Shining a light on species delimitation in the tree genus Engelhardia Leschenault ex Blume (Juglandaceae)

Enhanced efficacy in species delimitation is critically important in biology given the pending biodiversity crisis under global warming and anthropogenic activity. In particular, delineation of traditional classifications in view of the complexity of species requires an integrative approach to effectively define species boundaries, and this is a major focus of systematic biology. Here, we explored species delimitation of Engelhardia in tropical and subtropical Asia. In total, 716 individuals in 71 populations were genotyped using five chloroplast regions, one nuclear DNA region (nrITS), and 11 nuclear simple sequence repeats (nSSR). Phylogenetic trees were constructed and relationships among species were assessed. Molecular analyses were then combined with 14 morphological characteristics of 720 specimens to further explore the species boundaries of Engelhardia. Integrating phylogenetic and morphological clusters provided well-resolved relationships to delineate seven species. The results suggested that: first, that E. fenzelii, E. roxburghiana, E. hainanensis, E. apoensis, and E. serrata are distinct species; second, E. spicata var. spicata, E. spicata var. aceriflora, E. spicata var. colebrookeana, and E. rigida should be combined under E. spicata and treated as a species complex; third, E. serrata var. cambodica should be raised to species level and named E. villosa. We illuminated that bias thresholds determining the cluster number for delimiting species boundaries were substantially reduced when morphological data were incorporated. Our results urge caution when using the concepts of subspecies and varieties in order to prevent confusion, particularly with respect to species delimitation for tropical and subtropical species. In some cases, re-ranking or combining subspecies and/or varieties may enable more accurate species delimitation.