Molecular area effects in Cepaea.

Enzyme polymorphisms in the land snail Cepaea nemoralis in the central Pyrenees show concordant geographic patterns of strong differentiation that are not correlated with the distributions of characters of shell color and banding or with the major pattern of variation in climate and vegetation type. Three regions of relative genetic uniformity separated by steep clines in allele frequencies are designated as "molecular area effects." A model of allopatric differentiation of populations in temporary geographic isolation during the last period of Pleistocene glaciation, followed by invasion of the Pyrenees and hybridization in secondary contact, is proposed to account for the present-day pattern of genetic differentiation. The genetic structure of the Pyrenean populations of C. nemoralis is not interpretable in terms of stasipatric or parapatric models of speciation.