Am Nat. 2022 Mar;199(3):313-329. doi: 10.1086/717813. Epub 2022 Jan 11.
AbstractWith the twofold cost of sex, derived asexual organisms have an immediate reproductive advantage over their sexual sisters. Yet the "twiggy" phylogenetic distribution of asexual lineages implies that they become extinct relatively quickly over evolutionary time. Meanwhile, bacteria and archaea have persisted for billions of years without requiring sexual reproduction. A simple explanation for this difference is that prokaryotes have very large population sizes that are not subject to the accumulation of deleterious mutations, but this implies that drift and mutational meltdown dominate derived asexual populations. Here, we explore a different hazard, quantifying the degree to which genetic variation is lost in asexual populations experiencing selective sweeps. Even though large populations generate diversity by mutation during sweeps, we find that populations that are safe from mutational meltdown may still be reduced to dangerous effective population sizes by sweeps. Thus, ironically, adaptation itself reduces further adaptive potential and may predispose asexual populations to extinction. We derive a simple approximation for the effective population size after a hard sweep and explore the impact of recent sweeps on evolutionary rescue. These factors may help to explain the phylogenetic twigginess of asexuals, the maintenance of sex and recombination, and the evolutionary persistence of prokaryotes.
摘要
由于性别的双重代价，无性生殖生物相较于其有性姐妹具有直接的生殖优势。然而，无性谱系的“枝状”系统发育分布意味着它们在进化时间内相对较快地灭绝。与此同时，细菌和古菌已经持续存在了数十亿年，而不需要有性繁殖。对于这种差异的一个简单解释是，原核生物的种群规模非常大，不会积累有害突变，但这意味着漂变和突变崩溃主导了衍生的无性种群。在这里，我们探讨了一个不同的危险，即量化经历选择清除的无性种群中遗传变异丧失的程度。尽管大种群在清除过程中通过突变产生多样性，但我们发现，免受突变崩溃影响的种群仍可能因清除而减少到危险的有效种群规模。因此，具有讽刺意味的是，适应本身会降低进一步的适应潜力，并可能使无性种群灭绝。我们推导出硬清除后有效种群大小的简单近似，并探讨最近清除对进化拯救的影响。这些因素可能有助于解释无性生物的系统发育枝状、性和重组的维持以及原核生物的进化持续。
