The recent revival of interest in modes of speciation without geographic isolation (reviewed by Bush, 1975; Endler, 1977; White, 1978) makes it appropriate to reinvestigate mechanisms for clinal speciation first outlined in genetic terms by Fisher (1930, Ch. 6). He noted that following adaptive divergence of geographical or ecological races, if hybrids are at a disadvantage, genetic variation in mating preferences (and/or habitat preferences) could lead to the evolution of a decreased frequency of hybridization. This mechanism for the evolution of sexual isolation was popularized by Dobzhansky (1940, 1951, 197 0 , Ch. 11) who believed it could occur only after secondary contact between races which had become partially reproductively isolated by postmating mechanisms (see also Sturtevant, 1938; Muller, 1940; Sibley, 1961; Mayr, 1963, p. 525-526, 548-559). Fisher (1915, 1930) also described a process of runaway sexual selection which can produce exponentially increasing rates of evolution in male secondary sexual characters and female mating preferences. As argued below, the latter mechanism is potentially much more powerful for initiating sexual isolation and character divergence along a cline than is selection against hybridization. Population genetic models have shown that when hybrids between geographic races are at a substantial disadvantage a narrow hybrid zone will evolve, followed perhaps by formation of reversed clines (i.e., reproductive character displacement) around the contact zone, and slow or limited diffusion of the genetic differences to remote regions by migration (Crosby, 1970; Endler, 1977; Caisse and Antonovics, 1978). However, if populations in the contact zone have comparatively low mean fitness, in part due to hybrid disadvantage, and if there is a net immigration from surrounding regions, gene flow may prevent the evolution of sexual isolation or may limit it to the contact zone (Littlejohn, 1969; Hall, 1973). Hybrid zones of apparent long standing are known in various animal taxa where neither widespread sexual isolation nor localized reproductive character displacement has occurred (Mayr, 1963, p. 548-550; Bigelow, 1965; White, 1978, Ch. 6). In contrast with the localized origin and slow or restricted spread of sexual isola-