ADAPTIVE EVOLUTION AND THE FOOTPRINTS OF HISTORY

摘要

The genetic structure of a population at any given time is a reflection of the combined effects of many factors such as past selection history, ongoing selection, ancestry and chance, in the form of random divergence among lineages derived from a common ancestor. In this paper, I describe results from two experiments on the bacterium Escherichia coli and the fruit-fly Drosophila melanogaster, respectively, in which the contribution of these various factors to adaptive evolution was quantified in rigorous laboratory experiments. In both species, current ongoing selection obliterated the effects of past selection and ancestry on fitness-related traits. In the asexually reproducing E. coli, the effects of history on traits not related to fitness were still important after 1000 generations, whereas in the sexually reproducing D. melanogaster, traits uncorrelated with fitness showed most variation (similar to 95%) to be among individuals within populations. When an effect of history was seen, it was largely due to past selection experienced by a population rather than ancestry per se. The time scale of adaptive evolution was much faster in D. melanogaster, and the obliteration of the effects of history much more complete, suggesting that genetic recombination may play a major role in removing historical constraints and facilitating adaptive evolution in sexually reproducing species. [References: 30]