Data from: Hybridization between Townsend’s Dendroica townsendi and black-throated green warblers D. virens in an avian suture zone

Hybrid zones between species provide natural systems for the study of processes involved in divergence, reproductive isolation and speciation. Townsend’s Dendroica townsendi and black-throated green D. virens warblers are phenotypically and genetically divergent groups that occur in western and eastern North America respectively, with potential for range contact in the Rocky Mountains of British Columbia, where other west-east avian pairs come into contact. Although one potential hybrid (a phenotypic Townsend’s warbler with the black-throated green mitochondrial DNA) has been previously reported, there have been no studies of interactions between the taxa in potential areas of sympatry. To determine whether interbreeding between these species is a regular occurrence we examined variation in individuals across the area of putative range overlap. Analysis of plumage, morphology, and mitochondrial (COI) and nuclear molecular markers (CHD1Z and numt-Dco1) shows surprisingly extensive hybridization between these species, with at least 38% of individuals in the hybrid zone being either hybrids or backcrosses. Each of the traits displays a sigmoidal cline centred along the eastern slope of the Rocky Mountains (molecular cline centres averaging 50 km east of the crest of the Rockies, ranging from 41 to 56 km). The clines are narrow (average molecular cline width is 60 km, ranging from 40 to 87 km) relative to the dispersal distance of related warbler species, suggesting that selection is maintaining the hybrid zone; we discuss possible sources of selection. Given the narrowness of the zone we recommend the two forms should continue to be treated as separate taxonomic species. Townsend’s warblers also form an extensively studied hybrid zone with their more closely related southern relative, the hermit warbler D. occidentalis. The combined system of three discrete forms separated by narrow hybrid zones provides an excellent system for the study of hybridization, reproductive isolation and speciation.物种间的杂交带为研究分化、生殖隔离与物种形成过程提供了天然实验体系。汤森林莺（Dendroica townsendi）与黑喉绿林莺（Dendroica virens）为表型与遗传均存在分化的类群，分别分布于北美西部与东部，二者的分布范围可能在不列颠哥伦比亚省的落基山脉区域交汇——该区域也是其他东西向鸟类类群的接触地带。此前虽有一则疑似杂交个体的报道（即表型为汤森林莺，但携带黑喉绿林莺线粒体DNA的个体），但尚未有针对这两个类群在潜在同域分布区域内相互作用的研究。为明确二者是否存在常态化的基因交流，我们对推测分布重叠区域内的个体进行了变异分析。对羽色、形态学特征，以及线粒体（COI）与核分子标记（CHD1Z和numt-Dco1）的分析显示，这两个物种间存在出人意料的广泛杂交，杂交带中至少38%的个体为杂交后代或回交个体。各性状均呈现以落基山脉东坡为中心的S形渐变群，分子渐变群的中心平均位于落基山脉山脊以东50公里处，区间为41至56公里。相较于近缘林莺类群的扩散距离，该渐变群较为狭窄（平均分子渐变群宽度为60公里，区间40至87公里），提示选择压力维持了该杂交带的存在；我们还讨论了潜在的选择压力来源。鉴于该杂交带的狭窄性，我们建议仍将这两个类群视为独立的分类学物种。汤森林莺还与其亲缘关系更近的南部类群隐士林莺（Dendroica occidentalis）形成了被广泛研究的杂交带。这三个由狭窄杂交带分隔的离散类群组成的复合体系，为杂交、生殖隔离与物种形成的研究提供了极佳的实验模型。