Spontaneous origin of an incipient species in the Drosophila paulistorum complex.

Abstract

The snperspecies Drosophila paulistorum is a complex of several incipient species. Strains of the same incipient species cross easily and produce fertile hybrids; strains of the different ones exhibit strong ethological (sexual) isolation; and the F1 hybrids are fertile females and sterile males. The male sterility continues in the blackcrosses. A strain established in the laboratory from flies collected in the Llanos of Colombia in 1958 behaved at first as a member of the Orinocan incipient species. By 1963 it changed so that its male F1 hybrids with other Orinocan strains became completely sterile. The backcross hybrids, however, continue fertile, and no strong ethological isolation from Orinocan strains has arisen. The reproductive isolation between Llanos and other incipient species has not been altered. Some possible causes of the emergence of the new incipient species are indicated.