Uplift and formation of a mountain range divides a freshwater snail species into two isolated populations. Erosion eventually lowers the mountain range and brings the two populations together again, but when they mate, the resulting hybrids have sterile young. This is and example of which type of reproductive barrier

Post reproductive isolation → Hybrid sterility

Explanation:

The biological concept of species states that individuals of a species can not mate and reproduce with individuals of another species. But if they get to reproduce, the progeny will not be viable or fertile. There will not be any reproductive success.

There are different reproductive isolation mechanisms, which are barriers that inhibit or interrupt the genetic flow between different species.

Reproductive barriers are isolation mechanisms that prevent mating between two or more species. The prezygotic mechanism avoids fertilization between individuals of different species, while the postzygotic mechanism impedes the zygote to develop and reach the adult stage.

Postzygotic mechanisms or barriers include

In the exposed example, sympatric speciation occurs. It seems that the mountains separating the snails´ populations made a place for speciation and the development of postzygotic barriers, specifically hybrid sterility. After the erosion process, both populations got to meet again. Snails from one population get to mate and produce offspring with the snails of the other population, but their progeny is sterile.  

c. heat from the poles back down towards the equator.