A colony of Cepaea nemoralis polymorphic for banding and white or dilute pigmented lip has been re-surveyed after 44 years. Frequencies of banding and lip colour are unchanged, although mid-banded has declined in frequency. Aberrant frequencies of banding in intermediate years, when the snails were rare, suggest that density-dependent visibility bias may occur. The peculiar association of lip colour and banding indicates that the colony was founded by unusual mutant individuals. Lip colour variation and frequency suggests that the allele concerned may show great breadth of expressivity.
morph frequency change