Genic variation in isolated marginal populations of the Roman snail, Helix pomatia L.

Abstract

Four Finnish and five Swedish populations of Helix pomatia, living at the ecological margin of the species, have been studied electrophoretically. The populations are of different origins, small and completely isolated. However, their polymorphisms are similar at four loci, Aph, Idh and two Mdh loci. At a fifth locus, α - Gpdh, the Finnish and Swedish populations differ considerably, the difference being explainable only by selection towards a new adaptive peak in the young Finnish populations. Taking into account monomorphic loci (at most 22), three methods indicate that 96–98% of all genic variation can be found in an average population. Variation between the populations of a country give rise to 0.6-2%, while the differences between the populations of the two countries account for 1–2%. It is suggested that selection has prevented reorganization of the gene pools by random drift. Both genetical and population ecological information (polymorphism patterns and changes in reproductive rates) are consistent with the hypothesis that the populations studied are responding to extreme ecological conditions as such, not to a particular environmental factor such as average temperature conditions. A careful distinction should be made between different types of marginal populations.