Data from: The length of adaptive walks is insensitive to starting fitness in Aspergillus nidulans

Adaptation involves the successive substitution of beneficial mutations by selection, a process known as an adaptive walk. Gradualist models of adaptation, which assume that all mutations are small relative to the distance to a fitness optimum, predict that adaptive walks should be longer when the founding genotype is less well adapted. More recent work modelling adaptation as a sequence of moves in phenotype or genotype space predicts, by contrast, much shorter adaptive walks irrespective of the fitness of the founding genotype. Here we provide what is, to the best of our knowledge, the first direct test of these alternative models, measuring the length of adaptive walks in evolving lineages of fungus that differ initially in fitness. Contrary to the gradualist view, we show that the length of adaptive walks in the fungus Aspergillus nidulans is insensitive to starting fitness and involves just two mutations on average. This arises because poorly adapted populations tend to fix mutations of larger average effect than those of better-adapted populations. Our results suggest that the length of adaptive walks may be independent of the fitness of the founding genotype and, moreover, that poorly adapted populations can quickly adapt to novel environments.适应性进化指自然选择逐次固定有益突变的过程，这一过程被称为适应性行走（adaptive walk）。渐进主义适应性进化模型假定，所有突变相对于抵达适应度最优点的距离均为小幅突变，该模型预测：奠基基因型的适应度越低，适应性行走的长度便越长。与之相对，近期将适应性进化建模为表型或基因型空间中一系列位移的研究则认为，无论奠基基因型的适应度如何，适应性行走的长度都会显著更短。本研究据我们所知首次对这两种对立模型开展了直接检验：以初始适应度存在差异的真菌进化谱系为对象，测量其适应性行走的长度。与渐进主义模型的观点相悖，我们发现构巢曲霉（Aspergillus nidulans）的适应性行走长度对起始适应度并不敏感，平均仅涉及两次突变。这一现象的成因在于：适应度较低的种群所固定的突变，其平均效应显著高于适应度较高的种群所固定的突变。我们的研究结果表明，适应性行走的长度或许与奠基基因型的适应度无关；不仅如此，适应度较低的种群能够快速适应全新环境。