Both hybridization and intraspecific morphological variation across environmental gradients complicate species delineation. We aimed to discern both possibilities that may blur species boundaries in the Carpinus viminea–Carpinus laxiflora–Carpinus londoniana species complex. We conducted statistical analyses on 535 specimens encompassing the entire distribution of this species complex to identify phenotypic clusters. Additionally, we analyzed genetic divergence and probable hybridization between clusters using 76 individuals from 37 populations. Based on phenotypic and genetic clusters, we tentatively recognized four species: C. viminea, C. fargesii, C. laxiflora, and C. londoniana. Except for rare overlapping distributions between C. fargesii and C. londoniana, the redefined four species are mostly allopatric to each another based on their distributions. The morphological delimitation, species boundary and distribution of each species differ distinctly from past taxonomic treatments. For example, specimens previously identified under C. viminea, in fact, belong to three different species. Hybrids between C. fargesii and C. londoniana exhibit morphological traits similar to C. viminea, thereby contributing to difficulties in determining species boundaries and outlining species distributions. These findings suggest that local selection and geographical isolation may together have promoted both phenotypic and genetic divergences within this species complex. However, interspecific hybridization blurs species boundaries by producing hybrids with phenotypic similarity in addition to intraspecific variation. This study emphasizes the importance of statistical analyses of population-level morphological and genetic variations across major distributional ranges for an integrative delimitation of species boundaries and the identification of hybridization and hybrids.

中文翻译：

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一种角树物种复合体（桦木科：Carpinus）内物种边界的综合界定

跨环境梯度的杂交和种内形态变化都使物种描述变得复杂。我们的目的是辨别两种可能模糊Carpinus viminea – Carpinus laxiflora – Carpinus londoniana物种复合体物种界限的可能性。我们对涵盖该物种复合体整个分布的 535 个标本进行了统计分析，以确定表型簇。此外，我们还使用来自 37 个种群的 76 个个体分析了簇之间的遗传差异和可能的杂交。根据表型和遗传簇，我们初步识别了四个物种：C. viminea、C. fargesii、C. laxiflora和C. londiana。除了C. fargesii和C. londoniana之间罕见的重叠分布外，重新定义的四个物种根据其分布大多是异域的。每个物种的形态界定、物种边界和分布与过去的分类学处理明显不同。例如，之前在C. viminea下鉴定的标本实际上属于三个不同的物种。C. fargesii和C. londoniana之间的杂交种表现出与C. viminea相似的形态特征，从而导致确定物种边界和概述物种分布的困难。这些发现表明，当地选择和地理隔离可能共同促进了该物种复合体的表型和遗传差异。然而，种间杂交通过产生除了种内变异之外还具有表型相似性的杂交种，模糊了物种界限。这项研究强调了对主要分布范围内的种群水平形态和遗传变异进行统计分析对于物种边界综合划定以及杂交和杂种识别的重要性。