Two species of frogs belonging to the same genus occasionally mate, but the embryos stop developing after a day and then die. These two frog species separate by .

the postzygotic barrier called hybrid inviability

Explanation:

Some times when two members of the same genus mate occasionally then they are not able to produce a viable offspring and the offspring dye very early which is called hybrid inviability which is a postzygotic barrier.

This prevents the gene flow between the two species because the offspring fitness is very low as compared to the fitness shown by a pure breed individual. So nature does not allow the less fit breed to survive. Therefore the two frog species are separated by postzygotic barrier.  

yo