POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE MEDITERRANEAN LAND SNAIL ALBINARIA CORRUGATA (PULMONATA: CLAUSILIIDAE).

The amount of gene flow among local populations partly determines the relative importance of genetic drift and natural selection in the differentiation of such populations. Land snails, because of their limited powers for dispersal, may be particularly likely to show such differentiation. In this study, we directly estimate gene flow in Albinaria corrugata, a sedentary, rock-dwelling gastropod from Crete, by mark-recapture studies. In the same area, 23 samples were taken and studied electrophoretically for six polymorphic enzyme loci. The field studies indicate that the population structure corresponds closely to the stepping-stone model: demes are present on limestone boulders that are a few meters apart, and dispersal takes place mainly between adjacent demes. Average deme size (N) is estimated at 29 breeding individuals and the proportion of migrants per generation at 0.195 (Nm = 5.7). We find no reason to assume long-distance dispersal, apart from dispersal along occasional stretches of suitable habitat. Genetic subdivision of the population, as derived from F values, corresponds to the direct estimate only at the lowest spatial level (distance between sample sites < 10 m), where values for Nm of 5.4 and 17.6 were obtained. In contrast, at the larger spatial scales, F values give gene-flow estimates that are incompatible with the expected amount of gene flow at these scales. We explain these discrepancies by arguing that gene flow is in fact extremely limited, making correct estimates of Nm from F impossible at the larger spatial scales. In view of these low levels of gene flow, it is concluded that both genetic drift and natural selection may play important roles in the genetic differentiation of this species, even at the lowest spatial scales.