NATURAL HYBRIDIZATION BETWEEN THE SYMPATRIC HAWAIIAN SPECIES DROSOPHILA SILVESTRIS AND DROSOPHILA HETERONEURA

Two newly formed, morphologically distinct species of Drosophila from the island of Hawaii have been found to form fertile hybrids in two areas of sympatry. Both F₁ and backcross hybrids have been recognized in nature; in one case, the hybridization events extended over three years. Original hybridizations involved one or more D. silvestris females mating with D. heteroneura males. Female F₁ hybrids from this cross have participated in backcrosses to D. silvestris. In any one locality, less than 2% hybrids have been found in nature. A hybrid swarm was not formed; selection appears to favor a strict maintenence of morphologies characteristic of the separate species. This result is attributed to pervasive sexual selection, which serves to preserve the syndromes of sexual characteristics that arose during past allopatric divergence. Populations of D. silvestris both within and outside the present range of D. heteroneura often display heritable variation in color patterns involving the abdomen, pleurae, legs, and wings. Genes effecting variation in these characters may be derived from genes involved in a past introgression from D. heteroneura. Independent evidence for past hybridization between these species comes from study of mitochondrial DNA. Although the inferred direction of the cross is the opposite of that observed in the recent case described here, both reciprocal crosses have been obtained experimentally in the laboratory. Accordingly, we suggest that these species may have been open to hybridization since their first sympatic encounters following their inception in allopatry. That they remain as strictly recognizable morphological entities is due both to their current partial allopatry and to the action of sexual selection in maintaining two separate major modes of efficient reproduction. There is no reason to invoke specific reinforcing selection that has imposed reproductive isolation.