Recent evolutionary dynamism in three pulmonate gastropods from south-eastern Australia

Over broad geographic scales, estuarine species generally display substantial inter-regional differentiation, such as significant allelic frequency differences and/or the presence of haplotypes that are markedly genetically-divergent from those found in other regions.There is some evidence, including data from three gastropod species from south-eastern Australia, that there may be a “low-divergence” population genetic structure in some estuarine species. These species are characterised by having one or a few widely distributed high frequency haplotypes with large numbers of closely-related low frequency haplotypes. The species exhibit low levels of nucleotide divergence between widely-separated populations and, quite frequently, evidence of recent evolutionary upheavals.To examine how frequently the low-divergence structure is found in the gastropods of southern eastern Australian, we have investigated the population structures of three species of the closely-related genera Ophicardelus and Pleuroloba, the most abundant eupulmonates in the region's estuaries. The three species each exhibit a “moderate divergence” structure with high numbers of moderately-related haplotypes occurring at low frequencies in restricted areas. All three show significant deviations from the expectations of neutral variation in a constant-sized population. The structure is consistent with demographic expansion from a small population size although such an expansion cannot be readily related to known historical factors. Both the low and moderate divergence structures can be explained by selective sweeps, albeit that they may have occurred at different times.