Data from: Very low levels of direct additive genetic variance in fitness and fitness components in a red squirrel population

A trait must genetically correlate with fitness in order to evolve in response to natural selection, but theory suggests that strong directional selection should erode additive genetic variance in fitness and limit future evolutionary potential. Balancing selection has been proposed as a mechanism that could maintain genetic variance if fitness components trade off with one another and has been invoked to account for empirical observations of higher levels of additive genetic variance in fitness components than would be expected from mutation–selection balance. Here, we used a long-term study of an individually marked population of North American red squirrels (Tamiasciurus hudsonicus) to look for evidence of (1) additive genetic variance in lifetime reproductive success and (2) fitness trade-offs between fitness components, such as male and female fitness or fitness in high- and low-resource environments. “Animal model” analyses of a multigenerational pedigree revealed modest maternal effects on fitness, but very low levels of additive genetic variance in lifetime reproductive success overall as well as fitness measures within each sex and environment. It therefore appears that there are very low levels of direct genetic variance in fitness and fitness components in red squirrels to facilitate contemporary adaptation in this population.

性状必须与适合度（fitness）存在遗传相关，才能响应自然选择发生演化；但理论研究表明，强烈的定向选择会侵蚀适合度的加性遗传方差（additive genetic variance），并限制种群未来的演化潜力。平衡选择（balancing selection）曾被提出为一种潜在机制：若适合度组分间存在相互权衡，则该机制可维持遗传方差；同时它也被用于解释实验观测到的、适合度组分的加性遗传方差水平高于突变-选择平衡（mutation–selection balance）预期的现象。本研究基于对个体标记的北美红松鼠（Tamiasciurus hudsonicus）种群开展的长期研究，旨在寻找两方面的证据：（1）终生繁殖成功率（lifetime reproductive success）的加性遗传方差；（2）适合度组分间的权衡关系，例如雄性与雌性适合度的权衡，或高资源与低资源环境下适合度的权衡。针对多世代谱系开展的动物模型（Animal model）分析显示，适合度存在适度的母体效应（maternal effects），但整体的终生繁殖成功率，以及各性别、各环境下的适合度指标，其加性遗传方差水平均极低。由此可见，北美红松鼠种群的适合度及其组分的直接遗传方差水平极低，难以支撑该种群的当代适应性演化。