Silander Olin K, Tenaillon Olivier, Chao Lin
Division of Biology, University of California San Diego, La Jolla, California, United States of America.
PLoS Biol. 2007 Apr;5(4):e94. doi: 10.1371/journal.pbio.0050094.
The most consistent result in more than two decades of experimental evolution is that the fitness of populations adapting to a constant environment does not increase indefinitely, but reaches a plateau. Using experimental evolution with bacteriophage, we show here that the converse is also true. In populations small enough such that drift overwhelms selection and causes fitness to decrease, fitness declines down to a plateau. We demonstrate theoretically that both of these phenomena must be due either to changes in the ratio of beneficial to deleterious mutations, the size of mutational effects, or both. We use mutation accumulation experiments and molecular data from experimental evolution to show that the most significant change in mutational effects is a drastic increase in the rate of beneficial mutation as fitness decreases. In contrast, the size of mutational effects changes little even as organismal fitness changes over several orders of magnitude. These findings have significant implications for the dynamics of adaptation.
二十多年来实验进化中最一致的结果是，适应恒定环境的种群适应性并非无限增加，而是达到一个平稳期。通过噬菌体实验进化，我们在此表明反之亦然。在足够小的种群中，随机漂变压倒选择并导致适应性下降，适应性会下降至一个平稳期。我们从理论上证明，这两种现象必定要么归因于有益突变与有害突变比例的变化、突变效应的大小，要么归因于两者。我们利用突变积累实验和实验进化的分子数据表明，随着适应性下降，突变效应最显著的变化是有益突变率急剧增加。相比之下，即使生物体适应性在几个数量级上发生变化，突变效应的大小变化也很小。这些发现对适应动态具有重要意义。
