Faculty of Mathematics, University of Vienna, Vienna, Austria.
Leiden University Medical Center, Leiden, The Netherlands.
J Anim Ecol. 2018 Jan;87(1):11-23. doi: 10.1111/1365-2656.12598. Epub 2016 Nov 10.
It is now widely accepted that genetic processes such as inbreeding depression and loss of genetic variation can increase the extinction risk of small populations. However, it is generally unclear whether extinction risk from genetic causes gradually increases with decreasing population size or whether there is a sharp transition around a specific threshold population size. In the ecological literature, such threshold phenomena are called 'strong Allee effects' and they can arise for example from mate limitation in small populations. In this study, we aim to (i) develop a meaningful notion of a 'strong genetic Allee effect', (ii) explore whether and under what conditions such an effect can arise from inbreeding depression due to recessive deleterious mutations, and (iii) quantify the interaction of potential genetic Allee effects with the well-known mate-finding Allee effect. We define a strong genetic Allee effect as a genetic process that causes a population's survival probability to be a sigmoid function of its initial size. The inflection point of this function defines the critical population size. To characterize survival-probability curves, we develop and analyse simple stochastic models for the ecology and genetics of small populations. Our results indicate that inbreeding depression can indeed cause a strong genetic Allee effect, but only if individuals carry sufficiently many deleterious mutations (lethal equivalents). Populations suffering from a genetic Allee effect often first grow, then decline as inbreeding depression sets in and then potentially recover as deleterious mutations are purged. Critical population sizes of ecological and genetic Allee effects appear to be often additive, but even superadditive interactions are possible. Many published estimates for the number of lethal equivalents in birds and mammals fall in the parameter range where strong genetic Allee effects are expected. Unfortunately, extinction risk due to genetic Allee effects can easily be underestimated as populations with genetic problems often grow initially, but then crash later. Also interactions between ecological and genetic Allee effects can be strong and should not be neglected when assessing the viability of endangered or introduced populations.
现在人们普遍认为，遗传过程（如近亲繁殖衰退和遗传变异丧失）会增加小种群的灭绝风险。然而，通常不清楚遗传原因导致的灭绝风险是否会随着种群数量的减少而逐渐增加，或者是否存在特定的种群规模阈值的急剧转变。在生态学文献中，这种现象被称为“强阿利效应”，例如，在小种群中，由于配偶限制，就会出现这种现象。在这项研究中，我们旨在：(i) 提出一个有意义的“强遗传阿利效应”概念；(ii) 探讨在什么条件下，由于隐性有害突变引起的近亲繁殖衰退可能会产生这种效应；(iii) 量化潜在遗传阿利效应与众所周知的求偶阿利效应之间的相互作用。我们将强遗传阿利效应定义为一种遗传过程，它使种群的生存概率成为其初始规模的一个 S 形函数。该函数的拐点定义了临界种群规模。为了描述生存概率曲线，我们开发并分析了小种群的生态学和遗传学的简单随机模型。我们的研究结果表明，近亲繁殖衰退确实会导致强遗传阿利效应，但前提是个体携带足够多的有害突变（致死当量）。受遗传阿利效应影响的种群通常会先增长，然后随着近亲繁殖衰退的出现而下降，然后随着有害突变的清除而可能恢复。生态和遗传阿利效应的临界种群规模似乎通常是可加的，但也可能存在超加性相互作用。许多已发表的鸟类和哺乳动物的致死当量数量估计值都落在预期出现强遗传阿利效应的参数范围内。不幸的是，由于遗传阿利效应导致的灭绝风险很容易被低估，因为存在遗传问题的种群通常会先增长，但后来会崩溃。此外，生态和遗传阿利效应之间的相互作用可能很强，在评估濒危或引入种群的生存能力时不应忽视。
