Data from: Effects of assortative mate choice on the genomic and morphological structure of a hybrid zone between two bird subspecies

Phenotypic differentiation plays an important role in the formation and maintenance of reproductive barriers. In some cases, variation in a few key aspects of phenotype can promote and maintain divergence; hence the identification of these traits and their associations with patterns of genomic divergence are crucial for understanding the patterns and processes of population differentiation. We studied hybridization between the alba and personata subspecies of the white wagtail (Motacilla alba), and quantified divergence and introgression of multiple morphological traits and 19,437 SNP loci on a 3000 km transect. Our goal was to identify traits that may contribute to reproductive barriers and to assess how variation in these traits corresponds to patterns of genome-wide divergence. Variation in only one trait – head plumage patterning – was consistent with reproductive isolation. Transitions in head plumage were steep and occurred over otherwise morphologically and genetically homogeneous populations, whereas cline centers for other traits and genomic ancestry were displaced over one hundred kilometers from the head cline. Field observational data show that social pairs mated assortatively by head plumage, suggesting that these phenotypes are maintained by divergent mating preferences. In contrast, variation in all other traits and genetic markers could be explained by neutral diffusion, although weak ecological selection cannot be ruled out. Our results emphasize that assortative mating may maintain phenotypic differences independent of other processes shaping genome-wide variation, consistent with other recent findings that raise questions about the relative importance of mate choice, ecological selection and selectively neutral processes for divergent evolution.

表型分化（Phenotypic differentiation）在生殖隔离（reproductive barriers）的形成与维持中发挥着核心作用。在部分情形下，少数关键表型性状的变异即可推动并维持种群趋异；因此，识别这些性状及其与基因组分化（genomic divergence）模式的关联，对于理解种群分化（population differentiation）的格局与过程至关重要。本研究以白鹡鸰（Motacilla alba）的alba亚种与personata亚种间的杂交为研究对象，在长达3000公里的样带（transect）上，对多项形态性状以及19437个单核苷酸多态性（SNP）位点的分化与基因渐渗（introgression）情况进行了量化分析。本研究的目标为识别可能参与生殖隔离形成的性状，并评估这些性状的变异与全基因组分化模式的对应关系。结果显示，仅头部羽色图案这一个性状的变异与生殖隔离相符：头部羽色的转变十分陡峭，且发生在形态与遗传上均同质化的种群中；而其余性状的渐变群（cline）中心与基因组祖先成分的渐变群中心，均偏离头部羽色渐变群中心超过100公里。野外观测数据表明，配对个体根据头部羽色进行选型交配（assortative mating），这暗示此类表型由分化的交配偏好所维持。与之相对，其余所有性状与遗传标记的变异均可通过中性扩散（neutral diffusion）解释，尽管微弱的生态选择（ecological selection）无法被完全排除。本研究结果强调，选型交配可在不依赖塑造全基因组变异的其他演化过程的前提下维持表型差异，这与近期其他研究结果一致——后者对配偶选择、生态选择以及选择性中性过程在趋异演化（divergent evolution）中的相对重要性提出了质疑。