Data from: Repeatability of adaptation in experimental populations of different sizes

The degree to which evolutionary trajectories and outcomes are repeatable across independent populations depends on the relative contribution of selection, chance, and history. Population size has been shown theoretically and empirically to affect the amount of variation that arises among independent populations adapting to the same environment. Here we measure the contribution of selection, chance, and history in different-sized experimental populations of the unicellular alga Chlamydomonas reinhardtii adapting to a high salt environment to determine which component of evolution is affected by population size. We find that adaptation to salt is repeatable at the fitness level in medium (Ne = 5 x 10^3) and large (Ne = 4 x 10^4) populations because of the large contribution of selection. Adaptation is not repeatable in small (Ne = 5 x 10^2) populations because of large constraints from history. The threshold between stochastic and deterministic evolution in this case is therefore between effective population sizes of 10^2 and 10^3. Our results indicate that diversity across populations is more likely to be maintained if they are small. Experimental outcomes in large populations are likely to be robust, and can inform our predictions about outcomes in similar situations.进化轨迹与演化结果在独立种群间的可重复程度，取决于选择、随机遗传漂变与历史遗留效应的相对贡献。已有理论与实证研究证实，种群大小会影响适应同一环境的独立种群间产生的变异总量。本研究以适应高盐环境的单细胞藻类莱茵衣藻（Chlamydomonas reinhardtii）不同有效种群大小（effective population size, Ne）的实验种群为研究对象，量化选择、随机遗传漂变与历史遗留效应的贡献占比，以明确种群大小对演化各组分的影响。 我们发现，在中等有效种群大小（Ne = 5 × 10³）与大型种群（Ne = 4 × 10⁴）中，由于选择的贡献占比极高，高盐适应在适合度层面具备可重复性；而在小型种群（Ne = 5 × 10²）中，由于历史遗留效应带来的强约束，适应过程不具备可重复性。据此，本研究中随机演化与确定性演化的阈值区间为有效种群大小10²至10³之间。 本研究结果表明，若种群规模较小，种群间的遗传多样性更易得以维持；大型种群的实验结果更具稳健性，可为同类情境下的演化结果预测提供科学参考。