system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are two different species capable of breeding fertile offspring not considered the same species?

From what I learned at school, one of the criteria to differentiate species was that two different species couldn't produce fertile offspring. For example, a donkey and a mare can produce a mule, but it's sterile. Our canine friends and a wolf can breed fertile offspring, but one (Canis lupus familiaris) is a subspecies of the other(Canis lupus).

But I found out about the Beefalo, the offspring of a cow and a bison, which is said to be a fertile hybrid. But those are two different species, two different genera actually (Bos Taurus x Bison bison). How can they breed fertile offspring and not be considered the same species?
gpt
That is one definition of a species. It does have the benefit of being quite simple to understand and test. However as you points out it does not always fit. Your example of cows and bison is one example. And even cross bread animals like mules can sometimes be fertile. Then there are so called ring species where one species can bread with two other but those two can not bread with each other. So biologists nowadays use other definitions that are more complex but work better. But your definition is still being taught in a lot of schools because it is more simple.