Recent history of melanism in American peppered moths.

Abstract

Industrial melanism in peppered moths has been studied most intensively in Britain. The first melanic phenotype (effectively solid black) was recorded near Manchester in 1848. By 1895 about 98% of the specimens near Manchester were melanic, and this once rare phenotype had spread across regions of the country blackened by industrial soot. In rural, unpolluted regions, well away from industrial centers, the pale phenotype (peppered with white and black scales) remained the predominant form. During the latter half of the 20th century, following legislation designed to improve air quality, melanics began to decline in frequency and are now rare where once they had been common. Similar evolutionary changes have occurred elsewhere, but records from outside Britain are fragmentary. We have extended previous surveys of American peppered moth populations and present a composite picture of the recent decline in melanism in northern industrial states-Michigan and Pennsylvania-where melanic phenotypes decreased from more than 90% in 1959 to 6% by 2001. We contrast these changes to the near absence of melanism in a southern state-Virginia-during that same period. As in Britain, the decline in melanism in American peppered moths followed clean air legislation.