Sexual Isolation, Speciation and the Direction of Evolution

Various concepts of evolutionary biology are reviewed to emphasize the significance of premating isolation barriers in processes of speciation. Concepts such as sexual selection, genetic drift, and the Founder Principle appear to lend support to Muller's views that premating isolating mechanisms arise as by-products of natural selection in allopatry. Examples are presented in support of this view and in contrast to that held by Fisher and Dobzhansky, who envisioned such mechanisms as arising upon recontact of populations in which some postmating isolating mechanism had arisen. Additionally, a hypothesis proposed by Kaneshiro (1976) which enables interpretation of the one-sided mating preference (asymmetrical isolation) observed between populations of Drosophila is discussed. Such a hypothesis, if valid, permits inferences of direction of evolution among closely related species.