system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that two different species like a horse and a donkey reproduce to make a mule, but other instances of interspecial breeding do not produce offspring?
gpt
There is actually no globally applicable, waterproof definition of what a "species" is. One common definition is "if it can't interbreed, it's a different species", but cases like horse/donkey (and tiger/lion) show its limit. Another problem is that it can only apply to populations that existed at the same time: How can you say that horses today and horses 500 years ago are the same species?

Definitions aside, two populations can interbreed if their genes are compatible, but that is really more a continuum than a yes/no question. There are, for example genetic defects where people who carry the defect cannot have viable offspring with each other, but can with anyone who doesn't. In the case of horses and donkeys, their genes are almost compatible, but there are differences which cause the offspring to be infertile.