Abstract: 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.

Key words: genetic cluster, Hornbeam, hybridization, integrative species delimitation, phenotypic cluster, species boundary