The Violet family (Violaceae Batsch) comprises about 25 genera with 900 species and is distributed worldwide. Viola is by far the largest genus with 600 species. Viola grahamii and V. hookeriana are two closely related species in the Mesoamerican subsection Mexicanae. Both species grow sympatrically around Lake Pátzcuaro in the state of Michoacán, Mexico. On this mountain several individuals with intermediate morphological characteristics have suggested hybridization and perhaps introgression. This study used morphological evidence, ecological variation and molecular evidence from ISSRs (inter-simple sequence repeat markers) to evaluate the hypothesis of gene flow between V. hookeriana and V. grahamii. The results indicate that lack of spatial isolation, strong overlapping blooming times, and weak ecological isolation favor extensive hybridization and directional gene flow towards V. grahamii. Most hybrid derivatives are morphologically, ecologically and phenologically similar to V. grahamii. The proportion of hybrids is as high as V. grahamii and much higher than V. hookeriana. Very low levels of pollination and high levels of cleistogamy during the present study, complex variation, and substantial numbers of fixed hybrid-specific alleles suggest that selfing of hybrid derivatives, accompanied by segregation and recombination of parental genotypes, could be common. The initial hybridization and introgression event may have occurred in the past, and hybrid-derived populations may maintain themselves indefinitely as long as post-mating limitations are not so severe as to prohibit viability and fecundity of later-generation offspring. The contribution of seed set from cleistogamous flowers and the proportion of vegetative reproduction in the populations are two factors that should be explored to evaluate their effect in this hybrid swarm.

Key words: cleistogamy, introgression, isolation mechanisms, ISSRs, Mexico, Viola