Variation in the reversibility of evolution

How reversible is adaptive evolution(1-6)? Studies of microbes give mixed a nswers to this question(6-11). Reverse evolution has been little studied in sexual populations(12-14), even though the population genetics of sexual p opulations may be quite different. In the present study, 25 diverged replic ated populations of Drosophila melanogaster are returned to a common ancest ral environment for 50 generations. Here we show that reverse evolution bac k to the ancestral state occurs, but is not universal, instead depending on previous evolutionary history and the character studied. Hybrid population s showed no greater tendency to undergo successful reverse evolution, sugge sting that insufficient genetic variation was not the factor limiting rever se evolution. Adaptive reverse evolution is a contingent process which occu rs with only 50 generations of sexual reproduction.