What occurs when species' gametes may fuse, but the embryo does not develop normally?

When gametes from different species successfully fuse but the resulting embryo fails to develop normally, this phenomenon is referred to as hybrid inviability. Hybrid inviability is a specific type of reproductive barrier that occurs after fertilization, preventing the development of a viable offspring. This can happen because the genetic differences between the two species prevent the embryo from developing properly, leading to its early death or incapacity to thrive.

In contrast, hybrid sterility refers to situations where the offspring may develop but are incapable of reproducing themselves, such as mules, which are hybrids of horses and donkeys. Post-zygotic barriers encompass any factors that lead to reduced viability or reproductive capacity of hybrid offspring, including hybrid inviability as a subset. Pre-zygotic barriers, on the other hand, are mechanisms that operate before fertilization occurs, preventing mating or fertilization between species. Thus, hybrid inviability specifically addresses the scenario where fusion of gametes occurs but results in an embryo that cannot develop normally, making this choice the most fitting response to the question.