Agrawal Aneil F
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada.
PLoS Biol. 2006 Aug;4(8):e265. doi: 10.1371/journal.pbio.0040265.
For over 25 years, many evolutionary ecologists have believed that sexual reproduction occurs because it allows hosts to change genotypes each generation and thereby evade their coevolving parasites. However, recent influential theoretical analyses suggest that, though parasites can select for sex under some conditions, they often select against it. These models assume that encounters between hosts and parasites are completely random. Because of this assumption, the fitness of a host depends only on its own genotype ("genotypic selection"). If a host is even slightly more likely to encounter a parasite transmitted by its mother than expected by random chance, then the fitness of a host also depends on its genetic similarity to its mother ("similarity selection"). A population genetic model is presented here that includes both genotypic and similarity selection, allowing them to be directly compared in the same framework. It is shown that similarity selection is a much more potent force with respect to the evolution of sex than is genotypic selection. Consequently, similarity selection can drive the evolution of sex even if it is much weaker than genotypic selection with respect to fitness. Examination of explicit coevolutionary models reveals that even a small degree of mother-offspring parasite transmission can cause parasites to favor sex rather than oppose it. In contrast to previous predictions, the model shows that weakly virulent parasites are more likely to favor sex than are highly virulent ones. Parasites have figured prominently in discussions of the evolution of sex, but recent models suggest that parasites often select against sex rather than for it. With the inclusion of small and realistic exposure biases, parasites are much more likely to favor sex. Though parasites alone may not provide a complete explanation for sex, the results presented here expand the potential for parasites to contribute to the maintenance of sex rather than act against it.
25多年来，许多进化生态学家一直认为有性生殖的发生是因为它使宿主能够在每一代改变基因型，从而躲避与其共同进化的寄生虫。然而，最近有影响力的理论分析表明，虽然寄生虫在某些情况下可以选择促进有性生殖，但它们往往也会选择抑制有性生殖。这些模型假设宿主与寄生虫之间的接触是完全随机的。基于这一假设，宿主的适应性仅取决于其自身的基因型（“基因型选择”）。如果宿主遇到由其母亲传播的寄生虫的可能性比随机预期的略高，那么宿主的适应性还取决于其与其母亲的遗传相似性（“相似性选择”）。本文提出了一个群体遗传模型，该模型同时包含基因型选择和相似性选择，从而能够在同一框架内对它们进行直接比较。结果表明，就有性生殖的进化而言，相似性选择比基因型选择的作用要强大得多。因此，即使相似性选择在适应性方面比基因型选择弱得多，它也能够推动有性生殖的进化。对明确的共同进化模型的研究表明，即使母代向子代的寄生虫传播程度很小，也会使寄生虫倾向于有性生殖而非反对有性生殖。与之前的预测相反，该模型表明，弱毒力的寄生虫比强毒力的寄生虫更有可能倾向于有性生殖。寄生虫在有性生殖进化的讨论中一直占据显著地位，但最近的模型表明，寄生虫往往选择抑制而非促进有性生殖。如果纳入微小且符合实际的暴露偏差，寄生虫更有可能倾向于有性生殖。虽然仅靠寄生虫可能无法对有性生殖提供完整的解释，但本文给出的结果扩大了寄生虫有助于维持有性生殖而非阻碍有性生殖的可能性。
