Speciation in Vireos. I. Macrogeographic Patterns of Allozymic Variation in the Vireo solitarius Complex in the Contiguous United States

Using allozymes, I examined genetic variation in 23 populations of four cur- rently recognized subspecies of the Solitary Vireo (Vireo solitarius solitarius. V. s. alticola, V. s. cassinii, and V. s. plumbeus), with emphasis on the latter two taxa. Eighteen of 38 genetic loci (47.4%) were polymorphic. Intrataxon Nei's genetic distances were low (D = 0.0008) among populations of V. s. cassinii and I/ s. plumbeus. Average intertaxon Nei's D ranged from zero (V. s. solitarius vs. V. s. alticola) to 0.0030 (V s. solitarius vs. V. s. cassinii), 0.0033 (V. s. cassinii vs. V. s. alticola), 0.0283 (V. s. plumbeus vs. V. s. solitarius), 0.0288 (V. s. plumbeus vs. V. s. alticola), and 0.0294 (V. s. plumbeus vs. V. s. cassinii). Nei's D between the Yellow-throated Vireo (Vireo flavifrons) and the four taxa of Solitary Vireo ranged from 0.0430-0.0743. A mean F,, value of 0.289 over all populations of V. solitarius, and across combined populations of V. s. cassinii and V. s. plumbeus, indicated pronounced genetic discontinuity between these two forms. Gene flow estimates between populations representing the subset of V. s. cassinii and V. s. plumbeus ranged from one individual every two generations (Wright's 195 1 formula) to an average of 12 immigrants per generation over all populations (Slatkin's rare allele method). Based on strong allozymic divergence from the three other allopatric forms treated here, the Plumbeous Vireo (Vireo plumbeus) is a species. Although similar allozymically, the Cassin's Vireo (Vireo cassinii) and the Blue- headed Vireo (V. s. solitarius plus the "Mountain Vireo," V. s. alticola) also may deserve species status because data published by others demonstrate trenchant differences in voice and mtDNA base sequences.