Evolutionary stability since the Pleistocene illustrated by reproductive compatibility between Swedish and Spanish Helophorus lapponicus Thomson (Coleoptera, Hydrophilidae)

Spanish and Swedish populations of Helophorus lapponicus are shown on fossil evidence to have been isolated from one another for about 10 000 years, since the end of the Pleistocene. When cross-bred they show some evidence of heterosis in the F₁, with reduced viability in the F₂ and back-cross generations. The level of compatibility of these populations is similar to that found when different populations of Lepidoptera species are crossed, and quite different from the incompatibility revealed by studies of interspecific hybridization. The degree of evolutionary stability revealed is consistent with the apparent absence of evolution revealed by studies of Pleistocene fossil Coleoptera.