Change in a secondary sexual character as evidence of incipient speciation in Drosophila silvestris

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Abstract
Search for genetic changes that are pivotal in species formation led to intraspecific studies of D. silvestris, a giant species found only on the geologically new island of Hawaii [USA]. Males bear large, curved, modified bristles or cilia on the dorsal surface of the foreleg tibia and tarsus. In males from the south and west parts of the island, there are 2 rows of cilia separated by a naked area. In males from the north and east there is a mean of 20-30 additional cilia between the 2 major rows on the tibia. These extra cilia are absent in closely related species of this subgroup, including the sympatric species D. heteroneura and 3 spp. from adjacent islands. Males use the foreleg tibiae in vibratory movements against the female''s abdomen during courtship, so this character difference is likely to be important in the reproductive biology of the species. Inversion polymorphisms are similar in both northeast and southwest populations; they show large and strikingly parallel altitudinal shifts in frequency distributions involving the same inversions. Populations from various parts of the island cannot be distinguished by routine electrophoresis of soluble proteins encoded by 25 loci. Thus, the extra cilia character is superimposed on a more ancient genetic background of similarity involving both chromosomal and electrophoretic polymorphisms. The extra cilia are interpreted as a specific new embellishment of a secondary sexual character brought about by altered sexual selection occurring very recently in 1 part of the species range. This suggests incipient speciation.