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We tested the effects of hybridization and introgression on the levels of leaf fluctuating asymmetry (FA) in a hybrid zone between Quercus magnoliifolia and Quercus resinosa at the Tequila volcano, Jalisco state, Mexico, in which the species are distributed along an altitudinal gradient ranging from 1400 to 2100 m. Bayesian clustering analysis was performed with STRUCTURE on data for eight nuclear microsatellite loci to assign individuals from reference populations and from the hybrid zone to pure or hybrid genotypic classes. To test the performance of the assignment procedure and to determine optimal thresholds for genetic assignment pure, hybrid and backcrossed genotypes were simulated (from the allelic frequencies found in real pure populations of the two species) and reanalyzed with STRUCTURE. Leaf FA and morphological identification of parental and hybrid individuals were obtained using geometric …