industrial melanism

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 lichen 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, is a popular teaching example in Darwinian evolution, providing evidence for natural selection. Kettlewell's results have been challenged by zoologists, creationists and the journalist Judith Hooper, but later researchers have upheld Kettlewell's findings.

Industrial melanism is widespread in the Lepidoptera (butterflies and moths), involving over 70 species such as '' Odontopera bidentata'' (scalloped hazel) and ''Lymantria monacha'' (dark arches), 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".

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.

History

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.

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.

In 1924, the evolutionary biologist J. 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.

From 1955 onwards, the geneticist Bernard Kettlewell conducted a series of experiments exploring the evolution of melanism in the peppered moth. He used a capture-mark-recapture technique to show that dark forms survived better than light ones.

By 1973, pollution in England had begun to decrease, and the dark ''carbonaria'' form had declined in frequency. This provided convincing evidence, gathered and analysed by Kettlewell and others such as the entomologist and geneticist Michael Majerus and the population geneticist Laurence M. Cook, that its rise and fall had been caused by natural selection in response to the changing pollution of the landscape.

Taxonomic range

Industrial melanism is known from over 70 species of moth that Kettlewell found in England, and many others from Europe and North America.
Among these, '' Apamea crenata'' (clouded border brindle moth) and ''Acronicta rumicis'' (knot grass moth) are always polymorphic, though the melanic forms are more common in cities and (like those of the peppered moth) are declining in frequency as those cities become less polluted.

Among other insects, industrial melanism has been observed in a beetle, '' Adalia bipunctata'', the two-spot ladybird.

In the vertebrates, industrial melanism is known from the turtle-headed seasnake ''Emydocephalus annulatus'', and may be present in urban feral pigeons.

Camouflage

Originally, peppered moths lived where light-colored lichen from predators against that clean background, they had generally light coloration. 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. This provided a selective advantage to the gene responsible for melanism, and the darker-colored moths increased in frequency. The melanic phenotype of '' Biston betularia'' has been calculated to give a fitness advantage as great as 30 per cent. By the end of the 19th century it almost completely replaced the original light-coloured type (var. ''typica''), forming a peak of 98% of the population in 1895.

Melanic ''B. betularia'' have been widely observed in North America. In 1959, 90% of ''B. betularia'' in Michigan and Pennsylvania were melanic. By 2001, melanism dropped to 6% of the population, following clean air legislation. The drop in melanism was correlated with an increase in species diversity of lichen