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

Abstract

The 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 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 moths at rest by day on lichen-encrus ted 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 moths. Industrial melanism in the Lepidoptera ranks among the best examples of observable evolution by natural selection. The most thoroughly documented case is that of the geometrid moth Biston betularia, with the fundamental details of its recent evolutionary history described in virtually every biology textbook. Its common name, peppered moth, describes the appearance of the so-called form or phenotype which is covered with white scales peppered with black over the wings and body. Several other pigmentation phenotypes are known. A melanic form, named carbonaria, is nearly solid black. Intermediates between the pale and fully melanic phenotypes are called insularia. The range in variation from lightest to darkest is continuous, but the phenotypic differences result from multiple alleles at a single locus which exhibit an approximate dominance hierarchy: the carbonaria allele shows complete dominance over the insularia and typical alleles, and