Gene frequency changes in Cepaea snails on the Marlborough Downs over 25 years

In 1985 we resurveyed the sites on the Marlborough Downs in southern Englan d at which Cain and Currey in 1960/61 sampled Cepaea snails and thence intr oduced the term 'area effects' to describe large areas of uniform morph fre quency. Some sites no longer harboured Cepaea and at others the species com position had changed, with a general spread of Cepaea hortensis at the expe nse of Cepaea nemoralis. The majority, however, permitted comparison of mor ph frequencies between the two surveys. In C. nemoralis, we detected a sign ificant overall decrease in the frequency of the brown morph and estimate s election as 5-9% per generation. There was no apparent change in frequencie s of banded morphs. In C. hortensis we detected a significant overall incre ase in the frequency of unbanded shells (1-3% selection per generation) and an almost significant decrease in the frequency of fusions within the band ed class. There was insufficient colour polymorphism in C. hortensis to all ow analysis of colour morph frequencies. These changes - all in the directi on of reduced absorption of solar energy - resemble others detected in both species at other localities in southern England. Possible explanations inc lude large-scale climatic effects and changes in vegetation. (C) 1998 The L innean Society of London.