The rise and fall of the Carbonaria form of the peppered moth

The evidence for change in frequency of the melanic carbonaria morph in the peppered moth Biston betularia (L.) (Lepidoptera: Geometridae) in England and Wales is reviewed. At mid-20th century a steep cline of melanic phenotype frequency running from the north of Wales to the southern coast of England separated a region of 5 % or less to west from 90 % or more to northeast. By the 1980s the plateau of 90 % frequency had contracted to northern England. The frequency has since continued to drop so that the maximum is now less than 50 % and in most places below 10 %. There have been similar declines in Europe and North America. Evidence from surveys and from two-point records shows the change to require 5 % to 20 % selection against the melanic. The melanic is more disadvantageous in regions where its frequency was initially high than in regions where it was low. Experiments to investigate predation by birds show a net advantage to carbonaria morphs in regions where typical frequencies were low at the time of the experiment, and a disadvantage where typical frequencies were high. This would be expected if environment and frequency were associated, and selective predation played a part in generating the association. The cryptic advantage of carbon-aria was large in areas of heavy pollution where typical frequencies were 20 % or less. The moth usually has a low density but is relatively highly mobile. The ability of present information to explain the patterns has been tested in simulations. They indicate a system under strong selection that has always been in a dynamic state without equilibria.