Drosophila heteroneura and Drosophila silvestris: Head Shapes, Behavior and Evolution

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Abstract
It is postulated that D. heteroneura and D. silvestris are the descendants of a migrant from the D. differens population of Molokai [Hawaii, USA] which colonized the young Hualalai volcano. The character of the Hualalai forest was such that the males of the colonizing population were subjected to intense predation when occupying their courtship arenas (leks). The males abandoned their ancestral upright fighting posture and kept their horizontal with and close to the substrate and then rushed head on against an opponent. This shift in behavior produced the selection pressure resulting in the evolution of the hypertrophied maxillary palps and the unique head structure which the D. heteroneura male now exhibits. The evolution of the hypertrophied maxillary palps preceded in time that of the head shape. When the Hualalai population had evolved the hypertrophied male maxillary palps but had not fully acquired the unique head shape of present day D. heteroneura, a migrant from this population colonized the then mature Niole volcano. The Niole forests at that time possessed a character similar to Molokai and the selection pressure that was operating on the Hualalai population was eliminated. Evolution of the head shape ceased but the hypertrophied maxillary palps were retained. Subsequent growth of Mauna Kea and especially Mauna Loa resulted in the fusions of the 7 volcanoes that comprise the present-day island of Hawaii and allowed the 2 spp. to expand their areas and become sympatric.