Gene flow tends to impede the accumulation of genetic divergence. Here, we determine the limits for the evolution of postzygotic reproductive isolation in a model of two populations that are connected by gene flow. We consider two selective mechanisms for the creation and maintenance of a genetic barrier: local adaptation leads to divergence among incipient species due to selection against migrants, and Dobzhansky–Muller incompatibilities (DMIs) reinforce the genetic barrier through selection against hybrids. In particular, we are interested in the maximum strength of the barrier under a limited amount of local adaptation, a challenge that many incipient species may initially face. We first confirm that with classical two-locus DMIs, the maximum amount of local adaptation is indeed a limit to the strength of a genetic barrier. However, with three or more loci and cryptic epistasis, this limit holds no longer. In particular, we identify a minimal configuration of three epistatically interacting mutations that is sufficient to confer strong reproductive isolation.

This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers’.

基因流动倾向于阻止遗传差异的积累。在这里，我们确定了通过基因流连接的两个种群的模型中合子后生殖分离进化的极限。我们考虑了创建和维持遗传屏障的两种选择性机制：由于对移民的选择，局部适应导致初始物种之间的差异；而Dobzhansky-Muller不相容性（DMI）通过选择杂交来增强遗传屏障。特别是，我们对在有限的局部适应条件下屏障的最大强度感兴趣，这是许多初期物种可能最初面临的挑战。我们首先确认，对于经典的双基因座DMI，最大的局部适应性确实限制了遗传屏障的强度。然而，具有三个或更多位点和隐性上位的患者，此限制不再成立。特别是，我们确定了三个上位相互作用的突变的最低限度的配置，足以赋予强大的生殖隔离。

本文是主题“走向物种形成：生殖隔离超越第一个障碍的演变”的一部分。