted areas after the Industrial Revolution.^[1]

Industrial melanism is an evolutionary effect prominent in several arthropods, where dark pigmentation (melanism) has evolved in an environment affected by industrial pollution, including sulphur dioxide gas and dark soot deposits. Sulphur dioxide kills lichens, leaving tree bark bare where in clean areas it is boldly patterned, while soot darkens bark and other surfaces. Darker pigmented individuals have a higher fitness in those areas as their camouflage matches the polluted background better; they are thus favoured by natural selection. This change, extensively studied by Bernard Kettlewell (1907–1979), is a popular teaching example in Darwinian evolution, providing evidence for natural selection.^[2] Kettlewell's results have been challenged by zoologists, creationists and the journalist Judith Hooper, but later researchers have upheld Kettlewell's findings.^[3]

Industrial melanism is widespread in the Lepidoptera (butterflies and moths), involving over 70 species such as Odontopera bidentata (scalloped hazel)^[1] and Lymantria monacha (dark arches),^[4] but the most studied is the evolution of the peppered moth, Biston betularia. It is also seen in a beetle, Adalia bipunctata (two-spot ladybird), where camouflage is not involved as the insect has conspicuous warning coloration, and in the seasnake Emydocephalus annulatus where the melanism may help in excretion of trace elements through sloughing of the skin. The rapid decline of melanism that has accompanied the reduction of pollution, in effect a natural experiment, makes natural selection for camouflage "the only credible explanation".^[1]

Other explanations for the observed correlation with industrial pollution have been proposed, including strengthening the immune system in a polluted environment, absorbing heat more rapidly when sunlight is reduced by air pollution, and the ability to excrete trace elements into melanic scales and feathers.