Biological Journal of the Linnean Society (1998), 65: 233–255 Article ID: bj980250 Gene frequency changes in Cepaea snails on the Marlborough Downs over 25 years ROBERT H. COWIE* AND J. S. JONES Galton Laboratory, University College London, 4 Stephenson Way, London NW1 2HE Received 3 April 1998; accepted for publication 24 May 1998 In 1985 we resurveyed the sites on the Marlborough Downs in southern England at which Cain and Currey in 1960/61 sampled Cepaea snails and thence introduced the term ‘area effects’ to describe large areas of uniform morph frequency. Some sites no longer harboured Cepaea and at others the species composition had changed, with a general spread of Cepaea hortensis at the expense of Cepaea nemoralis. The majority, however, permitted comparison of morph frequencies between the two surveys. In C. 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. In C. hortensis we 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 in C. hortensis to 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.  1998 The Linnean Society of London ADDITIONAL KEY WORDS:—natural selection – area effects – polymorphism – climate – Gastropoda (Pulmonata: Helicidae). CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . 234 Methods . . . . . . . . . . . . . . . . . . . . . . . . 234 Survey and scoring . . . . . . . . . . . . . . . . . . . 234 Statistical analysis . . . . . . . . . . . . . . . . . . . 235 Selection coefficients . . . . . . . . . . . . . . . . . . 236 Results . . . . . . . . . . . . . . . . . . . . . . . . 236 Cepaea nemoralis . . . . . . . . . . . . . . . . . . . . 236 Cepaea hortensis . . . . . . . . . . . . . . . . . . . . . 239 Discussion . . . . . . . . . . . . . . . . . . . . . . . 240 Acknowledgements . . . . . . . . . . . . . . . . . . . . 243 References . . . . . . . . . . . . . . . . . . . . . . . 243 Appendix 1 . . . . . . . . . . . . . . . . . . . . . . . 245 Appendix 2 . . . . . . . . . . . . . . . . . . . . . . . 252 * Correspondence to Robert H. Cowie, present address: Bishop Museum, 1525 Bernice Street, Honolulu, Hawaii 96817-2704, U.S.A. 233 0024–4066/98/110233+23 $30.00/0  1998 The Linnean Society of London