Perlitz M, Stephan W
Department of Zoology, University of Maryland, College Park 20742-4415, USA.
J Math Biol. 1997 Nov;36(1):1-23. doi: 10.1007/s002850050087.
Tight linkage may cause a reduction of nucleotide diversity in a chromosomal region if an advantageous mutation appears in that region which is driven to fixation by directional selection. This process is usually called genetic hitchhiking. If selection is strong, the entire process takes place during a time period of length s/2 1n (2N) that is very short relative to 2N generations [s is the selection coefficient of the advantageous mutation and N the effective diploid population size]. On the time scale of 2N generations, which is characteristic for neutral evolution, we may therefore call this process a hitchhiking event. Using coalescent methods, we analyzed a model in which a hitchhiking event occurred in a chromosomal region of zero-recombination in the past at time x. Such a hitchhiking "catastrophe" wipes out completely genetic variation that existed in a population before that time. Standing variation observed at present must therefore be due to mutations that have arisen since time point x. Assuming that all newly arising mutations are neutral, we derived expressions for the expectation, variance and also for the higher moments of the number of nucleotide sites segregating in a sample of n genes as a function of x. The result for the first moment is then used to estimate the time back to the last hitchhiking event based on DNA polymorphism data from Drosophila. Assuming that directional selection is the sole determinant of the level of genetic variation in the gene regions surveyed, we obtained estimates of x that were typically in the order of 0.1N generations.
如果一个有利突变出现在一个染色体区域并通过定向选择被固定下来，紧密连锁可能会导致该区域核苷酸多样性的降低。这个过程通常被称为遗传搭便车。如果选择强度很大，整个过程会在长度为s/2 ln(2N)的时间段内发生，相对于2N代来说这个时间段非常短[s是有利突变的选择系数，N是有效二倍体种群大小]。在以2N代为特征的中性进化时间尺度上，因此我们可以将这个过程称为一次搭便车事件。使用溯祖方法，我们分析了一个模型，其中在过去时间x时，在一个零重组的染色体区域发生了一次搭便车事件。这样一次搭便车“灾难”会完全消除在该时间之前种群中存在的遗传变异。因此，目前观察到的现存变异必定是由于自时间点x以来出现的突变。假设所有新出现的突变都是中性的，我们推导了作为x的函数的在n个基因样本中分离的核苷酸位点数量的期望、方差以及高阶矩的表达式。然后，基于来自果蝇的DNA多态性数据，利用一阶矩的结果来估计距离上一次搭便车事件的时间。假设定向选择是所研究基因区域遗传变异水平的唯一决定因素，我们得到的x估计值通常在0.1N代的量级。
