Change in morph frequency in the snail Cepaea nemoralis on the Marlborough Downs

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 estimating selective coefficients over a few generations have been applied to data on the frequency of brown at the shell colour locus, showing an average selective disadvantage of 3-6% per generation. The integration method overestimated 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 of homogenization of frequency over the period of the study. Both tendencies are also seen when banded vs. unbanded at the banding locus is examined. Possible reasons are discussed. Between-colony migration may be involved, at levels higher than usually assumed to prevail.