Postcopulatory sexual selection can generate evolutionary arms races between the sexes resulting in the rapid coevolution of reproductive phenotypes. As traits affecting fertilization success diverge between populations, postmating prezygotic (PMPZ) barriers to gene flow may evolve. Conspecific sperm precedence is a form of PMPZ isolation thought to evolve early during speciation yet has mostly been studied between species. Here, we show conpopulation sperm precedence (CpSP) between Drosophila montana populations. Using Pool-seq genomic data we estimate divergence times and ask whether PMPZ isolation evolved in the face of gene flow. We find models incorporating gene flow fit the data best indicating populations experienced considerable gene flow during divergence. We find CpSP is asymmetric and mirrors asymmetry in non-competitive PMPZ isolation, suggesting these phenomena have a shared mechanism. However, we show asymmetry is unrelated to the strength of postcopulatory sexual selection acting within populations. We tested whether overlapping foreign and coevolved ejaculates within the female reproductive tract altered fertilization success but found no effect. Our results show that neither time since divergence nor sperm competitiveness predicts the strength of PMPZ isolation. We suggest that instead cryptic female choice or mutation-order divergence may drive divergence of postcopulatory phenotypes resulting in PMPZ isolation.

This article is part of the theme issue ‘Fifty years of sperm competition’.

交配后的性选择可以在性别之间产生进化军备竞赛，从而导致生殖表型的快速共同进化。由于影响受精成功的性状在种群之间存在差异，因此可能会进化出合子后（PMPZ）基因流的障碍。同种精子优先是PMPZ分离的一种形式，认为它在物种形成过程中会早期发展，但大多数物种之间已进行了研究。在这里，我们显示了果蝇蒙大拿州之间的人口精子优先（CpSP）人口。使用Pool-seq基因组数据，我们估计了发散时间，并询问PMPZ分离是否在面对基因流时进化了。我们发现整合基因流的模型最适合数据，表明群体在分化过程中经历了可观的基因流。我们发现CpSP在非竞争性PMPZ隔离中是不对称的并且镜像不对称，这表明这些现象具有共同的机制。但是，我们显示不对称性与人口中进行交配后性选择的强度无关。我们测试了女性生殖道内重叠的异物和共同射出的射精是否改变了受精成功率，但未发现效果。我们的结果表明，自发散以来的时间或精子竞争力都无法预测PMPZ隔离的强度。

本文是“精子竞争五十年”主题问题的一部分。