DOES ASYMMETRICAL MATING PREFERENCE SHOW THE DIRECTION OF EVOLUTION

The analysis of present behavior patterns is often a useful tool in our attempts to decipher the past. Studies of reproductive behavior have proven particularly fruitful in this regard, for the extent and type(s) of ethological barriers between two or more species can sometimes offer clues concerning key events in their evolutionary history. The usual approach is to obtain data on the relative willingness of individuals to mate, indicated by some type of isolation index. This index may actually be a function of both sex drive and discrimination. The strength of the isolation present-as measured by the isolation index-together with other relevant information, is then used to determine evolutionary patterns. Such investigations have shown that certain species pairs exhibit asymmetrical isolation. Asymmetrical isolation is said to exist when sexual isolation between the females of one population (population A) and the males of another population (population B) is greater than that found in the reciprocal crosses (females of population B with males of population A). One might expect that the rate of the development of sexual isolation between two populations would, by chance, occasionally be asymmetrical. Indeed, where isolation is not complete, equality of isolation between populations may be the exception rather than the rule. On the other hand, since the development of reproductive isolation is crucial to the speciation process, the presence of asymmetrical mating preferences may actually reflect the mechanism by which speciation has occurred. Toward this end, Kaneshiro (1976) and Watanabe and Kawanishi (1979) proposed models whereby they suggested that the direction of evolution can be determined by the examination of patterns of asymmetrical isolation. Kaneshiro (1976) and later Ohta (1978) uncovered asymmetry in mating preferences among several Hawaiian Drosophila species. Kaneshiro's model, upon which Ohta elaborated, suggested that the females of the putative ancestral species are more isolated from males of the putative derived species than are the derived females from the ancestral males. In 1979 Watanabe and Kawanishi offered a contrasting model based on their experimental work with the Drosophila melanogaster species group (Watanabe and Kawanishi, 1979). Here they had found an asymmetrical mating preference whereby the females of the putative derived species were more isolated from the males of the putative ancestral species than were the ancestral females from the derived males. This seemed also to be true of the published work on the virilis group. Thus, for any given set of asymmetrical isolation indices, the direction of evolution proposed by Watanabe and Kawanishi is exactly opposite to that suggested by Kaneshiro. If either of these two models is generally correct, a very powerful tool is available to researchers. Both are adequate to explain the data for which they were designed. A good test of these models would be to apply them to new data. For this purpose we will use the results of a study made on the Drosophila arizonensis-Drosophila majavensis species pair in which we investigated the reproductive relationships between various allopatric and sympatric populations of the two species (Wasserman and Koepfer, 1977). We found a much higher level of sexual isolation between sympatric populations