If two individuals from different species attempt to mate but fertilization fails to occur, this is likely the result of . However, if such a mating produces a zygote that fails to form a viable embryo, this is likely the result of .

Prezygotic reproductive barriers; Postzygotic reproductive barriers

Explanation:

Prezygotic reproductive barriers are reproductive barriers that involve mechanisms that prevent the successful mating and fertilization between two individuals of a species. Prezygotic factors include gamete isolation, difference in mating seasons, habitat isolation.

Postzygotic reproductive barriers, on the other hand, involve mechanisms that prevent the reproduction of viable offspring that are fertile, even when mating and fertilization between two different species occurs.