Data from: Population size, habitat fragmentation, and the nature of adaptive variation in a stream fish

Whether and how habitat fragmentation and population size jointly affect adaptive genetic variation and adaptive population differentiation are largely unexplored. Owing to pronounced genetic drift, small, fragmented populations are thought to exhibit reduced adaptive genetic variation relative to large populations. Yet fragmentation is known to increase variability within and among habitats as population size decreases. Such variability might instead favour the maintenance of adaptive polymorphisms and/or generate more variability in adaptive differentiation at smaller population size. We investigated these alternative hypotheses by analysing coding-gene, single-nucleotide polymorphisms associated with different biological functions in fragmented brook trout populations of variable sizes. Putative adaptive differentiation was greater between small and large populations or among small populations than among large populations. These trends were stronger for genetic population size measures than demographic ones and were present despite pronounced drift in small populations. Our results suggest that fragmentation affects natural selection and that the changes elicited in the adaptive genetic composition and differentiation of fragmented populations vary with population size. By generating more variable evolutionary responses, the alteration of selective pressures during habitat fragmentation may affect future population persistence independently of, and perhaps long before, the effects of demographic and genetic stochasticity are manifest.

生境破碎化与种群大小是否以及如何共同影响适应性遗传变异与适应性种群分化，目前尚未得到充分探索。鉴于显著的遗传漂变效应，相较于大种群，小型破碎化种群的适应性遗传变异被认为有所降低。但众所周知，随着种群规模缩小，生境内部及生境间的异质性会因破碎化而加剧；这种异质性反而可能有助于维持适应性多态性，或在更小种群规模下催生更多适应性分化的变异。本研究通过分析不同规模破碎化溪鳟种群中与不同生物学功能相关的编码基因单核苷酸多态性（Single-nucleotide polymorphisms），对这两种对立假说展开了验证。结果显示，相较于大种群之间，小种群与大种群之间，或是小种群内部的推定适应性分化程度更高。相较于以种群统计指标衡量的种群规模，以遗传指标衡量的种群规模所呈现的上述趋势更为显著，且该现象在伴随显著遗传漂变的小型种群中依然存在。研究结果表明，生境破碎化会对自然选择产生影响，且破碎化种群的适应性遗传组成与分化所产生的变化随种群规模而异。生境破碎化过程中选择压力的改变会催生更多样化的演化响应，这可能独立于种群统计随机性与遗传随机性的影响，甚至在这些随机性效应显现之前很久，就已对种群未来的存续能力产生影响。