Problem : The product of mating between a horse and a donkey is a mule. Mules usually develop normally into strong, healthy adults, but they are fertile. How is this an example of reproductive isolation?

The mule is an example of an infertile hybrid. Even though horses and donkeys can mate and produce healthy offspring, those offspring cannot pass their genetic information down to future generations, so they are a genetic dead-end. There is no gene flow through infertile hybrids.

Problem : What are the three main problems found in hybrid offspring?

Most hybrid offspring have one of three problems that prevents them from mating. Some do not develop normally and can never reach sexual maturity. Others reach sexual maturity but do not produce gametes and are infertile. Many hybrids have the potential to reach sexual maturity, but are unhealthy and do not survive to reproductive age.

Problem : How does post-zygotic reproductive isolation differ from pre-zygotic reproductive isolation?

In pre-zygotic isolation, no zygote is formed, and in most cases mating does not take place. Little energy is wasted. However, in post-zygotic isolation, mating does take place and offspring are produced at a considerable cost of energy. Unfortunately, this mating is still unsuccessful, and this energy is wasted.

Problem : A species of flowering plant is successfully fertilized with pollen from a different species of plant. Seeds form and are dispersed by the wind. These seeds germinate and give rise to young hybrid plants. However, these hybrid plants are not very strong and are easily killed by drought and disease and rarely survive to reproductive age. What type of reproductive isolation is this?

This is an example of low hybrid viability.