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

Populations of C. nemoralis from the Channel Islands show strongbetween-island variation in their shell colour and banding polymorphism,despite the general similarity of the habitats sampled on eachisland. Morph-frequency distributions do not, in general, matchthose found in similar open habitats in England and Wales inregions where visual selection for crypsis is effective. Somechanges have occurred over the last 30 years, reflecting changingpatterns of land-use. A detailed comparison is made with C.nemoralis populations on the Isles of Scilly, where inter-islanddifferences are much less marked, but where morph-frequencydistributions are even more unlike those found in similar habitatson the British mainland. These patterns can be interpreted interms of rising Holocene sea-levels: the Scillies were probablycolonized once, when a single larger island existed, while manyof the Channel Islands have a history of separate isolationsfrom differing parts of the French mainland. Population historieshave an effect on present genetic make-up, even for loci onwhich selection can and does operate. (Received 20 January 1997; accepted 12 May 1997)