Commentary on three papers on clines for melanism and polymorphism in moths

The three papers that follow are interdependent. They present an additional approach to the existence of genetic polymorphism and clines for melanism in the night-flying moths Biston betularia and Phigalia pilosaria . In both species the character is dominant and is determined by the substitution of a single allele. The spread of melanic forms of Biston betularia after they were first noted near Manchester in 1848 has been claimed to be the most striking evolutionary change ever to be witnessed. In spite of this there was a notable lack of appreciation of the reasons for the phenomenon. Kettlewell, however, in his classic experiments (1952–1955; Kettlewell 1973), showed that the species rests exposed on surfaces during the day. Melanics are better camouflaged from insectivorous birds on the dark backgrounds available in smoke-polluted areas than are the non-melanic or typical moths. The reverse is true in unpolluted countryside. Kettlewell also reviewed the sketchy evidence for other forms of selection acting to influence morph frequency in the species.