The populations diverged before coming into contact. In contrast with secondary contact following allopatric divergence, which may be limited in time before fusion of populations or speciation by secondary reinforcement, hybrid zones can be stable. If, in a hybrid zone, hybrids suffer selective disadvantage and both races are adapted to conditions different from those in the hybrid zone, the region may be called a tension zone . If a stable hybrid zone is a tension zone, conditions should favor reinforcement of reproductive isolation. If variation in mate preferences is present, selection should favor reproductive isolation.
Hybrid zones are common.
Evidence for reinforcement is less so. Both allopatric and paratric speciation are well supported by theory. Sympatric speciation has long been more controversial. Sympatric speciation is theoretically possible. Polymorphism can be supported by assortative mating.