Industrial melanism
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.
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.
Industrial melanism was first noticed in 1900 by the geneticist William Bateson; he observed that the colour morphs were inherited, but did not suggest an explanation for the polymorphism.
[1][6] In 1906, the geneticist Leonard Doncaster described the increase in frequency of the melanic forms of several moth species from about 1800 to 1850 in the heavily industrialised north-west region of England.
B. S. Haldane constructed a mathematical argument showing that the rapid growth in frequency of the carbonaria form of the peppered moth, Biston betularia, implied selective pressure.
[18] During the Industrial Revolution in England, sulphur dioxide pollution in the atmosphere reduced the lichen cover, while soot blackened the bark of urban trees, making the light-colored moths more vulnerable to predation.
[29][30][31][32] Michael Majerus however found that Kettlewell was basically correct in concluding that differential bird predation in a polluted environment was the primary cause of industrial melanism in the peppered moth.
[35] Zoologists including L. M. Cook, B. S. Grant, Majerus and David Rudge however all upheld Kettlewell's account, finding that each of Hooper's and the creationists' claims collapsed when the facts were examined.
[3][33][36][37][38][39] It has been suggested that the demonstrated relationship between melanism and pollution can not be fully proven because the exact reason for increase in survivability can not be tracked and pin-pointed.
Cook and J. R. G. Turner have concluded that "natural selection is the only credible explanation for the overall decline",[1] and other biologists working in the area concur with this judgement.
A proposed explanation is that the melanic forms have a thermal advantage directly linked to the pollution aspect of industrialization, since smoke and particulates in the air reduce the amount of sunlight that reaches the habitats of these species.
