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 phenetic pattern of variation in skeletal morphometrics generally matched traditional taxonomic groupings (based on plumage patterns) with the exceptions of T redivivum, which because of its large size clusters outside of its proper evolutionary group (lecontei), and T occelatum, which did not cluster with T curvirostre. Skull characters contributed highly to species discrimination, suggesting that unique feeding adaptations arose in different species groups. Although genetic variation was detected at isozyme loci (average 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: rufum group (T. rufum, T longirostre, and T. guttatum), lecontei group (T lecontei, 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 external appearances and differ by 5.0%. Toxostoma guttatum is restricted to Cozumel Island and often is considered a subspecies of T. longirostre; it differs 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 phenotypically 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%), contrary to expectation. Because neither allozymes nor mtDNA could unambiguously resolve the placement of T. occelatum and T. curvirostre, a scenario involving contemporaneous speciation is suggested. Application of a molecular clock suggested that most speciation occurred in the late Pliocene or early Pleistocene. Received 12 June 1998, accepted 17 February 1999. THE AVIAN GENUS Toxostoma (thrashers) includes 10 formally recognized species distributed throughout much of North America (Fig. 1). Species' distributional areas vary in extent from eastern North America (T rufum) to Cozumel Island (T guttatum). Most species inhabit arid or semiarid regions. Engels (1940) evaluated patterns of bill shape and plumage pattern (especially presence and shape of ventral spots) and recognized four species groups: (1) the rufum group (T rufum [Brown Thrasher], T guttatum [Cozumel Thrasher], and T longirostre 3E-mail: [email protected] [Long-billed Thrasher]); (2) the cinereum group (T cinereum [Gray Thrasher] and T bendirei [Bendire's Thrasher]); (3) the occelatum group (T occelatum [Occelated Thrasher] and T. curvirostre [Curve-billed Thrasher]); and (4) the lecontei group (also known as the sickle-billed group; T lecontei [Le Conte's Thrasher], T crissale [Crissal Thrasher], and T redivivum [California Thrasher]). Toxostoma rufum and T bendirei are migratory, whereas the other species are thought to be generally sedentary. All species tend to be secretive and terrestrial; many run rather than fly when startled. Hubbard (1973) noted parallels in the distri-