SPECIATION IN VIREOS .1. 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 fou r currently recognized subspecies of the Solitary Vireo (Vireo solitar ius solitarius, V. s. alticola, V. s. cassinii, and V. s. plumbeus), w ith 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 V. s. plumbeus. Averag e intertaxon Nei's D ranged from zero (V. s. solitarius vs. V. s. alti cola) to 0.0030 V. s. solitarius vs. V. s. cassinii, 0.0033 (V. s. cas sinii 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. plumb eus vs. V. s. cassinii). Nei's D between the Yellow-throated Vireo (Vi reo flavifrons) and the four taxa of Solitary Vireo ranged from 0.0430 -0.0743. A mean F-st value of 0.289 over all populations of V. solitar ius, and across combined populations of V. s. cassinii and V. s. plumb eus, indicated pronounced genetic discontinuity between these two form s. Gene how estimates between populations representing the subset of V . s. cassinii and V. s. plumbeus ranged from one individual every two generations (Wright's 1951 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. Altho ugh 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 seque nces.