The evolutionary processes generating population genetic structures may differ depending on the spatial scale. We examined the population structure of the freshwater snail Radix balthica on a local scale in Saanen Valley, Switzerland. We used a combination of a direct approach by simulating dispersal with parameters gained out of experiments in an artificial pond and indirect molecular genetic approaches by using mitochondrial (COI) and nuclear (microsatellite) marker at all sites harbouring R. balthica in this restricted area.

Contrary to the expectations from the direct methods, populations in homogeneous water bodies were found to be genetically heterogeneous, independently of geographic distance between sites. Instead, several selfing lineages coexisted with little local gene flow among them.

The population structure observed in the valley could thus not be explained by active dispersal alone. It was rather characterized by independent, probably bird-mediated, colonisation events from several sources, in combination with high metapopulation dynamics and a mixed mating system preserving this structure. This population structure has rarely been reported before, but might nevertheless be typical for passively dispersed, patchily distributed taxa (e.g., freshwater invertebrates).