A local survey of the distribution of industrial melanic forms in the moth Biston betularia and estimates of the selective values of these in an industrial environment

Abstract

The survey shows that there is a rapid decline in the frequency of the industrial melanic carbonaria of the moth Biston betularia from a value of about 97% in Liverpool to less than 10% 50 miles to the west in North Wales. The decline in the frequency of the intermediate phenotype insularia in this area, controlled by an allelomorph at the same locus, is from about 14% on the Wirral (no reliable frequency is available for Liverpool) to about 4% 30 miles to the west. The frequency of carbonaria has declined in one locality (Caldy) since 1959, perhaps as the result of the introduction of smokeless zones to the east of it. It is argued on the basis of experimental data that the typical form is not at an advantage with respect to visual predation by birds at this locality, but it is at less of a disadvantage than formerly. The increase in its frequency is explained by postulating a compensating non-visual disadvantage of the carbonaria homozygote, for which there is independent evidence. This disadvantage may be of the order of 15%. Experiments using dead moths placed in life-like positions on tree trunks at Caldy and in Liverpool confirmed that carbonaria is better camouflaged on the blackened tree trunks of industrial areas. Estimates of the selective disadvantage of the typical form in Liverpool, using data from the survey and these experiments, together with a variety of assumptions, indicate values of the order of 60%, which is somewhat higher than previous estimates. At Caldy the typical form appears to have been at a disadvantage of about 50% prior to the introduction of the smokeless zones and is now at about a 20% disadvantage, using assumptions similar to those in the Liverpool estimates. Although these estimates are subject to considerable error, there is little doubt that they reflect the correct order of magnitude of the relative selective values.