Reproductive isolation in Damselflies

Paulson, D. R. (Department of Zoology, University of Washington, Seattle, Washington 98195) 1974. Reproductive Isolation in Damselflies. Syst. Zool. 23:40-49. -Experiments were performed in the field on males of five species and females of ten species of damselflies (Odonata: Zygoptera: Coenagrionidae) to determine the relative importance of visual and mechanical reproductive isolating mechanisms. Males of all five species did not distinguish visually their own female from heterospecific females of the same genus, but they were slightly less responsive to females of other genera, probably because of size differences. When the males attempted to mate with females of other species, they were usually prevented from doing so because their abdominal appendages were unable to secure a firm grip on the appropriate thoracic structures in the females. In only one species pair was the male able to achieve the tandem position successfully most of the time, and the reciprocal was unable to do so. Mechanical isolation is clearly very important in this group of odonates, and it is hypothesized that it will be found to be important in all odonates in which male appendages differ substantially between species while female coloration does not. [Reproductive isolation; damselflies. ] Recent discussions of reproductive isolation in animals (Littlejohn, 1969; Mayr, 1970; Dobzhansky, 1970; Stebbins, 1971) emphasize that there is very little evidence for the operation in nature of the phenomenon of mechanical isolation, in which the genitalia of a male of one species fail to fit with the genitalia of a female of another species, thus preventing mating. Mayr (1963, 1970) cited a study by Webb (1947) of polygyrid snails as perhaps the only good example of interspecific mating being prevented by the structure of the genitalia. That paper is not convincing, the most positive evidence of mechanical isolation being "observations wherein the mating has been noted to be obstructed by out-of-timing malfunctioning. In one case, persistently-faster-eversion prevented a common entwistment of the penis with that of the prospective mate" (Webb, 1947:137). This apparently was an attempted mating between individuals of the same species, and Webb went on to say that he had never observed attempts at "interspecific sementransfer by any of the polygyrin species studied. " In the related subfamily Triodopsinae cross-copulation between different species was observed, and in some of these species it was successful. Webb did not write that he observed individuals of different species attempting copulation but being prevented from copulating by the structure of their genitalia, and his paper is not evidence of mechanical isolation in animals. Mayr (1963) further cited two lines of evidence from a number of taxa against the existence of mechanical isolation: (1) species with different genitalia can interbreed; and (2) altering the male genitalia fails to prevent successful copulation and fertilization. Thus he concluded that "mechanical isolation plays a very minor role as an isolating mechanism in most groups of animals" (1963: 104). In fact, Mayr (1970) attempted to explain the differences among the genitalia of related species as incidental changes, the pleiotropic by-products of many changes in the genetic constitution of the species. He hypothesized that as long as any sort of apparatus could function during clasping and copulation, then variations would not be subject to selection. Dobzhansky went further in stating that the "usefulness of genitalia for distinguishing species does not necessarily mean that they are important in mechanical isolation. The reason for their usefulness is that the complexity of genitalic structures is often so great that species differences are more likely to be manifested in these structures than in