Low genetic differentiation between two morphologically and ecologically distinct giant-leaved Mexican oaks

Quercus magnoliifolia and Q. resinosa are two Mexican white oak species that have been taxonomically reported to exhibit morphological similarities and possible hybridization. The objective of this study was to compare the variation in Q. magnoliifolia and Q. resinosa throughout their distribution range to identify the degree of species differentiation using morphological, ecological and genetic tools. Morphological analysis showed differentiation in leaf shape between the species corresponding to the taxonomical identification of Q. magnoliifolia and Q. resinosa in almost all cases, but intermediate individuals were identified in the middle of the species ranges. Comparison of ecological niche models for Q. magnoliifolia and Q. resinosa showed non-equivalent ecological niches, high climatic niche differences and low to moderate spatial and environmental niche overlap, mainly along the Trans-Mexican Volcanic Belt where morphologically intermediate individuals between species were more frequently located, suggesting recent hybridization by secondary contact. In contrast, we found low but significant genetic differentiation between Q. magnoliifolia and Q. resinosa and lower interspecific than intraspecific genetic differentiation, and Bayesian clustering analysis (K = 2) failed to assign each species to a unique genotype, suggesting shared ancestral variation as the cause of genetic similarity between species due to recent divergence. In conclusion, although neutral molecular markers do not distinguish the species Q. magnoliifolia and Q. resinosa, we found morphological and ecological differentiation between these oaks that provide preliminary evidence for divergent selection.