Parallel Rise and Fall of Melanic Peppered Moths in America and Britain

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Abstract
The peppered moth (Biston betularia) is well known for the rapid rise in the frequency of alleles producing melanic phenotypes correlated with a general blackening of the environment following the nineteenth-century industrial revolution. In recent years the frequency of melanics has been dropping steadily in Britain in apparent response to improved air quality. Some regional American populations of this same species also experienced significant increases in melanics, by 1959 exceeding 90% in southeastern Michigan, but Michigan populations were not reexamined for over 30 years. In 1994 and 1995 we trapped moths in southeastern Michigan and here report that a parallel decline in melanism has occurred in American peppered moths. Furthermore, we document that changes in Michigan's air quality as measured by atmospheric sulfur dioxide (SO2) and suspended particulates also parallel the changes recorded in Britain. The traditional interpretation is that pale phenotypes of peppered moths at rest by day on lichen-encrusted trees are camouflaged from bird predators; industrial fallout kills lichens and darkens resting surfaces, favoring darker colored moths. However, the changes in allele frequencies in the moth populations we sampled have occurred in the absence of perceptible changes in the local lichen floras. We suggest that the role of lichens has been inappropriately emphasized in chronicles about the evolution of melanism in peppered moths.

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