Polymorphism in the Isopod Sphaeroma rugicauda

Abstract

The spatial variation of gene frequencies has been recorded in several polymorphic species, including Biston betularia (Kettlewell, 1958), Cepaea nemoralis (Lamotte, 1951, 1959; Cain and Sheppard, 1954), C. hortensis (Clarke, 1962), Littorina obtusata (Sacchi, 1961), Homo sapiens (Allison, 1955), Drosophila spp. (Dobzhansky, 1951), and Spaeroma serratum (Bocquet, Levi, and Teissier, 1951; Hoestlandt, various papers). In at least the first five species, there are indications of the reasons for the variation. In some species changes in gene frequencies have been recorded as well (see, for example, Sheppard, 1961, on Panaxia dominula; Dobzhansky, 1958; Clarke and Murray, 1962). The causes of spatial variation and changes in gene frequencies in natural populations are controversial, with more importance ascribed to selection (Cain and Sheppard, 1954; Fisher and Ford, 1947) or to random drift (Lamotte, 1959). As Wright (1948) has emphasized, however, these are not exclusive categories but may interact in certain circumstances. The purpose of this paper is to describe the genetics of the polymorphic isopod, Sphaeroma rugicauda Leach, and to consider some aspects of gene-frequency data from 46 British populations of the species which suggest that natural selection is responsible for differences between populations.