Genetic studies of three sibling species ofDrosophilawith relationship to theories of speciation

SUMMARY Drosophila melanogaster,D. simulansandD. mauritianaare closely related species, the first two cosmopolitan and the last restricted to the oceanic island of Mauritius.D. simulansandD. mauritianaare the most closely related pair, with the latter species probably resulting from a founder event. The relatedness of the three species and their ability to hybridize allow tests of recent theories of speciation. Genetic analysis of two characters differing betweenD. simulansandD. mauritiana(sex comb tooth number and testis colour) show that the differences are due to at least five and three loci respectively. Behavioural tests further demonstrate that sex combs are probably used by males at a crucial step in mating, and that the differences between the two species may be related to differences in their mating ability. These genetic studies and previous work indicate that differences among these species are polygenic and not (as proposed by recent theories) attributable to only one or two loci of large effect. Further studies of interspecific hybrids show that genetic divergence leading to developmental anomalies is more advanced in the older species pairD. simulans/D. melanogaeterthan in the younger pairD. simulans/D. mauritiana. This supports the neo-Darwinian contention that reproductive isolation is one step in a continuous process of genetic change among isolated populations, and does not support current theories that such change occurs only during the evolution of reproductive isolation. Finally, investigations of the degree of gonadal atrophy and its sensitivity to temperature inD. simulans/D. mauritianahybrids fail to support recent speculations that phenomena similar to hybrid dysgenesis (which causes such atrophy inD. melanogaster) play a role in speciation.