Abstract. Hybrid zones occur when two species are found in close proximity and interbreeding occurs, but the species characteristics remain distinct. These systems have been treated in the biology literature using partial differntial equations models. Here we investigate a stochastic spatial model and prove the existence of a stationary distribution that represents the hybrid zone in equilibrium. We calculate the width of the hybrid zone, which agrees with the PDE formula only in dimensions d > 2. Our results also give insight into properties of hybrid zones in patchy environments.
Revised version (5/19/06) to appear in Stochastic Processes and their Applications