The co-occurrence of hybrids and parental species in similar ecological niches poses a question on the role oftraits additivity and overdispersion (emergence of new traits) in microevolutionary processes. We analysed genet-ic polymorphism of Bromus benekenii, B. ramosusand the spontaneous hybrid B. benekenii× B. ramosusinsympatric and allopatric parts of the species distribution in Europe, based on non-coding regions of the taxongenomes (ISSR genetic fingerprinting). We tested 68 individuals in 7 populations, including a hybrid populationin N France. Altogether 233 polymorphic ISSR bands (loci) were obtained. We found that the parent species weregenetically distinct and the hybrids had an additive pattern of ISSR bands found in the putative parental species(NMDS, STRUCTURE); however, there was evidence of introgression towards B. ramosus(NEWHYBRIDS,UPGMA classifications, Nei's D genetic distance). Bromus benekeniihad 72, B. ramosus21 and the hybrids 9 private bands (genetic overdispersion), probably resulting from the rearranged genomes. Based on its lowgenetic divergence index DW, the hybrid population seems to be at a young age. We argue that in the face ofanthropogenic landscape transformations favouring secondary contacts, the hybrids may competitively replacethe parental species in sympatric areas.