Large scale patterns of genetic differentiation at enzyme loci in the land snails Cepaea nemoralis and Cepaea hortensis

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Abstract
Samples of 231 populations of the land snails Cepaea nemoralis and C. hortensis from Britain, France, Switzerland and Spain were analysed for genetic polymorphism in six enzyme systems. These sibling species show similar levels of variation detected by electrophoresis, and have a generally similar degree of local divergence over homologous enzyme loci. As well as extensive local and regional differentiation, both species show large-scale changes in allele frequency across Europe. In C. hortensis there is a continuous gradient in allele frequency from northern Britain to northern Spain, while in C. nemoralis north-south clines on the continent of Europe are reversed in direction in Britain. There are few obvious correlations of allele frequency change at single loci with components of the environment. Patterns of allele frequency variation in 24 sympatric populations of the two species are generally independent of each other, although there is a positive association of the frequencies of alleles at a leucine aminopeptidase locus in the two species. The statistical significance of this association depends on a single locality, and there is little indication of shared patterns of allele frequency change which might reflect a common response to natural selection. In addition, there is no evidence that the extensive geographical change in C. nemoralis and C. hortensis is a precursor of speciation.