Artificial Selection in Drosophila simulans Males for Increased and Decreased Sexual Isolation from D. melanogaster Females

Crosses of homozygous yellow Drosophila melanogaster females with wild-type D. simulans males were studied. When the initial stock contained genetic variability, selection was successful in developing two lines which differed significantly, and it is clear that male isolation from foreign females has a genetic basis. In the later generations matings were several times more frequent in the decreased isolation line than in the increased isolation line. Mating between these two species more readily occurs when young females are used. The factors determining the success of a courtship apparently operate within 24 h when one male is confined with one female, because there is no increase in matings with longer time intervals in these pair matings. These experiments and others reported earlier indicate a cycle of increased isolation between these two species in the summer and reduced isolation in the winter. Matings were several times as frequent in the October-March interval as in the April-September interval.