Evolution of Reproductive Isolation in Allopatric and Sympatric Populations

Experiments were designed to measure the rate of evolution in sympatric (30% gene flow) and allopatric populations of houseflies under similar types of divergent selection (+ or - geotaxis). Under 5% selective pressure on both sympatric and allopatric populations, both diverged rapidly, with the allopatric populations evolving slightly faster during the 38 generations of the experiment. Incipient reproductive isolation occurred in both sympatric and allopatric population systems. In one test, a greater tendency toward reproductive isolation existed in sympatric populations than in allopatric ones.