Geographic and temporal variation in the incidence of melanism in peppered moth populations in America and Britain

Industrial melanism in the peppered moth (Biston betularia) is the classic example of observable evolution by natural selection. The phenomenon has been most thoroughly documented in Britain, where once rare melanic phenotypes increased in frequency in concert with increased levels of atmospheric pollution generated by industrial development. Populations of the American subspecies of the peppered moth (B. betularia cognataria) also experienced increases in melanism, which are especially well documented in southern Michigan. Following a succession of governmental regulations designed to improve air quality, melanic phenotypes have been on the decline in moth populations in the United Kingdom and the United States. In both countries, the declines in melanism appear to be correlated primarily with reductions in atmospheric sulfur dioxide. Detailed studies of the evolutionary parallels have focused mainly on two locations: Caldy Common (near Liverpool) in England, and the George Reserve (near Detroit) in the United States. We have expanded the geographic range of American samples to include northeastern Pennsylvania and southwestern Virginia, and we have conducted a national survey in Britain to produce a current melanic frequency distribution map. The results clearly show that the decline in melanism is widespread in both countries, and the changes are generally consistent with decreases in levels of atmospheric SO2