經(jīng)常，任何一種特征的期望水平都處在過多和缺乏兩個極端的中間,。不過,，本期Nature Communications上發(fā)表的模擬研究卻顯示，一個個體的最佳生存狀態(tài)可能會稍微傾向于“過多”這個極端,。這一發(fā)現(xiàn)有可能解釋自體免疫疾病是怎樣演化和怎樣被保持的,。該發(fā)現(xiàn)的一個例子見于患自體免疫疾病的個體：它們能有效地防范敵人，但卻是以同時攻擊健康的組織為代價的,。在過敏性休克中也是這樣的：相對來說無害的過敏源會引發(fā)可能會致命的系統(tǒng)免疫反應,。
Mark Urban及其合作者提出一個理論框架，按此框架,，稍微傾斜的最佳生存就足以通過生態(tài)演化反饋促使超常防衛(wèi)的形成,。
對防衛(wèi)行為的過多投入表明，身體會對正常情況下應忽略的狀況和物質(zhì)做出反應,。該新模型表明,，一個個體的生存與其對防衛(wèi)的投入成指數(shù)關系，并且有一個閾限,，過此閾限之后受害者就不會受到傷害,。這一點，再加上隨著投入增加所發(fā)生的繁殖能力的線性降低,，會導致可能實現(xiàn)的最高防衛(wèi)水平,，它在繁殖能力上妥協(xié)最少。不過作者指出,，種群將始終都由適應性好的個體和適應性不好的個體組成,，同時他們還提出：適應性不好的個體將會促進敵人繁殖，從而促使種群作為一個整體在防衛(wèi)上投入甚至更多資源,。
生物谷推薦英文摘要：
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.
