Host-parasite interactions may often be subject to opposing evolutionary forces, which likely influence the evolutionary trajectories of both partners. Natural selection and genetic drift are two major evolutionary forces that act in host and parasite populations. Further, population size is a significant determinant of the relative strengths of these forces. In small populations, drift may undermine the persistence of beneficial alleles, potentially impeding host adaptation to parasites. Here, we investigate two questions: (a) can selection pressure for increased resistance in small, susceptible host populations overcome the effects of drift and (b) can resistance be maintained in small host populations? To answer these questions, we experimentally evolved the host Caenorhabditis elegans against its bacterial parasite, Serratia marcescens, for 13 host generations. We found that strong selection favouring increased host resistance was insufficient to counteract drift in small populations, resulting in persistently high host mortality. Additionally, in small populations of resistant hosts, we found that selection for the maintenance of resistance is not always sufficient to curb the loss of resistance. We compared these results with selection in large host populations. We found that initially resistant, large host populations were able to maintain high levels of resistance. Likewise, initially susceptible, large host populations were able to gain resistance to the parasite. These results show that strong selection pressure for survival is not always sufficient to counteract drift. In consideration of C. elegans natural population dynamics, we suggest that drift may often impede selection in nature.

中文翻译：

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寄主遗传漂移和适应在寄生虫抗性的进化和维持中。

宿主-寄生虫相互作用可能经常受到相反的进化力量的影响，这可能会影响双方的进化轨迹。自然选择和遗传漂变是在宿主和寄生虫种群中起作用的两种主要进化力量。此外，人口规模是这些力量相对强度的重要决定因素。在小种群中，漂移可能会破坏有益等位基因的持久性，从而可能阻碍宿主对寄生虫的适应。在这里，我们研究了两个问题：（a）在小的易感宿主群体中增加抗性的选择压力能否克服漂移的影响？（b）能否在小宿主群体中保持抗性？为了回答这些问题，我们通过实验进化了宿主秀丽隐杆线虫对抗其细菌寄生虫粘质沙雷氏菌，13 代宿主。我们发现有利于增加宿主抗性的强选择不足以抵消小种群中的漂移，导致宿主死亡率持续升高。此外，在抗性宿主的小群体中，我们发现选择维持抗性并不总是足以遏制抗性的丧失。我们将这些结果与大型宿主群体中的选择进行了比较。我们发现最初具有抗性的大型宿主种群能够保持高水平的抗性。同样，最初易感的大型宿主种群能够获得对寄生虫的抵抗力。这些结果表明，强大的生存选择压力并不总是足以抵消漂移。考虑到秀丽隐杆线虫的自然种群动态，