Molecular systematic relationships and evidence for a recent origin of the thermal spring endemic snails Physella johnsoni and Physella wrighti (Pulmonata: Physidae)

The evolutionary origins of Physella johnsoni and Physella wrighti, freshwater snails endemic to thermal springs in Alberta and British Columbia, were examined by sequence comparisons of mitochondrial genes coding for segments of the large subunit ribosomal RNA (16S) and cytochrome c oxidase I (COI) with those in the closely allied species Physella gyrina. Phylogenetic reconstructions based on combined and separate analyses of the datasets identified P. wrighti as the sister taxon to a P. johnsoni + P. gyrina clade. For both gene regions there was little or no sequence variation among conspecific individuals, but a number of diagnostic differences existed between each of the species pairs. Tree topologies and estimates of sequence divergence indicate that the divergence events which separated the three species occurred rather recently and at relatively brief intervals, and it is likely that P. wrighti is the source of the ancestral population from which P. johnsoni and P. gyrina were derived. Our findings support the recognition of P. johnsoni and P. wrighti as endemic species. The populations of P. johnsoni probably separated from P. gyrina about 10 000 years ago, when glaciers retreated from the Banff area and prehistoric lake levels dropped. The limited molecular divergence between P. johnsoni and P. gyrina argues against their current placement in different subgenera within the genus Physella.