Evolutionary patterns of morphometrics, allozymes, and mitochondrial DNA in thrashers (genus Toxostoma)

We examined patterns of variation in skeletal morphometrics (29 characters) , allozymes (34 loci), mitochondrial DNA (mtDNA) restriction sites (n = 74) and fragments (n = 395), and mtDNA sequences (1,739 bp from cytochrome b, ND2, ND6, and the control region) among all species of Toxostoma. The phene tic pattern of variation in skeletal morphometrics generally matched tradit ional taxonomic groupings (based on plumage patterns) with the exceptions o f T. redivivum, which because of its large size clusters outside of its pro per evolutionary group (lecontei), and T. occelatum, which did not cluster with T. curvirostre. Skull characters contributed highly to species discrim ination, suggesting that unique feeding adaptations arose in different spec ies groups. Although genetic variation was detected at isozyme loci (averag e heterozygosity = 3.6%), these data yielded little phylogenetic resolution . Similarly, mtDNA restriction sites were relatively uninformative; hence, phylogenetic conclusions were based on sequence data. Phylogenetic analyses confirmed the monophyly of these traditionally recognized assemblages: ruf um group (T. rufum, T. longirostre, and T. guttatum), lecontei group (T. le contei, T. crissale, and T. redivivum), and cinereum group (T. bendirei and T cinereum). The cinereum and lecontei groups appear to be sister lineages . Monophyly of the curvirostre group (which also includes T occelatum) was not confirmed. Sequence data suggest that T. occelatum and T. curvirostre, which differ by 7.7% sequence divergence, are probably most closely related to the rufum group. Toxostoma rufum and T. longirostre have similar extern al appearances and differ by 5.0%. Toxostoma guttatum is restricted to Cozu mel Island and of ten is considered a subspecies of T. longirostre; it diff ers by more than 5% from the other two members of the rufum group and is a distinct species constituting the basal member of this group. The phenotypi cally distinctive T. bendirei and T. cinereum differ in sequence divergence by only 1.6%. Overall, mtDNA distances computed from coding genes (mean 8. 5%) exceeded distances computed from the control region (mean 7.6%), contra ry to expectation. Because neither allozymes nor mtDNA could unambiguously resolve the placement of T. occelatum and T. curvirostre, a scenario involv ing contemporaneous speciation is suggested. Application of a molecular clo ck suggested that most speciation occurred in the late Pliocene or early Pl eistocene.