Evolutionary genetics of island and mainland species of Rhagada (Gastropoda: Pulmonata) in the Pilbara Region, Western Australia

Mainland species of the camaenid genus Rhagada, endemic to northern Western Australia, have relatively large, non-overlapping geographic ranges. In contrast, over much smaller distances in the Dampier Archipelago, several locally endemic, morphologically distinctive species occur with intermingled ranges. To test alternative origins of the unusual local diversity, we compared allozymes at 21 loci in 12 archipelago populations and 14 mainland populations, representing 14 species. Genetic distances were consistently low, averaging 0.019 (range 0.000-0.051) within species, and only 0.043 (range 0.001-0.133) between species. In the Dampier Archipelago, the average genetic distance between species was even smaller (0.023). This result was indistinguishable from the within-species comparisons, highlighting the disconnection between morphological diversification and levels of molecular genetic divergence. A pattern of isolation by distance among all comparisons within the archipelago also suggests a historic cohesiveness of the species in the Dampier Archipelago. Although providing no resolution of relationships among mainland populations, a neighbour-joining tree provided further support for an in situ morphological radiation in the Dampier Archipelago, transcending variation seen over much larger distances on the mainland.