THE BOUNDS OF SPECIES - HYBRIDIZATION IN THE BUFO-AMERICANUS GROUP OFNORTH-AMERICAN TOADS

Interspecific hybridization is widespread and frequent among frogs and toads. Hybrids are not abnormalities but are the normal results of pa rticular kinds of genetic interactions. Hybrid zones in the form of st eep dines 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 p opulations. Hybrid interactions among the members of the Bufo american us group of toads illustrate taxonomic difficulties engendered by comp eting interpretations. Hybrid zones in the form of dines between parap atric species have been used as evidence to synonymize taxa, yet sympa tric 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 differen tiation. Taxa which hybridize have vague borders since hybrids transgr ess the boundaries of species taxa. Natural hybridization poses a prob lem for taxonomic practice because it cannot be accommodated by the Li nnaean system that relies upon two-valued Aristotelian logic when addr essing 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.