That's a slippery concept. Lions and tigers are separate species but on rare occasions mate and produce offspring.
Is a mule a separate species from donkey and horse?
Horses and donkeys are seperate species, the existance of mules notwithstanding. The key being that mules (like the “ligers” you referred to) are sterile. Since they cannot reproduce, mules are not even a species biologically speaking. To be considered a single species, the offspring must be capable of reproduction.
“Lions and tigers are separate species but on rare occasions mate and produce offspring.”
Really, any kind of feline can breed with any other kind of feline, barring some physiological impediments like great size differences. There is so little genetic diversity in the felines that, even though we consider them separate species, it is obvious that they are all very closely related.
Deb: What's a liger?
Napoleon Dynamite: It's pretty much my favorite animal. It's like a lion and a tiger mixed... bred for its skills in magic.
Yes.
And horses and donkeys are not the same species.
They don't have the same number of chromosomes which leads to their having infertile offspring.