Final answer:
Reinforcement is more likely to occur in the hybrid zone when the hybrid offspring have lower fitness than purebred individuals.
Step-by-step explanation:
Reinforcement is more likely to occur in the hybrid zone when the hybrid offspring have lower fitness than purebred individuals. In this situation, the hybrids are less fit than either purebred species, and their low success reinforces the original speciation. Over time, this leads to further divergence between the two species, and eventually, hybridization can no longer occur.