History or current selection? A molecular analysis of ‘area effects’ in the land snail Cepaea nemoralis

We have used molecular variation in microsatellite and mitochondrial DNA to throw light on the origins of enigmatic geographical patterns, known as `area e¡ects’, in the shell polymorphisms of the land snail Cepaea nemoralis. Our aim was to assess the relative importance of recent selection and historical events in the formation of these patterns. On the Marlborough Downs inWiltshire, England, the t̀ype locality ’ for area e¡ects, the frequencies of microsatellites are signi¢cantly associated with the frequencies of alleles for shell banding. A less clear association is found between microsatellites and shell colour. Mitochondrial haplotypes show no signi¢cant relationships. Although the correlated geographical patterns could be the results of random genetic drift from an initially uniform array of populations, the magnitudes of the patterns, and of the correlations between them, seem too strong to have arisen by drift since the last glaciation. Our results suggest that invasions from refugia have been the most important factors in forming area e¡ects