HYBRID ZONES AND REPRODUCTIVE ISOLATION

Abstract

The interpretation of hybrid zones remains one of most difficult of taxonomic problems, despite striking advances in systematics of recent decades. Mayr (1963: 496) selected sexual isolation, hybrid fertility, and developmental compatibility as only criteria that will render possible unequivocal decisions (one species or two) in case of peripheral isolates, and same criteria should apply to hybrid zones. In hybrid zones, sexual isolation is always imperfect or absent, hybrids are often fertile, and developmental compatibility is sufficient to produce hybrids. All three criteria break down in hybrid zones without always rendering unequivocal decisions possible, and similar difficulties may be encountered in experimental analysis of peripheral isolates. These are anomalies that make decisions based on Mayr's well-known species definition difficult. The difficulties are due in part to gradual process of evolution itself, but they may also be due to a misunderstanding of definition, which states rather emphatically, at very outset, Species are groups of actually or potentially natural populations. In every hybrid zone, is actual and this opening phrase of definition promotes a natural tendency to consider question as settled once has been established. This feature of definition diverts attention from more essential remainder: . . which are isolated from other such groups. This is essence of Mayr's definition. Mere does not necessarily indicate an absence of reproductive isolation. Two good species will remain isolated however many sterile hybrids they might produce. The production of sterile or inviable hybrids may even be best possible evidence of full species status, as Mayr himself has pointed out. Proper understanding of Mayr's definition, of phenomenon of hybrid zones, and even of speciation process itself, depends upon clarity of our understanding of words reproductively isolated. Mayr (1959a) defines reproductive isolation in terms of the protective devices of a well-integrated and harmoniously coadapted gene pool against pollution by other gene pools, and he regards speciation as essentially completed when geographical isolation can be removed without resulting in genetic swamping of new daughter species by parental species. Dobzhansky (1950) omits word interbreeding from his species definition, and stresses reproductive community of gene pool, and later (1951: