Reproduction Isolation

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?

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

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

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?