School of Biotechnology & Biomolecular Sciences, and Evolution & Ecology Research Centre, University of New South Wales, NSW 2052 Australia.
J Theor Biol. 2014 Jan 7;340:222-31. doi: 10.1016/j.jtbi.2013.09.026. Epub 2013 Sep 25.
Populations can go extinct when their environments deteriorate, but evolutionary rescue occurs when a shrinking population adapts to the new environmental conditions. The emergence of resistance from a drug sensitive bacterial population under treatment can be regarded as an instance of evolutionary rescue. Understanding evolutionary rescue in a particular context such as drug resistance requires knowledge of how the environment changes and how selection coefficients change as a result. In this study, we propose a model for evolutionary rescue under three different scenarios of environmental change: abrupt change, periodic fluctuation and gradual decay. The model makes use of the notion of reaction norms to describe fitness values that depend on both genotype and environmental state. We find that although drug sensitive bacterial populations may be large, allowing them to generate resistant mutants frequently, a harsh abrupt change due to the drug usually drives them extinct. Evolutionary rescue occurs far more frequently under the milder forms of environmental change we investigated. Rescue is favoured when the absolute fitnesses of individuals remain sufficiently high over the range of environment qualities experienced by the population. The minimum environment quality, which is inversely related to drug dose in the antibiotic context, is thus an important factor. Interestingly, in the periodic fluctuation model, the inter-dose period is less influential in promoting rescue through resistance unless the minimum environment quality is in a particular range. We also investigated fitness trade-offs across environments including the case of a resistant allele not subject to any trade-off (a "superbug"). This fitness trade-off affects the probability of rescue in decaying environments, but surprisingly has only a weak effect in the periodic fluctuation scenario. Finally, we use the model to show how niche construction, whereby organisms are the source of environmental change, produces more complex dynamics.
当环境恶化时，种群可能会灭绝，但当一个缩小的种群适应新的环境条件时，就会发生进化拯救。在治疗下，药物敏感细菌种群产生抗药性可以被视为进化拯救的一个例子。在特定的背景下，如耐药性，了解进化拯救需要了解环境如何变化以及选择系数如何随之变化。在这项研究中，我们提出了一个在三种不同环境变化情景下进行进化拯救的模型：突然变化、周期性波动和逐渐衰减。该模型利用反应规范的概念来描述依赖于基因型和环境状态的适合度值。我们发现，尽管药物敏感的细菌种群可能很大，允许它们经常产生抗药性突变体，但由于药物的突然剧烈变化通常会导致它们灭绝。在我们研究的环境变化的较温和形式下，进化拯救发生的频率更高。当个体的绝对适合度在种群经历的环境质量范围内保持足够高时，就会有利于拯救。抗生素背景下与药物剂量成反比的最小环境质量因此是一个重要因素。有趣的是，在周期性波动模型中，除非最小环境质量处于特定范围，否则药物剂量的间隔时间对促进通过抗药性进行拯救的影响较小。我们还研究了跨环境的适应度权衡，包括不受任何权衡影响的抗性等位基因（“超级细菌”）的情况。这种适应度权衡会影响衰退环境中拯救的概率，但令人惊讶的是，在周期性波动场景中，它的影响很弱。最后，我们使用该模型展示了生物是环境变化的来源的生态位构建如何产生更复杂的动态。
