SEXUAL BEHAVIOR AND ISOLATION IN DROSOPHILA. I. THE MATING BEHAVIOR OF SPECIES OF THE WILLISTONI GROUP

Species maintain their distinctness from other related species by means of isolating mechanisms. Divers analyses of the isolating factors show them to be of sundry types operating at different levels anrd in various manners in the lives of the organisms. These mechanisms function and thereby become identifiable only when species come into contact. Obviously these isolating factors must have originated before this happens, or the species could notmaintain their integrity. This indicates that it is very-improbable that the student will ever see any of these isolating mechanisms come into existence. Their origin must therefore be studied by indirection. Of the various subdivisions of reproductive isolation that are to be found actively functioning in nature, sexual isolation appears to be one of the most effective. Most bisexual animals court and mate only with individuals of their own species; at the same time, they are able to disregard specimens of other closely related species that they happen to meet. Suchdiscrimination seems to depend upon innate abilities of the organisms which express themselves through a behavioral pattern that ensures sufficient successful matings for the survival of the species and simultaneously isolates the species. The investigation of the sexual behavior of a group of closely related species should not only furnish information concerning the specific behavioral mechanisms involved, but also such findings might be of general value in the whole problem of the nature of behavioral mechanisms. Such a study demands as a first step a detailed inventory of the whole set of actions and reactions involved in the sexual behavior. The cataloguing of such behavioral activities is of more than purely descriptive value since the degree and kind of differences between the species are measures of the relationships of the various species. These behavioral differences can then be compared with the morphological differences with the view of determining whether or not the two types of differences parallel or diverge from each other. Finally, strch, a study gives us anacquaintanceship with and possibly clues'to those specific activities that are the actual isolating mechanisms. For a variety of reasons, most studies of this type can not be done exclusively in the field but must be at least partially conducted in the laboratory. This necessitates the selection of material that can adequately be handled under laboratory existence without imposing upon the organisms conditions that patently call forth abnormal reactions. Various workers have used sundry materials, but none seems to be more suitable for such studies than members of the genus Drosophila. Not only are they amenable to laboratory conditions, but there is also known a great deal a-bout their biology, thanks to the work of many investigators. In addition, the work of the past decade has shown that there are within the genus many groups of closely related species which offer ideal materialfor an analysis of sexual behavior. The present study concerns itself with the willistoni group of the subgenus Sophophora of the genus Drosophila.