Experiments on the incipient species of the Drosophila paulistorum complex.

Drosophila paulistorum Dobzhansky and Pavan is a superspecies which consists of at least five races or incipient species, living in tropical Central and South America (DOBZHANSKY and SPASSKY 1959; DOBZHANSKY, EHRMAN, PAVLOVSKY and SPASSKY 1964). An ethological (sexual) isolation and a sterility of hybrid males impede the gene exchange between the incipient species. Although the ethological isolation varies in intensity from strain to strain, apparently no interbreeding takes place where the geographic distribution areas of two or even three incipient species overlap (EHRMAN 1965). The process of speciation has evidently been completed in these overlap zones. On the contrary, in northern Colombia there is found a Transitional race, some strains of which give fertile hybrids with more than a single incipient species. MALOGOLOWKIN-COHEN, SIMMONS and LEVENE (1965) have described also transitional strains, which give fertile hybrids with more than one incipient species. The differences between the Transitional race and transitional strains is that the former has its own geographic distribution area, while the latter occur in populations in which other strains do not show a transitional character. And finally, DOBZHANSKY and PAVLOVSKY ( 1966) have. in a preliminary communication, described the spontaneous origin in a laboratory strain of a hybrid sterility with strains with which it was formerly fertile. Experiments designed to elucidate some of the above complex and varied behavior are reported in the present article, as well as in the accompanying work of WILLIAMSON and EHRMAN (1967).