Intrinsic postzygotic barriers can play an important and multifaceted role in speciation, but their contribution is often thought to be reserved to the final stages of the speciation process. Here, we review how intrinsic postzygotic barriers can contribute to speciation, and how this role may change through time. We outline three major contributions of intrinsic postzygotic barriers to speciation. (i) reduction of gene flow: intrinsic postzygotic barriers can effectively reduce gene exchange between sympatric species pairs. We discuss the factors that influence how effective incompatibilities are in limiting gene flow. (ii) early onset of species boundaries via rapid evolution: intrinsic postzygotic barriers can evolve between recently diverged populations or incipient species, thereby influencing speciation relatively early in the process. We discuss why the early origination of incompatibilities is expected under some biological models, and detail how other (and often less obvious) incompatibilities may also serve as important barriers early on in speciation. (iii) reinforcement: intrinsic postzygotic barriers can promote the evolution of subsequent reproductive isolation through processes such as reinforcement, even between relatively recently diverged species pairs. We incorporate classic and recent empirical and theoretical work to explore these three facets of intrinsic postzygotic barriers, and provide our thoughts on recent challenges and areas in the field in which progress can be made.

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

内在的合子后屏障可以在物种形成中发挥重要且多方面的作用，但它们的贡献通常被认为仅用于物种形成过程的最后阶段。在这里，我们回顾了内在的后合子障碍如何促进物种形成，以及这种作用如何随时间变化。我们概述了内在后合子障碍对物种形成的三个主要贡献。(i)减少基因流动：内在的合子后屏障可以有效减少同域物种对之间的基因交换。我们讨论了影响不相容性在限制基因流动方面的有效性的因素。(ii)通过快速进化提早出现物种边界：内在的后合子障碍可以在最近分化的种群或早期物种之间进化，从而在过程中相对较早地影响物种形成。我们讨论了为什么在某些生物模型下会出现不相容性的早期起源，并详细说明其他（通常不太明显的）不相容性如何在物种形成的早期也成为重要的障碍。(三)加固：内在的后合子障碍可以通过强化等过程促进后续生殖隔离的进化，即使在相对较新的物种对之间也是如此。我们结合经典和最近的经验和理论工作来探索内在后合子障碍的这三个方面，并提供我们对最近的挑战和该领域可以取得进展的领域的看法。

这篇文章是主题问题“走向物种形成的完成：超越第一道障碍的生殖隔离的进化”的一部分。