Data from: Genomic divergence in a ring species complex

Ring species provide particularly clear demonstrations of how one species can gradually evolve into two, but are rare in nature. In the greenish warbler (Phylloscopus trochiloides) species complex, a ring of populations wraps around Tibet. Two reproductively isolated forms co-exist in central Siberia, with a gradient of genetic and phenotypic characteristics through the southern chain of populations connecting them. Previous genetic evidence has proven inconclusive, however, regarding whether species divergence took place in the face of continuous gene flow and whether hybridization between the terminal forms of the ring ever occurred. Here we use genome-wide analyses to show that, although spatial patterns of genetic variation are currently mostly as expected of a ring species, historical breaks in gene flow have existed at more than one location around the ring, and the two Siberian forms have occasionally interbred. Substantial periods of geographical isolation occurred not only in the north but also in the western Himalayas, where there is now an extensive hybrid zone between genetically divergent forms. Limited asymmetric introgression has occurred directly between the Siberian forms, although it has not caused a blending of those forms, suggesting selection against introgressed genes in the novel genetic background. Levels of reproductive isolation and genetic introgression are consistent with levels of phenotypic divergence around the ring, with phenotypic similarity and extensive interbreeding across the southwestern contact zone and strong phenotypic divergence and nearly complete reproductive isolation across the northern contact zone. These results cast doubt on the hypothesis that the greenish warbler should be viewed as a rare example of speciation by distance, but demonstrate that the greenish warbler displays a continuum from slightly divergent neighbouring populations to almost fully reproductively isolated species.

环物种可为物种如何逐步分化为两个物种提供极具说服力的例证，但这类物种在自然界中极为罕见。在暗绿柳莺（Phylloscopus trochiloides）物种复合体中，一系列种群环绕青藏高原形成环带。西伯利亚中部共存着两种生殖隔离的类群，而连接二者的南部种群链中存在遗传与表型特征的梯度变化。然而，此前的遗传学证据尚未能明确回答两个核心问题：一是物种分化是否在持续基因流的背景下发生，二是该环物种的末端类群之间是否曾发生杂交。本研究通过全基因组分析发现：尽管当前的遗传变异空间格局大体符合环物种的预期，但历史上环带周围存在多处基因流中断事件，且两种西伯利亚类群也曾偶尔发生杂交。不仅在北部，西喜马拉雅地区也存在过长期的地理隔离——该区域目前存在遗传分化类群间的大面积杂交带。两种西伯利亚类群之间虽发生了有限的不对称基因渐渗，但并未导致二者的遗传融合，这表明在新的遗传背景下，渐渗基因受到了选择排斥。环带各处的生殖隔离程度与基因渐渗水平，与表型分化程度相匹配：西南接触带类群表型相似且存在广泛杂交，而北部接触带类群表型分化显著且几乎完全生殖隔离。本研究结果对“暗绿柳莺可作为距离依赖型物种形成的罕见例证”这一假说提出了质疑，但同时证实暗绿柳莺呈现出从亲缘关系相近的轻度分化种群，到几乎完全生殖隔离的物种的完整演化连续体。