DROSOPHILA PAULISTORUM, A CLUSTER OF SPECIES IN STATU NASCENDI

Abstract

The geographic distributions of the 6 groups or subspecies are distinct. In 3 localities, representatives of 2 groups occur together sympatrically and yet remain incapable of interbreeding. These localities are: (1) Barro Colorado Island, Panama, where groups B and D have been found; (2) Llanos near Villavicencio, Colombia, groups C and D, and (3) Georgetown, British Guiana, groups D and E. In all these cases the sympatric populations yield no hybrid progenies and no cross-insemination. Inability of sympatric Mendelian populations to interbreed and exchange genes is prima facie evidence that these populations belong to different species. The authors conclude, however, that D. paulistorum is, considered as a whole, a single species. Compelling evidence comes from the existence of bridging populations, which do produce fertile hybrids when crossed to other populations which are reproductively isolated from each other. In addition to being 1 species, D. paulistorum is also a cluster of species in statu nascendi and in the authors'' opinion bespeaks the correctness of Darwin''s opinion that "each species first existed as a variety".