Behavioural isolation between two closely related Hawaiian Drosophila species: the role of courtship

Behavioural isolation plays a critical role in several recent models of spe ciation. A detailed understanding of the process of speciation requires ana lysis of taxonomic groups that have not completed reproductive isolation. W e studied D. silvestris and D. heteroneura because they are still in the pr ocess of divergence: behavioural isolation between them is incomplete, and neither postzygotic nor ecological isolation has been detected. Behavioural isolation is due to the failure of courtships between male D. silvestris a nd female D. heteroneura: there is no postzygotic isolation from either par ental species. The Fl hybrids are as successful in courtship with parental individuals as same-species pairs, which suggests that the hybrids resemble male D. heteroneura or female D. silvestris in some behaviour patterns tha t are crucial to mating success. We searched for this crucial resemblance b y examining courtship between F1 hybrids and the parental adults. We found that successful F1 males are somewhat more similar than unsuccessful F1 mal es to D. heteroneura males, but nevertheless they were intermediate between males of the two species. We also found that in both species the presence of female wings is necessary for courtship to proceed to copulation. These results reinforce an earlier report that behavioural isolation between thes e species is largely attributable to the decision as to whether to court at all, rather than to the details of courtship. (C) 2000 The Association for the Study of Animal Behaviour.