Compare the fitness of hybrids in cases where reinforcement occurs versus cases where speciation by hybridization occurs.
When reinforcement occurs, hybrids have the same fitness; this creates fusion between the two populations because they can freely interbreed. In contrast, the hybrids in the case of speciation by hybridization have lower fitness, usually because they do not have the proper adaptations for certain ecological conditions.
When reinforcement occurs, a hybridization zone develops in a well-defined zone. Hybrids have higher fitness that stabilizes the hybrid zone. The hybrids in the case of speciation by hybridization also have higher fitness, usually because they happen to have adaptations for certain ecological conditions. They may form a new species.
When reinforcement occurs, hybrids have higher fitness; this is why increasing hybridization improves the fitness of reproductive individuals. In contrast, the hybrids in the case of speciation by hybridization have lower fitness; therefore, the separation of the two species is accelerated.
When reinforcement occurs, hybrids have low fitness; this is why avoiding hybridization improves the fitness of reproductive individuals (their reproductive efforts are not wasted). In contrast, the hybrids in the case of speciation by hybridization have higher fitness, usually because they happen to have adaptations for certain ecological conditions.