Final answer:
Reinforcement is the term used to describe the process where two species diverge further due to the low fitness of hybrid offspring, reducing their occurrence and enhancing reproductive barriers.
Step-by-step explanation:
The term used to describe the continued divergence of species based on the low fitness of hybrid offspring is reinforcement. This process occurs when hybrids produced by two closely related species are less fit than the parent species, which leads to reduced likelihood of producing more hybrid offspring over time. As these less fit hybrids become less common, the reproductive barriers between the parent species are strengthened, leading to further divergence until hybridization may not occur at all.
In a hybrid zone where two closely related species continue to interact and produce offspring, three different outcomes can be observed depending on the fitness of the hybrids and the strength of reproductive barriers: reinforcement, fusion, or stability. Reinforcement is the outcome when hybrids are less fit, fusion occurs if the hybrids are as fit or more fit than the parents resulting in the two species merging back into one, and stability is present when hybrids continue to be produced without significant changes to the overall population structure.