Data from: The power of evolutionary rescue is constrained by genetic load

Extinction risk of small isolated populations in changing environments can be reduced by rapid adaptation and subsequent growth to larger, less vulnerable sizes. Whether this process, called evolutionary rescue, is able to reduce extinction risk and sustain population growth over multiple generations is largely unknown. To understand the consequences of adaptive evolution as well as maladaptive processes in small isolated populations, we subjected experimental Tribolium castaneum populations founded with 10 or 40 individuals to novel environments, one more favorable, and one resource poor, and either allowed evolution, or constrained it by replacing individuals one-for-one each generation from a non-adapting large population to minimize both adaptive and non-adaptive evolutionary processes. Replacement individuals spent one generation in the target novel environment before use to standardize effects due to the parental environment. After 8 generations we mixed a subset of surviving populations to facilitate admixture, allowing us to estimate drift load by comparing performance of mixed to unmixed groups. Evolving populations had reduced extinction rates, and increased population sizes in the first four to five generations compared to populations where evolution was constrained. Performance of evolving populations subsequently declined. Admixture restored their performance, indicating high drift load that may have overwhelmed the beneficial effects of adaptation in evolving populations. Our results indicate that evolution may quickly reduce extinction risk and increase population sizes, but suggest that relying solely on adaptation from standing genetic variation may not provide long-term benefits to small isolated populations of diploid sexual species, and that active management facilitating gene flow may be necessary for longer-term persistence.

在变化的环境中，小型孤立种群的灭绝风险可通过快速适应并后续增长至更大、更不易受侵害的种群规模得以降低。这一被称为进化救援（evolutionary rescue）的过程能否降低灭绝风险并维持多世代的种群增长，目前仍尚不明确。为探究小型孤立种群中适应性进化与非适应性进化过程所带来的演化后果，我们将由10只或40只个体建立的实验性赤拟谷盗（Tribolium castaneum）种群置于两种全新环境中：一种环境条件更优越，另一种则资源匮乏；同时设置两类实验处理：允许种群自然发生进化，或是通过每一代从一个未发生适应的大种群中按个体数一对一替换种群个体，以最大限度削弱适应性与非适应性进化过程，从而限制种群的进化能力。替换所用的个体在投入实验前，会先在目标全新环境中培养一代，以标准化亲本环境带来的实验效应。经过8代培养后，我们将部分存活种群进行混合以促进基因交流，借此通过比较混合种群与未混合种群的适合度表现，我们得以估算漂移负荷（drift load）。相较于进化受到限制的对照组种群，发生进化的实验组种群灭绝率更低，且在前4至5代的种群规模显著更大。但在后续世代中，进化种群的适合度表现出现下降。种群混合修复了它们的表现，这表明种群内存在较高的漂移负荷，该负荷可能抵消了进化种群中适应性演化带来的有益效应。我们的研究结果表明，适应性进化或许能快速降低小型孤立种群的灭绝风险并提升其种群规模，但也提示，仅依赖现有遗传变异的适应性进化，可能无法为二倍体有性生殖物种的小型孤立种群提供长期生存优势；而借助促进基因流动的主动管理措施，或许是实现这类种群长期存续的必要手段。