MOLECULAR PHYLOGENY AND ECOLOGICAL DIVERSIFICATION IN A CLADE OF NEW-WORLD SONGBIRDS (GENUS VIREO)

We constructed a molecular phylogeny for a clade of eye-ringed vireos (Vireo flavifrons and the V. solitarius complex) to examine existing h ypotheses of speciation and ecological diversification. Complete seque nces of the mtDNA cytochrome b gene were obtained from 47 individuals of this group plus four vireonid outgroups. Mean levels of sequence di vergence in the clade varied from 0.29% to 5.7%. Differences were grea test between V. flavifrons and four taxa of 'V. solitarius'. The latte r separated into three taxonomic, geographical and ecological groups: V. plumbeus plumbeus, V. cassinii cassinii, and V. solitarius solitari us plus V. solitarius alticola. These differed by an average of 2.6-3. 2%. Populations within each group revealed low levels of sequence vari ation ((x) over bar = 0.20%) and little geographical structuring. The mtDNA data generally corroborate results from allozymes. V. plumbeus s hows a loss of yellow-green carotenoid pigmentation from the ancestral condition. The occupancy of relatively dry habitats by this species a nd V. cassinii represents a derived ecological shift from more-humid e nvironments occupied by other species of vireonids. Ecological diverge nce in this clade occurred in allopatry and is associated with generic -level stability in morphometrics and foraging styles. Migratory behav iour and seasonal habitat shifts apparently evolved multiple times in vireos breeding in temperate environments. Present geographical and ec ological distributions, and low levels of intrataxon genetic divergenc e, are hypothesized to be the result of postglacial regionalization of climate-plant associations and rapid northward expansion of breeding ranges.