Sexual selection has been suggested to accelerate local adaptation and promote evolutionary rescue through several ecological and genetic mechanisms. Condition-dependent sexual selection has mainly been studied in laboratory settings, while data from natural populations are lacking. One ecological factor that can cause condition-dependent sexual selection is parasitism. Here, we quantified ectoparasite load (Arrenurus water mites) in a natural population of the common bluetail damselfly (Ischnura elegans) over 15 years. We quantified the strength of sexual selection against parasite load in both sexes and experimentally investigated the mechanisms behind such selection. Then we investigated how parasite resistance and tolerance changed over time to understand how they might influence population density. Parasites reduced mating success in both sexes, and sexual selection was stronger in males than in females. Experiments show that male-male competition is a strong force causing precopulatory sexual selection against parasite load. Although parasite resistance and male parasite tolerance increased over time, suggestive of increasing local adaptation against parasites, no signal of evolutionary rescue could be found. We suggest that condition-dependent sexual selection facilitates local adaptation against parasites and discuss its effects in evolutionary rescue.

中文翻译：

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男性-男性竞争导致寄生虫介导的性选择，以适应当地环境。

已经提出性选择通过一些生态和遗传机制来加速局部适应并促进进化拯救。依赖条件的性别选择主要是在实验室中进行的，而缺乏自然种群的数据。可能导致条件依赖的性选择的一种生态因素是寄生。在这里，我们量化了常见蓝尾豆娘（Ischnura elegans）的自然种群中的体外寄生虫负荷（Arrenurus水螨））超过15年。我们对两性中针对寄生虫负荷的性选择强度进行了量化，并通过实验研究了这种选择背后的机制。然后，我们调查了寄生虫抗性和耐受性如何随时间变化，以了解它们如何影响种群密度。寄生虫降低了男性和女性的交配成功率，并且男性的性别选择比女性强。实验表明，男性与男性之间的竞争是一种强大的力量，可引起针对寄生虫负荷的交配前性选择。尽管寄生虫抗性和雄性寄生虫耐受性随时间增加，这表明对寄生虫的局部适应性增加，但未发现进化拯救的信号。