Data from: Diversification and gene flow in nascent lineages of island and mainland North American tree squirrels (Tamiasciurus)

Pleistocene climate cycles and glaciations had profound impacts on taxon diversification in the Boreal Forest Biome. Using population genetic analyses with multilocus data we examined diversification, isolation, and hybridization in two sibling species of tree squirrels (Tamiasciurus douglasii and T. hudsonicus) with special attention to the geographically and genetically enigmatic population of T. hudsonicus on Vancouver Island, Canada. The two species differentiated only about 500,000 years ago, in the late Pleistocene. The island population is phylogenetically nested within T. hudsonicus according to our nuclear analysis but within T. douglasii according to mtDNA. This conflict is more likely due to historical hybridization than to incomplete lineage sorting, and it appears that bidirectional gene flow occurred between the island population and both species on the mainland. This interpretation of our genetic analyses is consistent with our bioclimatic modeling, which demonstrates that both species were able to occupy this region throughout the late Pleistocene. The divergence of the island population 40,000 years ago suggests that tree squirrels persisted in a refugium on Vancouver Island at the Last Glacial Maximum, 20,000 years ago. Our observations demonstrate how Pleistocene climate change and habitat shifts have created incipient divergence in the presence of gene flow.

更新世（Pleistocene）气候旋回与冰期（glaciations）对北方森林生物群系（Boreal Forest Biome）内的类群分化产生了深远影响。本研究采用基于多位点数据（multilocus data）的群体遗传学分析（population genetic analyses）方法，对树松鼠属的两个姊妹种（sibling species）——道格拉斯树松鼠（Tamiasciurus douglasii）与哈德逊树松鼠（Tamiasciurus hudsonicus）的分化、隔离及杂交事件展开探究，重点关注加拿大温哥华岛上地理与遗传特征均颇具特殊性的哈德逊树松鼠种群。这两个物种的分化时间仅距今约50万年，处于更新世晚期。核基因分析结果显示，该岛屿种群在系统发育上嵌套于哈德逊树松鼠类群中，但线粒体DNA（mtDNA）分析却将其归入道格拉斯树松鼠类群。这种系统发育冲突更可能源于历史杂交事件，而非不完全谱系分选（incomplete lineage sorting），且该岛屿种群与大陆上的两个物种均存在双向基因流（bidirectional gene flow）。本研究的遗传分析结论与生物气候建模（bioclimatic modeling）结果相一致，二者均表明，在整个更新世晚期，两个物种均可在该区域存续。该岛屿种群的分化时间距今约4万年，这暗示在末次盛冰期（Last Glacial Maximum，距今2万年）时，树松鼠曾在温哥华岛的避难所中持续存活。本研究结果阐明了更新世气候变化与生境转移如何在基因流存在的条件下促成类群的初期分化（incipient divergence）。