经常，任何一种特征的期望水平都处在过多和缺乏两个极端的中间。不过，本期Nature Communications上发表的模拟研究却显示，一个个体的最佳生存状态可能会稍微倾向于“过多”这个极端。这一发现有可能解释自体免疫疾病是怎样演化和怎样被保持的。该发现的一个例子见于患自体免疫疾病的个体：它们能有效地防范敌人，但却是以同时攻击健康的组织为代价的。在过敏性休克中也是这样的：相对来说无害的过敏源会引发可能会致命的系统免疫反应。
Mark Urban及其合作者提出一个理论框架，按此框架，稍微倾斜的最佳生存就足以通过生态演化反馈促使超常防卫的形成。
对防卫行为的过多投入表明，身体会对正常情况下应忽略的状况和物质做出反应。该新模型表明，一个个体的生存与其对防卫的投入成指数关系，并且有一个阈限，过此阈限之后受害者就不会受到伤害。这一点，再加上随着投入增加所发生的繁殖能力的线性降低，会导致可能实现的最高防卫水平，它在繁殖能力上妥协最少。不过作者指出，种群将始终都由适应性好的个体和适应性不好的个体组成，同时他们还提出：适应性不好的个体将会促进敌人繁殖，从而促使种群作为一个整体在防卫上投入甚至更多资源。
生物谷推荐英文摘要：
Nature Communications doi:10.1038/ncomms3085
Asymmetric selection and the evolution of extraordinary defences
Mark C. Urban, Reinhard Bürger，Daniel I. Bolnick
Evolutionary biologists typically predict future evolutionary responses to natural selection by analysing evolution on an adaptive landscape. Much theory assumes symmetric fitness surfaces even though many stabilizing selection gradients deviate from symmetry. Here we revisit Lande’s adaptive landscape and introduce novel analytical theory that includes asymmetric selection. Asymmetric selection and the resulting skewed trait distributions bias equilibrium mean phenotypes away from fitness peaks, usually toward the flatter shoulder of the individual fitness surface. We apply this theory to explain a longstanding paradox in biology and medicine: the evolution of excessive defences against enemies. These so-called extraordinary defences can evolve in response to asymmetrical selection when marginal risks of insufficient defence exceed marginal costs of excessive defence. Eco-evolutionary feedbacks between population abundances and asymmetric selection further exaggerate these defences. Recognizing the effect of asymmetrical selection on evolutionary trajectories will improve the accuracy of predictions and suggest novel explanations for apparent sub-optimality.
