High mutation rates select for the evolution of mutational robustness where populations inhabit flat fitness peaks with little epistasis, protecting them from lethal mutagenesis. Recent evidence suggests that a different effect protects small populations from extinction via the accumulation of deleterious mutations. In drift robustness, populations tend to occupy peaks with steep flanks and positive epistasis between mutations. However, it is not known what happens when mutation rates are high and population sizes are small at the same time. Using a simple fitness model with variable epistasis, we show that the equilibrium fitness has a minimum as a function of the parameter that tunes epistasis, implying that this critical point is an unstable fixed point for evolutionary trajectories. In agent-based simulations of evolution at finite mutation rate, we demonstrate that when mutations can change epistasis, trajectories with a subcritical value of epistasis evolve to decrease epistasis, while those with supercritical initial points evolve towards higher epistasis. These two fixed points can be identified with mutational and drift robustness, respectively.

高突变率选择突变稳健性的进化，其中种群居住在平坦的适应度峰值，几乎没有上位性，保护它们免受致命突变的影响。最近的证据表明，一种不同的作用通过有害突变的积累来保护小种群免于灭绝。在漂移稳健性方面，种群倾向于占据陡峭侧翼和突变之间正上位的峰。然而，当突变率高且种群规模小的同时发生时，不知道会发生什么。使用具有可变上位性的简单适应度模型，我们表明平衡适应度具有最小值作为调整上位性的参数的函数，这意味着该临界点是进化轨迹的不稳定固定点。在基于代理的有​​限突变率进化模拟中，我们证明了当突变可以改变上位性时，具有亚临界值的上位性的轨迹会演变为降低上位性，而具有超临界初始点的轨迹会朝着更高的上位性发展。这两个固定点可以分别通过突变鲁棒性和漂移鲁棒性来识别。