Mules, the offspring of donkeys and horses, are commonly used as pack animals. Despite the existence of mules, horses and donkeys are considered separate species.Which discovery would change that thinking and define horses and donkeys as the same species?

A common biological species rule is that members of the same species can mate and produce fertile offspring. Mules are hybrids of horses and donkeys, and they are usually sterile. If mules could produce offspring, that would support treating horses and donkeys as the same species. Their appearance does not determine whether they belong to the same species.

AI-written and checked against the verified answer.