Final answer:
Hybrids that have parents from different species are usually less fit than the parents, leading to further divergence and reinforcement of the two species. However, in some cases, two species may produce hybrid individuals that are as fit or more fit than the parents, leading to fusion into one species.
Step-by-step explanation:
Hybrids can be either less fit than the parents, more fit, or about the same. Usually hybrids tend to be less fit; therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process called reinforcement. This term is used because the low success of the hybrids reinforces the original speciation. If the hybrids are as fit or more fit than the parents, the two species may fuse back into one species. Scientists have also observed that sometimes two species will remain separate but also continue to interact to produce some hybrid individuals; this is classified as stability because no real net change is taking place.