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）上进化的小型无性繁殖种群，其适应度可能高于大型种群，这一现象源于自适应路径的异质性提升。本文引入一类单倍体三基因座适应度景观，可用于精准且定量地研究该演化场景。我们通过解析推导得到的核心结论显示：选择初始适应度增幅最大路径的概率随种群规模的变化规律。这一规律使得大型种群更易被困于适应度局部峰值，同时意味着小型种群在中等时间尺度上具备演化优势。该效应发挥作用的种群规模范围，与克隆干扰（clonal interference）的发生区间相重合。借助随机适应度景观集合开展的补充研究表明，针对特定三基因座景观参数得到的研究结果具备鲁棒性，且随着基因座数量增加依然成立。本研究显示，只要适应度景观存在局部极大值，小型种群便大概率具备演化优势。该优势出现在中等时间尺度：此时时长已足够让种群被困于适应度局部极大值，但尚未长到可通过多突变体生成实现峰逃逸的程度。