Schultz Stewart T, Lynch Michael
Department of Biology, University of Miami, Coral Gables, Florida, 33124.
Department of Biology, University of Oregon, Eugene, Oregon, 97403.
Evolution. 1997 Oct;51(5):1363-1371. doi: 10.1111/j.1558-5646.1997.tb01459.x.
Recent theoretical studies have illustrated the potential role of spontaneous deleterious mutation as a cause of extinction in small populations. However, these studies have not addressed several genetic issues, which can in principle have a substantial influence on the risk of extinction. These include the presence of synergistic epistasis, which can reduce the rate of mutation accumulation by progressively magnifying the selective effects of mutations, and the occurrence of beneficial mutations, which can offset the effects of previous deleterious mutations. In stochastic simulations of small populations (effective sizes on the order of 100 or less), we show that both synergistic epistasis and the rate of beneficial mutation must be unrealistically high to substantially reduce the risk of extinction due to random fixation of deleterious mutations. However, in analytical calculations based on diffusion theory, we show that in large, outcrossing populations (effective sizes greater than a few hundred), very low levels of beneficial mutation are sufficient to prevent mutational decay. Further simulation results indicate that in populations small enough to be highly vulnerable to mutational decay, variance in deleterious mutational effects reduces the risk of extinction, assuming that the mean deleterious mutational effect is on the order of a few percent or less. We also examine the magnitude of outcrossing that is necessary to liberate a predominantly selfing population from the threat of long-term mutational deterioration. The critical amount of outcrossing appears to be greater than is common in near-obligately selfing plant species, supporting the contention that such species are generally doomed to extinction via random drift of new mutations. Our results support the hypothesis that a long-term effective population size in the neighborhood of a few hundred individuals defines an approximate threshold, below which outcrossing populations are vulnerable to extinction via fixation of deleterious mutations, and above which immunity is acquired.
最近的理论研究已经阐明了自发有害突变作为小种群灭绝原因的潜在作用。然而，这些研究并未涉及几个遗传学问题，这些问题原则上可能对灭绝风险产生重大影响。其中包括协同上位性的存在，它可以通过逐步放大突变的选择效应来降低突变积累的速率，以及有益突变的发生，它可以抵消先前有害突变的影响。在小种群的随机模拟中（有效大小在100或更小的量级），我们表明，要通过有害突变的随机固定来大幅降低灭绝风险，协同上位性和有益突变的速率都必须高到不切实际的程度。然而，在基于扩散理论的分析计算中，我们表明，在大型异交种群中（有效大小大于几百），非常低水平的有益突变就足以防止突变衰退。进一步的模拟结果表明，在小到极易受到突变衰退影响的种群中，假设有害突变效应的平均值在百分之几或更低的量级，有害突变效应的方差会降低灭绝风险。我们还研究了将一个主要自交的种群从长期突变退化的威胁中解救出来所需的异交程度。关键的异交量似乎大于近专性自交植物物种中常见的量，这支持了这样一种观点，即这类物种通常注定会因新突变的随机漂变而灭绝。我们的结果支持这样一种假设，即几百个个体左右的长期有效种群大小定义了一个近似阈值，低于这个阈值，异交种群容易因有害突变的固定而灭绝，高于这个阈值则获得免疫。
