Data from: Slowly switching between environments facilitates reverse evolution in small populations

Natural populations must constantly adapt to ever-changing environmental conditions. A particularly interesting question is whether such adaptations can be reversed by returning the population to an ancestral environment. Such evolutionary reversals have been observed in both natural and laboratory populations. However, the factors that determine the reversibility of evolution are still under debate. The timescales of environmental change vary over a wide range, but little is known about how the rate of environmental change influences the reversibility of evolution. Here we demonstrate computationally that slowly switching between environments increases the reversibility of evolution for small populations, which are subject to only modest clonal interference. For small populations, slow switching reduces the mean number of mutations acquired in a new environment and also increases the probability of reverse evolution at each of these “genetic distances.” As the population size increases, slow switching no longer reduces the genetic distance, thus decreasing the evolutionary reversibility. We confirm this effect using both a phenomenological model of clonal interference and also a Wright-Fisher stochastic simulation that incorporates genetic diversity. Our results suggest that the rate of environmental change is a key determinant of the reversibility of evolution, and provides testable hypotheses for experimental evolution.自然种群必须持续适应不断变化的环境条件。一个尤为引人关注的科学问题是：将种群回归祖先环境后，这类适应性是否可以被逆转。目前已在自然种群与实验室种群中均观察到此类进化逆转现象，但决定进化可逆性的相关因素仍存在学术争议。环境变化的时间尺度跨度极大，但学界对环境变化速率如何影响进化可逆性的认知仍十分有限。本研究通过计算模拟证明：对于仅受适度克隆干涉（clonal interference）影响的小型种群而言，在不同环境间缓慢切换可提升其进化可逆性。对于小型种群，缓慢切换可降低其在新环境中获得的平均突变数量，同时提升在这些“遗传距离（genetic distance）”位点上发生反向进化的概率。随着种群规模扩大，缓慢切换不再能缩短遗传距离，反而会降低进化可逆性。我们分别采用克隆干涉现象学模型与纳入遗传多样性的赖特-费希尔（Wright-Fisher）随机模拟验证了这一效应。本研究结果表明，环境变化速率是决定进化可逆性的关键因素，并为实验进化研究提供了可检验的科学假说。