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ABSTRACT
Interspecific hybridization is widespread and frequent among frogs and toads. Hybrids are not abnormalities but are the normal results of particular kinds of genetic interactions. Hybrid zones in the form of steep clines may appear where two differentiated populations contact each other, frequently in association with an environmental gradient, but hybrids also occur among otherwise genetically independent sympatric populations. Hybrid interactions among the members of the Bufo americanus group of toads illustrate taxonomic difficulties engendered by competing interpretations. Hybrid zones in the form of clines between parapatric species have been used as evidence to synonymize taxa, yet sympatric hybrid zones between other related species have not. Examples of both kinds of hybrid zones in toads exhibit temporal dynamics and the range of hybrid interactions is best viewed as a continuum of differentiation. Taxa which hybridize have vague borders since hybrids transgress the boundaries of species taxa. Natural hybridization poses a problem for taxonomic practice because it cannot be accommodated by the Linnaean system that relies upon two-valued Aristotelian logic when addressing questions of categorical species membership. Continuous-valued (fuzzy) logic provides an underpinning for rigorous description of the boundaries of species where hybridization renders them obscure.