hybrid zones provide opportunities to study factors that cause reproductive isolationa hybrid zones is a region in which members of different species mate and produce hybridswhen closely related species meet in a hybrid zone, there are three possible outcomesreinforcement : strengthening reproductive barriersthe reinforcement of barriers occurs when hybrids are less fit than the parent speciesover time, the rate of hybridization decreaseswhere reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric speciesfusion : weakening reproductive barriersif hybrids are as fit as parents, there can be substantial gene flow between speciesif gene flow is great enough, the parent species can fuse into a single species