Nature of Selection in the Northern Flicker Hybrid Zone and Its Implications for Speciation Theory

Abstract Hybrid zones are regions of steep genotypic transition between populations that have reached a level of divergence comparable to that between “good” species. Remarkably, however, hybrid zones appear to be equilibrium configurations where the parental populations (whether they are called species, subspecies, or races) retain their taxonomic identity and integrity even though reproductive isolation is incomplete or, in many instances, wholly lacking. Although species definitions remain very much in contention (Mayr, 1957, 1987; Ehrlich, 1961; Sokal and Crovello, 1970; Mishler and Donoghue, 1982; Cracraft, 1983; Ghiselin, 1987; McKitrick and Zink, 1988; Endler, 1989; Templeton, 1989; Moor , 1990), we think there would be unanimity that an essential property of a species is that there is uniformity of genotype within the species and a discontinuity between the genotype of one species and that of another. In other words, genotypic variance within a species is low compared to the variance between species. Reproductive isolation, of course, would enforce this essential property of species, but the sustained cohesion (sensu Templeton, 1989) of the hybridizing taxa and the abrupt discontinuity of genotype seen across hybrid zones imply that forces more fundamental than reproductive isolation are involved in speciation. Hybrid zones pro vide natural experimental systems where modern genetic analysis can be applied to discern the forces that drive the origin of species.