Charlesworth B, Nordborg M, Charlesworth D
Department of Ecology and Evolution, University of Chicago, IL 60637, USA.
Genet Res. 1997 Oct;70(2):155-74. doi: 10.1017/s0016672397002954.
Levels of neutral genetic diversity in populations subdivided into two demes were studied by multilocus stochastic simulations. The model includes deleterious mutations at loci throughout the genome, causing 'background selection', as well as a single locus at which a polymorphism is maintained, either by frequency-dependent selection or by local selective differences. These balanced polymorphisms induce long coalescence times at linked neutral loci, so that sequence diversity at these loci is enhanced at statistical equilibrium. We study how equilibrium neutral diversity levels are affected by the degree of population subdivision, the presence or absence of background selection, and the level of inbreeding of the population. The simulation results are compared with approximate analytical formulae, assuming the infinite sites neutral model. We discuss how balancing selection can be distinguished from local selection, by determining whether peaks of diversity in the region of the polymorphic locus are seen within or between demes. The width of such diversity peaks is shown to depend on the total species population size, rather than local deme sizes. We show that, with population subdivision, local selection enhances between-deme diversity even at neutral sites distant from the polymorphic locus, producing higher FST values than with no selection; very high values can be generated at sites close to a selected locus. Background selection also increases FST, mainly because of decreased diversity within populations, which implies that its effects may be distinguishable from those of local selection. Both effects are stronger in selfing than outcrossing populations. Linkage disequilibrium between neutral sites is generated by both balancing and local selection, especially in selfing populations, because of linkage disequilibrium between the neutral sites and the selectively maintained alleles. We discuss how these theoretical results can be related to data on genetic diversity within and between local populations of a species.
通过多位点随机模拟研究了细分为两个亚群的种群中的中性遗传多样性水平。该模型包括全基因组位点上的有害突变，导致“背景选择”，以及一个通过频率依赖选择或局部选择差异维持多态性的单一位点。这些平衡多态性在连锁中性位点诱导长的合并时间，使得这些位点在统计平衡时的序列多样性增加。我们研究平衡中性多样性水平如何受到种群细分程度、背景选择的有无以及种群近亲繁殖水平的影响。将模拟结果与假设无限位点中性模型的近似解析公式进行比较。我们讨论如何通过确定多态性位点区域内的多样性峰值是在亚群内部还是之间出现，来区分平衡选择和局部选择。这种多样性峰值的宽度显示取决于物种的总种群大小，而不是局部亚群大小。我们表明，随着种群细分，局部选择即使在远离多态性位点的中性位点也会增强亚群间的多样性，产生比无选择时更高的FST值；在靠近选择位点的位点可以产生非常高的值。背景选择也会增加FST，主要是因为种群内多样性降低，这意味着其影响可能与局部选择的影响有所不同。在自交种群中，这两种影响都比异交种群更强。中性位点之间的连锁不平衡是由平衡选择和局部选择共同产生的，特别是在自交种群中，因为中性位点与选择性维持的等位基因之间存在连锁不平衡。我们讨论这些理论结果如何与一个物种的局部种群内部和之间的遗传多样性数据相关。
