Gene frequency changes inCepaeasnails on the Marlborough Downs over 25 years

In 1985 we resurveyed the sites on the Marlborough Downs in southern England at which Cain and Currey in 1960/61 sampledCepaeasnails and thence introduced the term «area effects» to describe large areas of uniform morph frequency. Some sites no longer harbouredCepaeaand at others the species composition had changed, with a general spread ofCepaea hortensisat the expense ofCepaea nemoralis. The majority, however, permitted comparison of morph frequencies between the two surveys. InC. nemoralis, we detected a significant overall decrease in the frequency of the brown morph and estimate selection as 5–9% per generation. There was no apparent change in frequencies of banded morphs. InC. hortensiswe detected a significant overall increase in the frequency of unbanded shells (1–3% selection per generation) and an almost significant decrease in the frequency of fusions within the banded class. There was insufficient colour polymorphism inC. hortensisto allow analysis of colour morph frequencies. These changes — all in the direction of reduced absorption of solar energy — resemble others detected in both species at other localities in southern England. Possible explanations include large-scale climatic effects and changes in vegetation.