Data from: Why we are not dead 100 times over

The possibility of pervasive weak selection at tens or hundreds of millions of sites across the genome, suggested by recent studies of silent site DNA sequence variation and divergence, raises the problem of the survival of the population in the face of the large genetic load that may result. Two alternative resolutions of this problem are presented for populations where recombination is sufficiently frequent that different sites under selection evolve independently. One invokes weak stabilizing selection, of the magnitude compatible with abundant silent site variability. This can be shown to produce only a modest genetic load, due to the effectiveness of even weak stabilizing selection in keeping the trait mean close to the optimum. The other invokes soft selection, whereby individuals compete for a limiting resource whose abundance determines the absolute fitness of the population. Weak purifying selection at a large number of sites produces only a small variance in fitness among individuals within the population, due to the fact that most sites are fixed rather than polymorphic. Even when it produces a large genetic load, it is compatible with the observations when selection is soft. It may be very difficult to distinguish between these two possibilities.

近期针对同义位点（silent site）DNA序列变异与趋异的研究提示，全基因组数千万乃至数亿个位点普遍存在弱选择的可能性，这一现象引出了种群在可能产生的巨大遗传负荷（genetic load）下如何存续的问题。针对重组频率足够高、使得受选择的不同位点能够独立演化的种群，本文提出了该问题的两种备选解决方案。其一为弱稳定选择（stabilizing selection），其强度与同义位点的丰富变异水平相符。研究表明，由于即便弱稳定选择也能有效维持性状均值处于最优值附近，因此这类选择仅会产生适度的遗传负荷。其二为软选择（soft selection），即个体为获取限制性资源展开竞争，而该资源的丰度决定了种群的绝对适合度（absolute fitness）。由于绝大多数位点处于固定状态而非多态状态，大量位点上的弱纯化选择（purifying selection）仅会在种群内个体间造成极小的适合度方差。即便这类选择会产生较大的遗传负荷，在软选择的情境下仍与观测结果相符。目前或很难区分这两种备选方案的优劣。