CEPAEA NEMORALIS (L.) IN THE CHANNEL ISLANDS: ISLAND HISTORIES AND GENETIC VARIATION

Populations of C. nemoralis from the Channel Islands show strong between-island variation in their shell colour and banding polymorphism, despite the general similarity of the habitats sampled on each island. Morph-frequency distributions do not, in general, match those found in similar open habitats in England and Wales in regions where visual selection for crypsis is effective. Some changes have occurred over the last 30 years, reflecting changing patterns of land-use. A detailed comparison is made with C. nemoralis populations on the Isles of Scilly, where inter-island differences are much less marked, but where morph-frequency distributions are even more unlike those found in similar habitats on the British mainland. These patterns can be interpreted in terms of rising Holocene sea-levels: the Scillies were probably colonized once, when a single larger island existed, while many of the Channel Islands have a history of separate isolations from differing parts of the French mainland. Population histories have an effect on present genetic make-up, even for loci on which selection can and does operate.