Variation in the reversibility of evolution.

How reversible is adaptive evolution? Studies of microbes give mixed answers to this question. Reverse evolution has been little studied in sexual populations, even though the population genetics of sexual populations may be quite different. In the present study, 25 diverged replicated populations of Drosophila melanogaster are returned to a common ancestral environment for 50 generations. Here we show that reverse evolution back to the ancestral state occurs, but is not universal, instead depending on previous evolutionary history and the character studied. Hybrid populations showed no greater tendency to undergo successful reverse evolution, suggesting that insufficient genetic variation was not the factor limiting reverse evolution. Adaptive reverse evolution is a contingent process which occurs with only 50 generations of sexual reproduction.