An East-West distribution of divergent mitochondrial haplotypes in Britishpopulations of the land snail, Cepaea nemoralis (Pulmonata)

Some continental European populations of the land snail Cepaea nemoralis ha ve mitochondrial haplotypes that differ by up to 20% at the 16S rRNA locus. I mapped the distribution of different lineages in populations from 36 dif ferent sites in Britain and Ireland. In 93% of individuals, one of two mito chondrial lineages was found, A or N, which differ from each other by about 6% using a 16S rRNA fragment (approximately 300 base pairs). The distribut ion of these two types is very striking-one is confined to Wales, West and central England, and Scotland, while the other is found mainly in East and central England. The two types meet in a transition zone. The most likely e xplanation for the distribution is that it reflects two routes of colonizat ion after the last ice age. Cepaea dispersal is leptokurtic, and only limit ed gene flow occurs between established populations, so that the original p attern could have been retained since the post-glacial colonization. Howeve r, many environmental gradients are orientated East-West, so alternative se lective explanations are possible. A distinct mitochondrial lineage, as wel l as fossil evidence, suggests that Ireland was colonized separately from B ritain. The implications of these distributions for the origins of the puzz ling geographical patterns of shell types known as 'area effects' is discus sed. (C) 2000 The Linnean Society of London.