Horses and donkeys can breed and produce sterile offspring known as mules. Horses and donkeys remain separate species because of this hybrid sterility, which is:

1. an allopatric barrier to reproduction.

2. a postzygotic barrier to reproduction.

3. a prezygotic barrier to reproduction.

4. a sympatric barrier to reproduction.

2. a post-zygotic barrier to reproduction.

Explanation:

Reproductive isolation does not allow the members of one biological species to interbreed with members of other species and produce viable and fertile progeny. Post-zygotic barriers to reproduction are operative after the formation of zygote. Fertilization sometimes occurs between gametes of two closely related species. However, post-zygotic barriers ensure reproductive failure.

For example, a mule is produced by mating between a female horse (2n = 64) and a male donkey (2n = 62). Its diploid chromosome number is 2n = 63). Mules are sterile because synapsis and segregation during meiosis cannot occur properly. Therefore, horses and donkeys may interbreed but remain separate species since their hybrid progeny is sterile. This represents post-zygotic reproductive isolation.