Hybrid cline or hybrid lineage: A genomic reevaluation of Sibley’s classic species conundrum in Pipilo towhees

Hybrid zones are often described as clines of genetic and phenotypic traits moving across species barriers through introgression. Yet, hybrid zones can also be spatially complex and shift over time, and dispersal and vicariance can isolate portions of a cline, potentially leading to hybrid lineage formation. We reassessed Sibley’s (1950) gradient between Collared Towhee (Pipilo ocai) and Spotted Towhee (P. maculatus) in Central Mexico to test whether it conformed to a typical tension-zone cline model. By comparing historical and modern data, we found that cline centers for genetic and phenotypic traits have not shifted over the course of 70 years. This equilibrium suggests that secondary contact between these species, which originally diverged over 2 million years ago, likely dates to the Pleistocene. Given the amount of mtDNA divergence, parental ends of the cline have very low autosomal nuclear differentiation (FST = 0.12). Dramatic and coincident cline shifts in mtDNA and throat colo..., Morphological data: traits measured from specimens with ruler and digital calipers
Genetic data: SNPs generated from sequence capture of ultraconserved elements and Illumina sequencing
Environmental data: extracted from environmental layers from localities associated with photo vouchers on iNaturalist,

杂交带（hybrid zones）常被描述为通过基因渐渗（introgression）跨越物种屏障、传递遗传与表型性状的渐变群（cline）。然而，杂交带也可呈现空间结构复杂性并随时间发生位移；扩散与地理分隔（vicariance）作用可使渐变群的部分区域发生隔离，进而可能形成杂交谱系。

本研究重新评估了西布利（Sibley，1950）提出的、分布于墨西哥中部的领唧鹀（Collared Towhee，学名*Pipilo ocai*）与斑唧鹀（Spotted Towhee，学名*P. maculatus*）之间的渐变群，以检验该渐变群是否符合典型的张力带渐变群模型。

通过对比历史与现代数据，本研究发现，遗传与表型性状对应的渐变群中心在70年间未发生位移。

这种平衡态表明，这两个物种的次生接触事件发生于更新世（Pleistocene）——二者的初始分化时间距今已超200万年。

结合线粒体DNA（mtDNA）的分化程度，该渐变群的两个亲本种群端的常染色体核分化程度极低（FST=0.12）。

线粒体DNA与喉部羽色[原文截断]的渐变群发生了显著且同步的位移。

形态学数据：通过直尺与数显卡尺对标本的性状进行测量获取。

遗传数据：通过超保守元件（ultraconserved elements）序列捕获与Illumina测序技术获取单核苷酸多态性（SNPs）位点。

环境数据：从iNaturalist平台上与凭证标本照片关联的样点对应的环境图层中提取获取。