UC Davis Department of Evolution and Ecology, and Center for Population Biology, University of California, One Shields Avenue, Davis, California, 95616.
Evolution. 2019 Jul;73(7):1375-1391. doi: 10.1111/evo.13767. Epub 2019 Jun 11.
Speciation occurs when reproductive barriers substantially reduce gene flow between lineages. Understanding how specific barriers contribute to reproductive isolation offers insight into the initial forces driving divergence and the evolutionary and ecological processes responsible for maintaining diversity. Here, we quantified multiple pre- and post-pollination isolating barriers in a pair of closely related California Jewelflowers (Streptanthus, Brassicaceae) living in an area of sympatry. S. breweri and S. hesperidis are restricted to similar serpentine habitats; however, populations are spatially isolated at fine-scales and rarely co-occur in intermixed stands. Several intrinsic postzygotic barriers were among the strongest we quantified, yet, postzygotic barriers currently contribute little to overall reproductive isolation due to the cumulative strength of earlier-acting extrinsic barriers, including spatial isolation, and flowering time and pollinator differences. Data from multiple years suggest that pre-pollination barriers may have different strengths depending on annual environmental conditions. Similarly, crossing data suggest that the strength of intrinsic isolation may vary among different population pairs. Estimates of total reproductive isolation in S. breweri and S. hesperidis are robust to uncertainty and variability in individual barrier strength estimates, demonstrating how multiple barriers can act redundantly to prevent gene flow between close relatives living in sympatry.
当生殖障碍大大减少谱系之间的基因流动时，就会发生物种形成。了解特定的障碍如何促成生殖隔离，有助于深入了解最初推动分歧的力量以及负责维持多样性的进化和生态过程。在这里，我们在一对生活在同域的加利福尼亚宝石花（苦苣苔科）中量化了多个授粉前和授粉后的隔离障碍。S. breweri 和 S. hesperidis 局限于相似的蛇纹石栖息地；然而，种群在小尺度上是空间隔离的，很少在混合的林分中共同出现。我们量化的几种内在的合子后障碍是最强的障碍之一，然而，由于早期作用的外在障碍的累积强度，包括空间隔离、开花时间和传粉者差异，合子后障碍目前对整体生殖隔离的贡献很小。来自多年的数据表明，授粉前障碍的强度可能取决于年度环境条件。同样，杂交数据表明，内在隔离的强度可能在不同的种群对之间有所不同。S. breweri 和 S. hesperidis 中总生殖隔离的估计值对单个障碍强度估计值的不确定性和可变性具有稳健性，这表明多个障碍可以冗余地阻止生活在同域的近亲之间的基因流动。
