While hybridization and introgression can have a strong adaptive importance, it can impede divergence of species. Quercus magnoliifolia and Q. resinosa are two endemic oak species distributed across the Mexican highlands. These species diverged ecological and morphologically; however, no nuclear genetic differentiation is evident. In this study, we determined the mechanisms that shape patterns of genetic variation and establish the role of migration and hybridization in the evolutionary history of these two oak species. To do this, Bayesian approaches were used for inference on migration rates and directionality and timing of divergence between species using chloroplast microsatellites. We then integrated species distribution models to infer the geographic distribution of Q. magnoliifolia and Q. resinosa during Last Interglacial, Last Glacial Maximum, and Mid-Holocene time frames. We failed in distinguishing a unique genetic composition for each species. Chloroplast differentiation was more congruent with geography than the taxonomic status of each species. Our study revealed that after the divergence (c. 10 Mya) of these two oak species, high rates of introgression took place at the end of the Pleistocene. Furthermore, past distribution models predicted that Q. magnoliifolia and Q. resinosa have likely been in sympatry presumable since Last Glacial Maximum and Q. resinosa probably expanded geographically towards its current distribution around Mid-Holocene. This expansion was supported by testing migration models, suggesting recent establishment of Q. resinosa to the north of Trans-Mexican Volcanic Belt. We hypothesized that after Q. magnoliifolia and Q. resinosa diverged, colonization events followed by hybridization between oaks and long-distance seed dispersal occurred, explaining the present-day patterns of distribution of chloroplast diversity. We propose that divergence of species remains mainly on loci under natural selection, providing evidence on the “porous” nature of species boundaries among oaks.

虽然杂交和基因渗入具有很强的适应性重要性，但它可能阻碍物种的分化。木兰栎和树脂栎是分布在墨西哥高地的两种特有橡树品种。这些物种在生态和形态上存在差异。然而，没有明显的核遗传分化。在这项研究中，我们确定了形成遗传变异模式的机制，并确定了迁移和杂交在这两种橡树物种进化史上的作用。为此，贝叶斯方法用于利用叶绿体微卫星推断物种之间的迁移率和方向性以及分化时间。然后，我们整合物种分布模型来推断地理分布末次间冰期、末次盛冰期和全新世中期期间的Q. magnoliifolia和Q. resinosa 。我们未能区分每个物种独特的遗传组成。叶绿体分化与每个物种的地理地位比与分类地位更一致。我们的研究表明，在这两个橡树物种分化（约10 Mya）之后，更新世末期发生了高频率的基因渗入。此外，过去的分布模型预测，自末次盛冰期以来，木兰栎和树脂栎可能处于同域关系。可能在地理上扩展到了全新世中期周围的当前分布。这种扩张得到了迁移模型测试的支持，表明最近在跨墨西哥火山带以北建立了树脂栎。我们假设木兰栎和树脂栎分化后，发生了定植事件，随后发生了橡树之间的杂交和远距离种子传播，这解释了当今叶绿体多样性的分布模式。我们认为物种的分化主要集中在自然选择下的基因座上，这为橡树之间物种边界的“多孔”性质提供了证据。