Interaction between area effects and variation with habitat in Cepaea

Populations of Cepaea nemoralis in Warwickshire occupy habitats of considerable temporal stability, most being at least 250 years old, and some much older. As expected from earlier work, shell pattern polymorphisms in these populations show variation with habitat of a kind suggesting the operation of visual selection for crypsis. They also show patterns of microgeographical variation unrelated to habitat. Although of a less extreme character, this variation resembles the ‘area effects’ seen in downland populations of Cepaea, in the lack of coincidence of variation at different loci, and in the existence of stronger and larger scale patterns in banding than in colour morphs. A similar explanation is advanced for their occurrence: previous bottlenecks and colonization from small relicts with founder effects. The less marked character of the variation is expected from the greater habitat stability and continuity in Warwickshire compared with downland. A clear colonization effect is seen in the inverse relationship between age of habitat and frequency of yellow in woodland populations. The results suggest that population histories affect variation in Cepaea even in areas of relative habitat stability.