Influence of spatial structure on genetic isolation in Plebejus argus populations (Lepidoptera: Lycaenidae)

Populations of Plebejus argus were sampled in southwest Finland, both on the mainland and on islands, and in and around the Doñana National Park in southwest Spain. A total of 453 individuals coming from 14 locations were investigated using allozyme electrophoresis on a total of 10 polymorphic allozyme loci. Contrary to earlier studies, all conducted in Britain, our samples showed little differentiation between sampled locations. In Spain, the populations of the Donaña area showed no differentiation despite being up to 36 km apart; only the population to the south of the Guadalquivir river showed a significant difference to the others. In Finland the population on one island showed marked genetic differentiation from all the others, which showed little or no difference from each other. No isolation‐by‐distance effect could be detected in either system. We hypothesise that emigration–immigration events are more frequent in the Spanish and Finnish populations than in the British ones. We did, however, find two isolated populations, one in Spain and one in Finland; both were small and geographically isolated and shared evident drift.