Industrial Melanism

The rapid life cycle and genetic variability of many butterfly and moth species makes them particularly useful for the study of evolution. One famous example is the classic investigations of the phenomenon of industrial melanism made by H. B. D. Kettlewell (1907-1979) in the early 1950's. Kettlewell worked with the peppered moth, a common species of moth throughout Britain and continental Europe. The moth was known to have two forms: a pale form that was very common and a dark form that was quite rare. Toward the end of the nineteenth century, this situation began to change. The first large-scale burning of coal, coke, and oil associated with the industrial revolution killed off the lichen cover of the unpolluted countryside downwind of manufacturing sites, visibly darkening the surfaces of trees. Coincident with these changes, naturalists and moth collectors alike began to notice that the previously rare dark form of this species (and others as well) was becoming more common. One way to explain this change can be stated with reference to natural selection. In unpolluted woods, the dark form is easily spotted by birds (the major predators on these moths) when it rests on pale, lichen-covered tree trunks, in contrast to the pale form, which is difficult to see. In polluted environments, the dark form is naturally camouflaged against visual predators when it rests on soot-darkened tree trunks, but the pale form is easily spotted. Kettlewell, using a series of investigations involving both experimental releases and field observations of differential bird predation, is generally credited as demonstrating that this explanation is indeed correct. It is interesting to note that since the advent of clean air legislation in Britain, the frequency of the dark form has declined once more.

Bibliography

Carter, David. Butterflies and Moths. New York: Dorling Kindersley, 1992. A comprehensive field guide to butterflies and moths throughout the world with brief descriptions, photographs, and distribution maps of over five hundred species. An excellent introduction to the field study of the Lepidoptera.

Douglas, Matthew M. The Lives of Butterflies. Ann Arbor: University of Michigan Press, 1986. A standard textbook on butterflies. This is a careful and comprehensive account of the natural history of butterflies that includes information on many American species, color photographs, and a glossary of terms.

Kettlewell, H. B. D. The Evolution of Melanism: The Study of a Recurring Necessity, with Special Reference to Industrial Melanism in the Lepidoptera. Oxford, England: Clarendon Press, 1973. Includes both a detailed discussion of Kettlewell's classic investigations on industrial melanism and his more general theory of the evolution of melanism throughout the animal kingdom.

Winter, William D. Basic Techniques for Observing and Studying Moths and Butterflies. Los Angeles: Lepidopterist's Society, 2000. A practical guide to the study of butterfly and moth anatomy, behavior, and ecology.

Young, Mark. The Natural History of Moths. San Diego, Calif.: Academic Press, 1997. Although its examples are confined primarily to British moths, this is an excellent introduction to the life history and ecology of moths.

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