Two species of mice belonging to the same genus occasionally mate (e. g. like the house mouse and the Algerian mouse from last Thursday's lecture), but most of the offspring fail to develop normally. What is the mechanism for keeping the two mouse species separate?

post zygotic barrier by hybrid inviability

Explanation:

Post zygotic barriers are involves creation of offsprings that will not survive either

Hybrid inviability - will not survive past the embryonic stages Hybrid sterility - Individuals will survive but they will fail to produce offsprings

In the case where mice belonging to the same genus mate but their offspring fail to develop normally is a good example of Post-zygotic barrier by hybrid inviability