Data from: Genomic divergence in a ring species complex

<b>Abstract</b><br/>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.

<b>摘要</b><br/>环形种（ring species）为物种如何逐步演化成两个物种提供了尤为清晰的例证，但在自然界中较为罕见。在绿柳莺（Phylloscopus trochiloides）物种复合种（species complex）中，一系列种群环绕西藏分布。两个生殖隔离（reproductive isolation）的形态在西伯利亚中部共存，而连接它们的南部种群链呈现出遗传与表型特征的梯度变化。然而，关于物种分化是否在持续基因流（gene flow）存在的情况下发生，以及环形种群末端形态是否曾发生杂交（hybridization），以往的遗传证据尚未得出明确结论。本研究通过全基因组分析（genome-wide analyses）表明，尽管当前遗传变异（genetic variation）的空间模式大体符合环形种的预期，但环上多个位置曾出现基因流的历史中断，且西伯利亚的两个形态曾偶尔杂交。长时间的地理隔离（geographical isolation）不仅发生在北部，也出现在喜马拉雅西部——如今该区域存在遗传分化形态间的广泛杂交区（hybrid zone）。西伯利亚形态间存在有限的不对称基因渗入（introgression），但这并未导致二者的融合，表明在新的遗传背景下，渗入基因受到选择压力。生殖隔离与基因渗入的程度与环周围表型分化（phenotypic divergence）的程度一致：西南接触区（contact zone）表型相似且杂交广泛，北部接触区表型分化显著且生殖隔离近乎完全。这些结果对绿柳莺作为距离成种（speciation by distance）罕见案例的假说提出了质疑，但证明绿柳莺展示了从轻微分化的邻近种群到近乎完全生殖隔离物种的连续谱。