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 'a rea effects', in the shell polymorphisms of the land snail Cepaea nemoralis . Our aim was to assess the relative importance of recent selection and his torical events in the formation of these patterns. On the Marlborough Downs in Wiltshire, England, the 'type locality: for area effects, the frequenci es of microsatellites are significantly associated with the frequencies of alleles for shell banding. A less clear association is found between micros atellites and shell colour. Mitochondrial haplotypes show no significant re lationships. Although the correlated geographical patterns could be the res ults of random genetic drift from an initially uniform array of populations , the magnitudes of the patterns, and of the correlations between them, see m too strong to have arisen by drift since the last glaciation. Our results suggest tl-lat invasions from refugia have been the most important factors in forming area effects.