Industrial Melanism

Abstract

Industrial melanism refers to the evolution of dark body colours in animal species that live in habitats blackened by industrial soot. The phenomenon has been documented in numerous species that hide from predators by blending in with their backgrounds. Peppered moths provide one example. Before the industrial revolution, peppered moths in the UK were pale grey, but after their habitats became polluted with soot from coal‐fired industries, melanic (black) phenotypes became numerous and spread to other regions. Away from industrial centres, the pale phenotype remained common. Following clean air legislation a century later, the atmosphere improved, soot‐damaged habitats gradually recovered, and the pale phenotype returned as the predominant form. Parallel changes have occurred in America. The melanic and pale phenotypes are determined by genes, and the changes in their percentages in populations reflect natural selection. Experiments identify bird predation on the moth phenotypes as the agent of selection.

Key Concepts:

  • The natural colour patterns of animals are adaptations produced by natural selection.

  • A change in frequency (percentage) of genetically determined phenotypes in natural populations is direct evidence of evolutionary change.

  • Mutations introduce new genetic variation to a population, but recurrent mutations occur too rarely to bring about rapid changes in the frequency of genes.

  • Random changes in the frequency of genes (genetic drift) are irregular and unpredictable in direction.

  • Directional, rapid changes in the frequency of genetically determined phenotypes in populations result from natural selection.

  • Historical records on phenotypic frequencies from population samples allow the assessment of natural selection.

  • Gene flow (migration) retards genetic differentiation among geographically widespread populations.

  • Clines indicate different selection pressures along environmental gradients; when selection is removed, migration homogenises the differences along a cline.

  • Parallel evolution is ‘nature's replicate experiment’.

Keywords: bird predation; Biston betularia; camouflage; carbonaria; crypsis; natural selection; observable evolution; parallel evolution; peppered moths; polymorphism

Figure 1.

Polytypism and polymorphism in wing patterns of peppered moths. The side‐by‐side comparison illustrates the differences and similarities of British Biston betularia betularia (left column) and the American subspecies, B. betularia cognataria. At rest the wings span 4–5 cm. The American ‘typical’ phenotype (top right) is generally darker than the British typical (top left) and exhibits continuous lines marking the wings. The lines marking British typicals are variously interrupted producing a mottled or speckled pattern of white and black. The bottom pair illustrates fully melanic phenotypes of both subspecies. They are both virtually solid black and are indistinguishable from one another. Intermediates also occur in natural populations. The phenotypes result from multiple alleles at the same locus in both subspecies (Grant, ). Illustrations by Derek Whiteley, from Grant et al. cover. Reproduced by permission of the American Genetics Association.

Figure 2.

The decline in frequency of melanic Biston betularia at Caldy Common in West Kirby, UK, from 1959 to 2002. Average annual sample sizes were 437. Data from Clarke et al., Grant and Clarke and 1999–2003 from S. Thompson (personal communication).

Figure 3.

A before‐and‐after comparison of the geographic distribution of melanic phenotypes of Biston betularia in Britain. The black segments of the pie charts represent the frequency of melanics at specific locations positioned on the maps, and the white segments represent the combined frequencies of typicals and ‘insularia’ (intermediates). The left map is abridged from the first national survey between 1952 and 1956 (summarised in Kettlewell, ). The locations selected from the early survey fall closest in position to the locations surveyed in 1996 graphed on the map to the right. Reproduced from Grant et al., with permission from American Genetic Association.

Figure 4.

A comparison of melanic (carbonaria) frequencies in population samples across northern Britain extending from Abersoch, Wales, to Leeds, England. Liverpool and Manchester are included in the transect at 120 and 160 km, respectively. Early samples taken during the 1960s and 1970s are indicated by solid and open circles, respectively. Recent samples (squares) are from 2002. The dashed line describes the early, steep cline. The solid line is the theoretical prediction after 30 generations of selection against carbonaria (s=0.19) and maximum likelihood estimates of dispersal based on low levels of genetic differentiation among nine independent microsatellite loci along the melanic cline. Reproduced with permission from Saccheri et al.. Copyright, National Academy of Sciences, USA.

Figure 5.

The decline of melanism in American peppered moths, Biston betularia cognataria, in the northern states – Michigan (solid circles) and Pennsylvania (open circles) – and the relative absence of melanism in a southern state – Virginia (inverted triangles). Data from Grant and references in Grant and Wiseman .

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Further Reading

Berry RJ (1990) Industrial melanism and peppered moths (Biston betularia L.). Biological Journal of the Linnean Society 39: 301–322.

Cook LM, Dennis RHL and Mani GS (1999) Melanic morph frequency in the peppered moth in the Manchester area. Proceedings of the Royal Society of London Series B 266: 293–297.

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Lees DR (1981) Industrial melanism: genetic adaptation of animals to air pollution. In: Bishop JA and Cook LM (eds) Genetic Consequences of Man‐made Change, pp. 129–176. New York: Academic Press.

Mallet J (2004) The peppered moth: a black and white story after all. Genetics Society News 50: 34–38.

Rudge DW (2005) Did Kettlewell commit fraud? Re‐examining the evidence. Public Understanding of Science 14: 369–375.

Rudge DW (2010) Tut‐tut Tutt, not so fast. Did Kettlewell really test Tutt's explanation of industrial melanism. History and Philosophy of the Life Sciences 32: 493–520.

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Grant, Bruce S(Jun 2012) Industrial Melanism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001788.pub3]