Coalescent‐based species delimitation, geometric morphometrics, and ecological niche modeling support the recognition of two species in the Quercus engleriana complex (Fagaceae)

Abstract

Species delimitation remains challenging in closely related complexes with continuous morphological variation. The Quercus engleriana complex, comprising 2–5 taxa distributed from the southeastern Himalayas to subtropical East Asia, exemplifies such uncertainty. To clarify species boundaries, we integrated phylogenomics, coalescent‐based delimitation (BP&P), geometric morphometrics, and ecological niche modeling. Phylogenomics and BP&P consistently recovered two well‐supported clades representing six evolutionarily significant units (ESUs): the QT clade ( Q. tungmaiensis and Q. kongshanensis ) and the QE clade ( Q. marlipoensis and three geographic groups of Q. engleriana , with Q. tatakaensis nested within one). Morphometrics distinguished Q. tungmaiensis from Q. engleriana . Within the QT clade, Q. kongshanensis was indistinguishable from Q. tungmaiensis . Within the QE clade, Q. marlipoensis had distinctive leaf morphology but shared nut traits with Q. engleriana , whereas Q. tatakaensis resembled the southeastern lineage of Q. engleriana . Ecological niche analyses revealed significant climatic divergence between the two clades and largely allopatric distributions. These results support recognizing two species: Q. tungmaiensis (with Q. kongshanensis reduced to synonymy) and Q. engleriana (with Q. marlipoensis treated as a subspecies and Q. tatakaensis as a synonym). The six ESUs highlight evolutionary structure within the complex and the value of genome‐wide SNP data for species delimitation and conservation.