Differences in Interactions between Selective Forces acting in the Wild on Certain Pleiotropic Genes of Cepaea

Abstract

THE snail Cepaea nemoralis is highly polymorphic for colour and banding varieties of its shell. The genetic basis of this polymorphism is fairly well known, and involves several loci, linked and unlinked, with alleles controlling clear-cut characters. The snail is also highly sedentary. It therefore offers a favourable opportunity for investigating local differences in the interaction of selective forces affecting the polymorphism. The colour and banding varieties of the shell have been shown already¹ to be subject to visual selection by predators, the more conspicuous shells being preferentially removed, so that the intercolony variation, which is considerable, of the phenotype frequencies is largely determined in relation to the visual properties of the backgrounds of the colonies. Since the polymorphism is stable, some non-visual selection, probably acting as heterozygote advantage, must maintain the balance, while visual selection sways it to different extents in different places. The majority of Cain and Sheppard's observations were made in the Oxford district; they pointed out that elsewhere a different general balance might be found because of regional differences in the overall non-visual selective values².