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Hybridization between the southwestern toad (Bufo microscaphus) and Woodhouse's toad (B. woodhousii) along the Agua Fria and Hassayampa rivers in central Arizona was investigated over a 3-yr period. Variation in allozymes and the pulse rate of male release calls was compared with morphological variation in these taxa and their hybrids. At two sites on the Agua Fria drainage, data on release calls, but not morphology, were concordant with allozyme evidence of hybridization. At a third site on the Agua Fria, morphological data, but not release calls, corroborated electrophoretic evidence of hybridization. Along the Hassayampa River, all lines of evidence indicated that there has not yet been any hybridization in an...
The Anaxyrus boreas species group currently comprises four species in western North America including the broadly distributed A. boreas, and three localized species, Anaxyrus nelsoni, Anaxyrus exsul and Anaxyrus canorus. Phylogenetic analyses of the mtDNA 12S rDNA, cytochrome oxidase I, control region, and restriction sites data, identified three major haplotype clades. The Northwest clade (NW) includes both subspecies of A. boreas and divergent minor clades in the middle Rocky Mountains, coastal, and central regions of the west and Pacific Northwest. The Southwest (SW) clade includes A. exsul, A. nelsoni, and minor clades in southern California. Anaxyrus canorus, previously identified as paraphyletic, has populations...