Department of Biology, Stanford University, Stanford, CA 94305, USA.
Proc Natl Acad Sci U S A. 2012 Mar 20;109(12):4684-9. doi: 10.1073/pnas.1018096109. Epub 2012 Mar 5.
Individuals within populations can differ substantially in their life span and their lifetime reproductive success but such realized individual variation in fitness components need not reflect underlying heritable fitness differences visible to natural selection. Even so, biologists commonly argue that large differences in fitness components are likely adaptive, resulting from and driving evolution by natural selection. To examine this argument we use unique formulas to compute exactly the variance in life span and in lifetime reproductive success among individuals with identical (genotypic) vital rates (assuming a common genotype for all individuals). Such individuals have identical fitness but vary substantially in their realized individual fitness components. We show by example that our computed variances and corresponding simulated distribution of fitness components match those observed in real populations. Of course, (genotypic) vital rates in real populations are expected to differ by small but evolutionarily important amounts among genotypes, but we show that such differences only modestly increase variances in fitness components. We conclude that observed differences in fitness components may likely be evolutionarily neutral, at least to the extent that they are indistinguishable from distributions generated by neutral processes. Important consequences of large neutral variation are the following: Heritabilities for fitness components are likely to be small (which is in fact the case), small selective differences in life histories will be hard to measure, and the effects of random drift will be amplified in natural populations by the large variances among individuals.
人群中的个体在寿命和终生繁殖成功率方面可能存在很大差异，但适应度成分的这种实际个体差异不一定反映可被自然选择观察到的潜在遗传适应度差异。即便如此，生物学家通常认为，适应度成分的巨大差异很可能是适应性的，是由自然选择驱动进化的结果。为了检验这一论点，我们使用独特的公式精确计算了具有相同（基因型）生命率的个体之间的寿命和终生繁殖成功率的差异（假设所有个体都具有相同的基因型）。这些个体具有相同的适应度，但在其实现的个体适应度成分方面存在很大差异。我们通过示例表明，我们计算的方差和相应的适应度成分的模拟分布与真实种群中观察到的分布相匹配。当然，真实种群中的（基因型）生命率在基因型之间可能会有小但具有进化意义的差异，但我们表明，这种差异只会适度增加适应度成分的方差。我们得出的结论是，观察到的适应度成分差异可能是进化中性的，至少在它们与中性过程产生的分布无法区分的程度上是如此。大量中性变异的重要后果如下：适应度成分的遗传力可能很小（实际上就是这种情况），生命史中的小选择性差异将难以测量，并且随机漂变的影响将通过个体之间的大方差在自然种群中放大。
