Morph frequencies in the snail Cepaea nemoralis: changes with time and their interpretation

Colonies of the polymorphic snail Cepaea nemoralis on the sand dune at Point of Air, north Wales, first sampled in 1962 by A.J. Cain and P.M. Sheppard, have been re-sampled after 35 years. The frequency of the mid-banded morph has increased. Overall, yellow frequency has decreased at the expense of pink, but the change is in different directions in different parts of the dune. Yellow mid-banded and pink five-banded, phenotype combinations controlled by alleles at unlinked loci, are in excess within colonies, which suggests an effect of selection. It is argued that changes in morph frequency are due, at least in part, to movement between colonies. Cepaea populations are slow to respond to environmental changes and displacement of individuals may be greater than is often assumed. It is therefore difficult to interpret frequency changes over time. Correlated changes in groups of colonies may not always be interpretable as indicating selection.