Origin and diversification of the soldier meadow springsnails (Hydrobiidae: Pyrgulopsis), a species flock in the northwestern Great Basin, United States

The large, western North American hydrobiid gastropod genusPyrgulopsis (commonly known as springsnails) includes a seriesof locally endemic faunas that are thought to be species flocks.Although these assemblages are of interest from the evolutionary,biogeographic and conservation perspectives, their monophylyand phylogenetic relationships have yet to be rigorously evaluated.Here we present a molecular phylogeny (based on mitochondrialsequence data) of a putative flock of four thermal spring-dwellingspringsnails that is distributed in many sites in Soldier Meadowand a single locality in Bog Hot Valley (northwestern Nevada).Our analyses support monophyly of this assemblage (‘SoldierMeadow clade’) and a close relationship with other regionalspecies and suggest that the invasion of thermal habitats bythese springsnails occurred independently of other such radiationswithin the genus. The divergence of the Soldier Meadow claderelative to its sister group is substantial (6.79–10.36%for COI, 10.35–15.88% for NDI), suggesting a split inthe early Pliocene, based on the application of a COI clockfor Pyrgulopsis. The splits within the Soldier Meadow cladeinto three main subunits also appear to be old events, basedon their 5.78–8.54% COI divergence relative to each other.These findings are consistent with a long history of springsnailevolution in Soldier Meadow, which is intriguing given thatthis basin was flooded by Lake Lahontan during periods of theearly and middle Pleistocene. We suggest that progenitors ofthe contemporary fauna survived in high elevation springs thatmay have been present in the basin during these pluvial periodsand subsequently colonized contemporary habitats following thetermination of the extreme Lake Lahontan highstands. We speculatethat the broadly disjunct population (of P. militaris) in BogHot Valley, which is consistently nested within the SoldierMeadow clade in our phylogenetic analyses, is either a vicariantrelict of a spring zone that may have once extended betweenthese two areas; or was founded by a past ‘jump’dispersal event from Soldier Meadow. Phylogeographic structureof springsnail populations in Soldier Meadow bears the strongstamp of geologically recent, allopatric diversification, perhapsreflecting the short time that basin floor habitats have beenoccupied. We describe a new species (P. varneri) for a seriesof recently discovered populations that are monophyletic, substantiallydivergent and morphologically distinctive. Additional studieswill be necessary to confidently assess the taxonomic statusof morphologically distinctive P. limaria and P. umbilicata,which are shown herein to be little divergent genetically; anda recently discovered minute springsnail that is morphologicallydivergent yet closely similar genetically to P. notidicola. (Received 20 December 2006; accepted 22 February 2007)