Department of Biology, University of Ottawa, Ottawa, Ontario, Canada.
PLoS Biol. 2009 Nov;7(11):e1000250. doi: 10.1371/journal.pbio.1000250. Epub 2009 Nov 24.
The rarity of beneficial mutations has frustrated efforts to develop a quantitative theory of adaptation. Recent models of adaptive walks, the sequential substitution of beneficial mutations by selection, make two compelling predictions: adaptive walks should be short, and fitness increases should become exponentially smaller as successive mutations fix. We estimated the number and fitness effects of beneficial mutations in each of 118 replicate lineages of Aspergillus nidulans evolving for approximately 800 generations at two population sizes using a novel maximum likelihood framework, the results of which were confirmed experimentally using sexual crosses. We find that adaptive walks do indeed tend to be short, and fitness increases become smaller as successive mutations fix. Moreover, we show that these patterns are associated with a decreasing supply of beneficial mutations as the population adapts. We also provide empirical distributions of fitness effects among mutations fixed at each step. Our results provide a first glimpse into the properties of multiple steps in an adaptive walk in asexual populations and lend empirical support to models of adaptation involving selection towards a single optimum phenotype. In practical terms, our results suggest that the bulk of adaptation is likely to be accomplished within the first few steps.
有益突变的稀有性一直以来都阻碍了人们开发定量适应理论的努力。最近的适应性漫步模型，即通过选择依次取代有益突变，做出了两个引人注目的预测：适应性漫步应该是短暂的，并且随着连续突变的固定，适应度的增加应该呈指数级减小。我们使用一种新颖的最大似然框架，在两个种群大小下，对 118 个重复谱系的 Aspergillus nidulans 进行了约 800 代的进化，估计了每个谱系中有益突变的数量和适应度效应，这些结果通过性杂交的实验得到了证实。我们发现，适应性漫步确实往往是短暂的，并且随着连续突变的固定，适应度的增加会减小。此外，我们表明，这些模式与随着种群适应而有益突变供应减少有关。我们还提供了在每个步骤固定的突变中适应度效应的经验分布。我们的结果首次揭示了无性种群中适应性漫步中多个步骤的性质，并为涉及朝着单个最优表型选择的适应模型提供了经验支持。实际上，我们的结果表明，大部分适应可能会在最初的几个步骤中完成。
