Speciation, the "mystery of mysteries", still remains one of the most challenging biological problems. Our primary interest is characterizing genes that cause reproductive isolation, particularly hybrid male sterility, between the fruitfly sibling species Drosophila simulans and D. mauriatiana. Genes that stabilize the Mendelian segregation appear to play a pivotal role in the speciation process. Whenever asymmetry of transmission exists, genes that cause non-Mendelian segregation can evolve. One such example we are studying is sex-ratio meiotic drive in D. simulans. Our working hypothesis is that genetic conflicts over Mendelian segregation are the major driving force in the formation of new species.