Final answer:
Fusion between two species can occur in a hybrid zone if the hybrids are as fit or more fit than their parents and reproductive barriers are weak, potentially leading to the merging of the species.
Step-by-step explanation:
Factors that lead to the occurrence of fusion between two species arise when closely related species interact within a hybrid zone after speciation. The extent to which these species may recombine into one species depends on the relative fitness of the hybrids produced and the strength of reproductive barriers.
If hybrids are as fit or more fit than their parent species, and if reproductive barriers weaken, then fusion can occur, resulting in the merging of the two species. Environmental factors, such as climate and prey choices, also influence the evolutionary path of a species, possibly leading to speciation, fusion, or stability.