Geographic cohesiveness versus associations with habitat: genetic subdivision of Bembicium vittatum Philippi (Gastropoda: Littorinidae) in the Houtman Abrolhos Islands

The littorine snail Bembicium vittatum has direct development and associated high levels of genetic subdivision. In the Houtman Abrolhos Islands, Western Australia, this species occupies a wide range of habitats. Replication of habitat types across the three major groups of islands, which are separated by water gaps of 10 to 15 km, separates patterns of genetic divergence related to gene flow from those expected from localized selection related to habitat. Allozyme frequencies at 14 loci were examined in samples of B. vittatum from 81 sites in the Abrolhos archipelago, at 72 of which descriptions of the physical and biological characteristics were made. Although shell shape and population density of B. vittatum varied with habitat, none of the allozymes showed such associations. Instead, there was spatial coherence of the allozyme variation, indicating connections among populations al two levels. On a larger scale, nearly half the interpopulation diversity was due to differences between island groups, including the association of rare alleles with island group. Al a smaller scale, variation within island groups showed coherence in the form of isolation by distance, the extent and intensity of which were related to expected patterns of gene now. This spatial coherence, independent of the mosaic of habitats, strongly favours the interpretation of patterns of genetic subdivision in B. vittatum in the Houtman Abrolhos as reflecting primarily the patterns of past and present gene flow. Contrasts with some results of an earlier study emphasize the importance of adequate sampling of sites and polymorphic loci. (C) 1996 The Linnean Society of London