Morphometric Evidence for Incipient Speciation in Drosophila silvestris from the Island of Hawaii

Abstract

A cluster of 5 closely related species of Hawaiian Drosophila [D. differens, D. hemipeza, D. heterorneura, D. planitibia and D. silvestris] display a male secondary sexual character consisting of rows of enlarged bristles (cilia) on the dorsal surface of the foreleg tibias. These cilia are used by the male to stimulate the female during courtship. In 4 of the 5 spp., this character is conservative and stable with low variances in number; cilia are deployed in 2 marginal rows separated by a bare area. D. silvestris of Hawaii Island, is strikingly different. In [South and West] populations, legs of males resemble those of the other species, but those from the [North and East] side of the island show a much larger number of cilia with greatly increased variance. These cilia are distributed as additions to one of the marginal rows and in a new irregular row in the area which is bare in the other species and in [South and West] populations of D. silvestris. Groupings identified by a posteriori test suggest that this change took place in 2 stages; males from 2 [North and East] localities (Kohala and Piihonua) show fewer new cilia than those from the more easterly populations. Kohala probably is an early derivative from [South and West] populations, as exemplified by the present-day population on Hualalai volcano. [North and East] silvestris probably is phylogenetically new, since flies from the older islands and other populations from Hawaii lack the embellishment. Intraspecific founder effects probably were conducive to a shift in the balance of the polygenes which serve as the genetic basis of the bristle character. This in turn may force the newly-formed allopatric population to shift from an old genetic equilibrium to a new one. This shift probably is mediated by an altered sexual selection favoring an increase in cilia number. The change may interpreted as one which could be important in species incipience although at first the change might have been purely an intrapopulational phenomenon involving the genetic coadaptation of the sexes.