Change in morph frequency in the snail Cepaea nemoralis on the MarlboroughDowns

Temporal change in morph frequency has been examined in Cepaea nemoralis (L .) from the Marlborough Downs, southern England, first studied by Cain and Currey and resurveyed after 25 years by Cowie and Jones. Three methods of e stimating selective coefficients over a few generations have been applied t o data on the frequency of brown at the shell colour locus, showing an aver age selective disadvantage of 3-6% per generation. The integration method o verestimated selection, whereas the ratio method gave values close to those obtained by iteration. There is heterogeneity in response between samples, indicating a patchy distribution of morph frequency change, and evidence o f homogenization of frequency over the period of the study. Both tendencies are also seen when banded vs. unbanded at the banding locus is examined. P ossible reasons are discussed. Between-colony migration may be involved, at levels higher than usually assumed to prevail.