Divergent selection for geotactic response and evolution of reproductive isolation in sympatric and allopatric populations of houseflies

Experimental populations of houseflies subjected to 95% selective pressure for geotactic preference under conditions of 50% potential gene flow and of allopatry evolved reproductive isolation after only 16 generations. No significant difference was found between sympatric and allopatric populations. Our study does not support the hypothesis that incipient reproductive isolation occurs more rapidly under conditions of sympatry than allopatry.