Data from: Evolutionary advantage of small populations on complex fitness landscapes

Recent experimental and theoretical studies have shown that small asexual populations evolving on complex fitness landscapes may achieve a higher fitness than large ones due to the increased heterogeneity of adaptive trajectories. Here we introduce a class of haploid three-locus fitness landscapes that allow the investigation of this scenario in a precise and quantitative way. Our main result derived analytically shows how the probability of choosing the path of the largest initial fitness increase grows with the population size. This makes large populations more likely to get trapped at local fitness peaks and implies an advantage of small populations at intermediate time scales. The range of population sizes where this effect is operative coincides with the onset of clonal interference. Additional studies using ensembles of random fitness landscapes show that the results achieved for a particular choice of three-locus landscape parameters are robust and also persist as the number of loci increases. Our study indicates that an advantage for small populations is likely whenever the fitness landscape contains local maxima. The advantage appears at intermediate time scales, which are long enough for trapping at local fitness maxima to have occurred but too short for peak escape by the creation of multiple mutants.近期的实验与理论研究表明，在复杂适应度景观（fitness landscape）上进行演化的小型无性繁殖种群，其适应度水平往往高于大型种群，这一现象源于适应性演化轨迹的异质性显著提升。本研究提出一类单倍体（haploid）三位点适应度景观，可用于精准定量地研究上述演化场景。我们通过解析推导得到的核心结论揭示了：初始适应度提升幅度最大的演化路径的选择概率，如何随种群规模发生变化。这使得大型种群更易陷入适应度局部峰值，同时意味着小型种群在中间时间尺度上具备演化优势。该效应生效的种群规模区间，与克隆干扰（clonal interference）的发生区间完全吻合。通过随机适应度景观集合开展的补充研究显示，针对特定三位点景观参数得到的研究结果具备稳健性，且随着位点数增加，该结论依然成立。本研究表明，只要适应度景观存在局部极大值，小型种群大概率会具备演化优势。该优势出现在中间时间尺度：此时时长足够长，种群已陷入适应度局部极大值，但又尚未足够长到可通过产生多重突变实现峰值逃离。