What is hybrid inviability?

Hybrid inviability refers to a situation where hybrids, often the result of mating between two different species, do not survive to adulthood or do not develop successfully past the early stages of embryonic development. This can occur due to genetic incompatibility between the two parent species, leading to developmental issues that prevent the embryo from reaching a mature state.

In the context of the provided options, the mention of an egg that fails to develop beyond the embryonic stage directly captures the essence of hybrid inviability. This concept is an interesting aspect of reproductive isolation, highlighting how certain species might be able to mate yet produce offspring that cannot thrive or survive, thus reinforcing the boundaries between species.

The other choices reference scenarios that do not align with the definition of hybrid inviability. The presence of fertile hybrids or viable hybrids with low fitness pertains to different aspects of hybrid reproduction, emphasizing aspects like fertility and adaptability rather than the inability of a hybrid to reach maturity.