A zone that exists between the populations of two related species, whose geographical ranges overlap, which contain hybrids resulting from matings between the two species. The hybrids, which typically have a reduced fertility, will tend to act as a barrier between further interspecific mating and hence will restrict gene flow between the two species. However, mating between hybrids and individuals of either parent species can also introduce novel genetic variation, which can be of adaptive value in a changing environment. This can lead to stabilizing of the hybrid zone over time or even to enhanced gene flow between the parent species, weakening the reproductive barrier and ultimately causing fusion of the hybridizing species into a single species. Hybrid zones can range in spatial form from simple bands to more complex patterns, dependent on local factors such as topography, vegetation, and climate. See also isolating mechanisms; parapatric speciation.