Frequency of insularia during the decline in melanics in the peppered moth Biston betularia in Britain.

Over the last three decades the frequency of the dark melanic form carbonaria of the peppered moth Biston betularia has declined in Britain. Data have been examined which show the intermediate phenotype insularia, controlled by alleles at the same locus, to have increased or remained level in frequency. Phenotype frequency of insularia does not always track allele frequency accurately because it is recessive to its alternative when carbonaria is common but dominant to the alternative when typical is common. It is shown that if insularia fitness lies between that of carbonaria and typical, and melanics replace typicals or vice versa, there will be a rise and fall in insularia allele frequency during a transitory period. The path followed is strongly affected by initial gene frequencies. During the high melanic period in Britain, differences in insularia frequency between localities may have been influenced by history of arrival of the novel morphs as well as by local selective conditions.