Systematics of the Cyprinella lutrensis group (Cyprinidae) from the southwestern United States as inferred from variation of mitochondrial DNA

A total of 197 mitochondrial DNA (mtDNA) restriction sites was surveyed amo ng samples representing the five species of the Cyprinella lutrensis group inhabiting the southwestern United States: C. formosa, C. lepida, C. cf lep ida, C. lutrensis, and C. proserpina. Average nucleotide sequence divergenc e between C. proserpina and the other four species was greater than that fo und between the other species and two species from the Cyprinella whipplei group (C. galactura and C. venusta) that were employed as outgroup taxa in phylogenetic analysis. These data coincide with other genetic data that sug gest C. proserpina is not closely related to these species of the C. lutren sis group. Alternatively, C. proserspina may have experienced heterogeneous , perhaps rapid, genomic evolution. Maximum-parsimony analysis (employing u nordered restriction-site characters) of the remaining four species (i.e., excluding C, proserpina) produced an unresolved tetrachotomy. Maximum-parsi mony analysis that employed Dollo parsimony and neighbor-joining analysis o f nucleotide sequence divergence estimates among the four species generated resolved but conflicting topologies. In the neighbor-joining analysis, bra nch lengths between species were short in comparison to branches to termina l taxa. Maximum-likelihood analysis generated a topology congruent with tha t generated by Dollo parsimony. Statistical comparisons (likelihood test of Kishino and Hasegawa), however, indicated that the conflicting topologies produced by Dollo parsimony (and maximum-likelihood) versus neighbor-joinin g were equally likely. The simplest interpretation of these data is that th e four species evolved near-synchronously from a series of vicariant events that occurred in the western Gulf Coastal Plain. This interpretation is co nsistent with the hypothesis that ancestors to the C. lutrensis group enter ed the western Gulf Coastal Plain via connections with the western Great Pl ains before the onset of Pleistocene glaciation.