Elena Santiago F, Lenski Richard E
Center for Microbial Ecology, Michigan State University, East Lansing, Michigan, 48824.
Evolution. 1997 Aug;51(4):1058-1067. doi: 10.1111/j.1558-5646.1997.tb03953.x.
Six replicate populations of the bacterium Escherichia coli were propagated for more than 10,000 generations in a defined environment. We sought to quantify the variation among clones within these populations with respect to their relative fitness, and to evaluate the roles of three distinct population genetic processes in maintaining this variation. On average, a pair of clones from the same population differed from one another in their relative fitness by approximately 4%. This within-population variation was small compared with the average fitness gain relative to the common ancestor, but it was statistically significant. According to one hypothesis, the variation in fitness is transient and reflects the ongoing substitution of beneficial alleles. We used Fisher's fundamental theorem to compare the observed rate of each population's change in mean fitness with the extent of variation for fitness within that population, but we failed to discern any correspondence between these quantities. A second hypothesis supposes that the variation in fitness is maintained by recurrent deleterious mutations that give rise to a mutation-selection balance. To test this hypothesis, we made use of the fact that two of the six replicate populations had evolved mutator phenotypes, which gave them a genomic mutation rate approximately 100-fold higher than that of the other populations. There was a marginally significant correlation between a population's mutation rate and the extent of its within-population variance for fitness, but this correlation was driven by only one population (whereas two of the populations had elevated mutation rates). Under a third hypothesis, this variation is maintained by frequency-dependent selection, whereby genotypes have an advantage when they are rare relative to when they are common. In all six populations, clones were more fit, on average, when they were rare than when they were common, although the magnitude of the advantage when rare was usually small (~1% in five populations and ~5% in the other). These three hypotheses are not mutually exclusive, but frequency-dependent selection appears to be the primary force maintaining the fitness variation within these experimental populations.
将六个大肠杆菌的重复群体在特定环境中繁殖超过10000代。我们试图量化这些群体中克隆之间相对适合度的差异，并评估三种不同的群体遗传过程在维持这种差异中的作用。平均而言，来自同一群体的一对克隆在相对适合度上彼此相差约4%。与相对于共同祖先的平均适合度增加相比，这种群体内差异较小，但具有统计学意义。根据一种假设，适合度的差异是短暂的，反映了有益等位基因的持续替代。我们使用费舍尔基本定理将每个群体平均适合度的观察变化率与该群体内适合度的差异程度进行比较，但未能发现这些量之间的任何对应关系。第二种假设认为，适合度的差异是由反复出现的有害突变维持的，这些突变导致了突变 - 选择平衡。为了检验这一假设，我们利用了六个重复群体中有两个群体进化出了突变体表型这一事实，这使得它们的基因组突变率比其他群体高约100倍。群体的突变率与其群体内适合度方差程度之间存在微弱的显著相关性，但这种相关性仅由一个群体驱动（而有两个群体的突变率升高）。根据第三种假设，这种差异是由频率依赖性选择维持的，即基因型在相对于常见时罕见时有优势。在所有六个群体中，克隆通常在罕见时比常见时更适合，尽管罕见时优势的幅度通常较小（五个群体中约为1%，另一个群体中约为5%）。这三种假设并非相互排斥，但频率依赖性选择似乎是维持这些实验群体内适合度差异的主要力量。
