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 I973), 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. This suggests that in regions where there is a transition from an urban to a rural environment there will be an associated change in the moth populations. City populations should largely consist of melanics while rural ones should be made up of typicals. There will be a zone of polymorphism, with a change in melanie frequency (a cline), on either side of the region where neither form is at an advantage. Theoretically, the cline is due to animals born in the favourable environment moving to the disadvantageous one. Clarke & Sheppard (1966) describe such a dline for the carbonaria melanic of Biston betularia between Liverpool and north Wales. The ecological components of this cline (differentlal predation and movement) were estimated by Bishop (1972). These variables could not account for the cline observed. Additional selective forces need to be invoked to explain its position and the continued polymorphism in both Liverpool and north Wales. Some theories were discussed by Bishop et al. (1978) but interpretations were not supported by sound data. The best evidence for selection other than that arising because of differential predation by birds comes from Phigalia pilosaria. A comprehensive study of the