posted by Nicole .
One family of mammals includes true horses (Equus), zebras (Hippotrigris and Dolichohippus), and donkeys and asses (Asinus). Zebroids are hybrid offspring of wild zebras and domesticated horses that were confined to the same pasture. The unnatural confinement breached the reproductive barriers between the two lineages. Those barriers have been in place since a divergence more than 3 million years ago. What does the breach say about the number of genetic changes required to attain reproductive isolation in nature?
Here at least are some sites to look through: