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MORPHOMETRIC EVIDENCE FOR INCIPIENT SPECIATION IN DROSOPHILA SILVESTRIS FROM THE ISLAND OF HAWAII

Carson and Bryant (1979) reported a case of variability in a secondary sexual character of males of Drosophila silvestris, a giant fly endemic to the high-altitude rainforests of the geologically very new island of Hawaii. In populations from the north and east (NE it is wholly lacking in females and shows high heritability in crosses (Bryant and Carson, 1979). Both chromosomal and electrophoretic distances among populations of silvestris are negligible and silvestris has therefore been considered a clear example of a single biological species (Carson and Bryant, 1979; Craddock and Johnson, 1979). The evidence for differentiation in the NE three of these are allopatric to silvestris and are endemic to separate, geologically older islands. The data support the notion that the radical new burst of evolutionary change has occurred relatively recently and solely within the N&E populations of D. silvestris, and that we are observing a case of incipient speciation. The discovery of a species in statu nascendi is not in itself unusual. Many examples of incipient species are known. The exciting implication of the D. silvestris situation is that here we have an example of the beginning of the development of a morphological character which is intimately concerned with sexual behavior. Male tibial cilia have a clearly-defined biological importance: the only function of this leg surface appears to be a ritualized stimulation of the female's abdomen during the final and crucial stage of courtship (Spieth, 1978). Although such a character might first arise simply in response to altered sexual selection in the population, the same character might later serve as an ethological barrier to hybridization. With newly-formed full species which are sympatric, it is impossible to know whether ethological barriers to interbreeding arose in allopatry or upon secondary contact (Futuyma, 1979). We are left to speculate. In present-day communities, where rich "species swarms" appear to be maintained through prezygotic mating barriers, we can only surmise that these barriers arose in the absence of gene flow. In D. silvestris, we see a mechanism by which behaviorally-related reproductive isolation may arise in allopatry.