SEXUAL ISOLATION, SPECIATION AND THE DIRECTION OF EVOLUTION.

Abstract

Kaneshiro (1976), in his study of behavioral isolation among four species of Hawaiian Drosophila, made the observation that isolation was asymmetrical between allopatric species. That is, between any allopatric pair of species, sexual isolation was strong in one direction but showed weak or no isolation in the other reciprocal. Based on the assumption that elements of behavior are lost during founder events due to severe drift conditions and the genetic revolution which accompanies such events, Kaneshiro proposed a hypothesis to infer the direction of evolution for the four species studied. Ohta (1978) analyzed sexual isolation among six populations of two other Hawaiian Drosophila species and observed striking asymmetry in pair-wise combinations of four of the six. By using Kaneshiro's (1976) hypothesis, Ohta was able to propose a uni-directional phylogeny for the six populations of the grimshawi complex of species. A preliminary search of the literature on ethological isolation between populations of Drosophila indicates that one-sided mating preference or asymmetrical isolation is not an uncommon phenomenon. Re-evaluation of these data in terms of the hypothesis proposed by Kaneshiro (1976) complements phylogenetic interpretations inferred from various other techniques. This paper elaborates the hypothesis proposed by Kaneshiro (1976). Additional evidence which appear to support his ideas will be presented. A discussion of the origin of premating isolation mechanisms is presented first to emphasize the role of sexual isolation in the speciation process.