Is sexual selection and species recognition a continuum? Mating behavior of the stalk-eyed fly Drosophila heteroneura.

If behavioral isolation between species can evolve as a consequence of sexual selection within a species, then traits that are both sexually selected and used as a criterion of species recognition by females should be identifiable. The broad male head of the Hawaiian picture-winged fly Drosophila heteroneura is a novel sexual dimorphism that may be sexually selected and involved in behavioral isolation from D. silvestris. We found that males with broad heads are more successful in sexual selection, both through female mate choice and through aggressive interactions. However, female D. heteroneura do not discriminate against hybrids on the basis of their head width. Thus, this novel trait is sexually selected but is not a major contributor to species recognition. Our methods should be applicable to other species in which behavioral isolation is a factor.