Evolutionary pattern and process within the Vertigo gouldii (Mollusca: Pulmonata, Pupillidae) group of minute North American land snails

A phylogenetic analysis of 19 sibling taxa in the Vertigo gouldii group was conducted on 73 individuals sampled across North America using DNA sequence data of the mitochondrial genes cytochrome oxidase subunit 1 (CO1) and 16S ribosomal RNA (16S), and the internal transcribed spacer-2 of the nuclear ribosomal RNA (ITS-2) gene. The results of these analyses were found incongruent with previous taxonomic concepts used to define the V. gouldii group and its composite taxa that were based entirely on conchological features. The mtDNA sequence data suggest that some previous members of the traditional V. gouldii group may be more closely related to V. modesta. They also suggest that V. gouldii may itself consist of seven species-level branches spread across two deeply rooted clades. Revision of geographical distributions on the basis of these analyses suggests that these Vertigo species may commonly possess continental-sized ranges in spite of their minute size and limited active dispersal ability. High levels of sympatry within the group are also confirmed, with up to four species being known to co-occur within single microsites. These data also suggest that rates of diversification have been non-constant. Assuming a 1%/my rate of base pair substitution, a 10-fold diversification pulse is indicated from 6.7–7.0 myBP, which would be co-incident with known mid-late Miocene global climate changes.