Animals adopt a range of avoidance strategies to reduce their exposure to parasites and the associated cost of infection. If strong selective pressures from parasites are sustained over many generations, avoidance strategies may gradually evolve from phenotypically plastic, or individually variable, to fixed, species-wide traits. Over time, host species possessing effective infection avoidance traits may lose parasite species. Indeed, if overcoming the avoidance strategies of a host species is too costly, i.e., if individuals of that species become too rarely encountered or difficult to infect, a generalist parasite may opt out of this particular arms race. From the host’s perspective, if avoidance traits are not costly or have been co-opted for other functions, they may persist in extant species even if ancestral parasites are lost, as signatures of past selection by parasites. Here, we develop the “ghost of parasitism past” hypothesis. We discuss how animal species with a lower number of parasite species than expected based on their ecological properties or phylogenetic affinities are a good starting point in the search for traces of past parasite-mediated selection. We then argue that the hypothesis explains the dynamic and inconsistent nature of the relationship between the expression of avoidance traits and relative infection risk in comparative analyses across host species. Finally, we propose some approaches to test the predictions of the hypothesis. Animal morphology and behavior show clear evidence of past selective pressures from predators; we argue that past selection from parasites has also left its imprint, though in more subtle ways.

中文翻译：

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古代寄生虫压力的进化特征，或寄生过去的幽灵

动物采用一系列避免策略来减少接触寄生虫和相关的感染成本。如果来自寄生虫的强大选择压力持续多代，回避策略可能会逐渐从表型可塑性或个体可变性演变为固定的、全物种性状。随着时间的推移，具有有效避免感染特性的宿主物种可能会失去寄生虫物种。事实上，如果克服宿主物种的回避策略成本太高，即，如果该物种的个体变得太少遇到或难以感染，则通才寄生虫可能会选择退出这种特殊的军备竞赛。从宿主的角度来看，如果回避性状的成本不高或已被用于其他功能，即使祖先的寄生虫消失，它们也可能在现存物种中持续存在，作为过去寄生虫选择的签名。在这里，我们提出了“寄生过去的幽灵”假说。我们讨论了寄生虫物种数量低于基于其生态特性或系统发育亲和力的预期动物物种如何成为寻找过去寄生虫介导的选择痕迹的良好起点。然后，我们认为该假设解释了在跨宿主物种的比较分析中回避性状的表达与相对感染风险之间关系的动态和不一致的性质。最后，我们提出了一些方法来检验假设的预测。动物的形态和行为清楚地表明过去来自捕食者的选择压力；我们认为，过去对寄生虫的选择也留下了印记，尽管以更微妙的方式。