The second category of isolating mechanism that may permit interspecific mating but ensures reduced viability is called post-mating isolating mechanism. This category could be subdivided into two types.

Interspecific steiility : Organisms belonging to different species may mate, but may not produce any offspring.

Hybrid sterility : Here the interspecific cross may result in an F1 offspring but the offspring is invariably sterile.

We shall now briefly look into each one of these sterility mechanisms with plant and animal examples.