Data from: Genetic load, inbreeding depression and hybrid vigor covary with population size: an empirical evaluation of theoretical predictions

Reduced population size is thought to have strong consequences for evolutionary processes as it enhances the strength of genetic drift. In its interaction with selection, this is predicted to increase the genetic load, reduce inbreeding depression, and increase hybrid vigour, and in turn affect phenotypic evolution. Several of these predictions have been tested, but comprehensive studies controlling for confounding factors are scarce. Here we show that populations of Daphnia magna, which vary strongly in genetic diversity, also differ in genetic load, inbreeding depression, and hybrid vigor in a way that strongly supports theoretical predictions: Inbreeding depression is positively correlated with genetic diversity (a proxy for Ne), and genetic load and hybrid vigour are negatively correlated with genetic diversity. These patterns remain significant after accounting for potential confounding factors and indicate that, in small populations, a large proportion of the segregation load is converted into fixed load. Overall, the results suggest that the nature of genetic variation for fitness-related traits differs strongly between large and small populations. This has large consequences for evolutionary processes in natural populations, such as selection on dispersal, breeding systems, ageing, and local adaptation.

种群规模缩减被认为对进化过程具有显著影响，因其会增强遗传漂变（genetic drift）的作用强度。在与自然选择的相互作用中，该效应理论上会提升遗传负荷（genetic load）、降低近交衰退（inbreeding depression）并增强杂种优势（hybrid vigour），进而影响表型进化。尽管已有部分此类预测得到验证，但针对混杂因素进行控制的综合性研究仍较为匮乏。本研究以遗传多样性差异显著的大型溞（Daphnia magna）种群为对象，发现其在遗传负荷、近交衰退与杂种优势方面同样存在显著差异，该结果与理论预测高度契合：近交衰退与作为有效种群大小（Ne）替代指标的遗传多样性呈正相关，而遗传负荷与杂种优势则与遗传多样性呈负相关。在控制潜在混杂因素后，上述关联仍具有统计学显著性，这表明在小型种群中，大量分离负荷（segregation load）会转化为固定负荷（fixed load）。总体而言，本研究结果显示，与适合度相关性状的遗传变异本质在大种群与小种群之间存在显著差异。该发现对自然种群的进化过程具有深远影响，例如对扩散策略、繁育系统、衰老以及局部适应的选择作用。