I grew up hearing that the main indicator of a species was that they only reproduced with one another. But Neanderthals and Humans cross bred. And they’re separate species. And in captivity Lions and Tigers can breed, but they’re obviously different species. Like I guess I’m just confused where the line gets drawn when deciding what is and isn’t a different species.
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“Breeding” is only part of it. “Breeding to create viable, fertile offspring that can reproduce with individuals similar to either parent or themselves” is a more accurate requirement.
Lions and tigers can produce offspring (known as ligers). But if they were the same species, a male liger would be able to mate with either a female lion, tiger, or liger to produce an offspring; and a female liger would be able to mate with either a male lion, tiger, or liger and birth a viable offspring. This isn’t true. The only viable offspring of ligers arise from mating a female liger with a male lion, and even those are barely viable.
This is the same with a donkey and a horse. While a jack and a mare can mate and produce a mule, mules have an odd number of chromosomes and are sterile. Mules cannot mate and produce offspring with either other mules, donkeys, or horses.
Compared to wolves and dogs, wolves and dogs are the same species (*Canis lupus*). A wolf can mate with a dog and produce offspring that are viable and fertile, being able to mate and produce viable offspring with wolves, dogs, or half-wolves. In humans, this is the level of difference between the races — in the same vein, designations such as “sub-Saharan African” or “Scandinavian” could be considered “breeds” of humans (this is typically considered racist so it’s bad to argue this, but that’s a social issue rather than a scientific one). We actually use words like those in breeds of dogs, such as “German shepherd”, “Siberian husky”, or “Rhodesian ridgeback”, named for the places they come from (Rhodesia is the colonial-era name for modern-day Zimbabwe)
In the world of fantasy, the rules of this change between universes. Most people have heard of a “half-elf”, the offspring of a union between a human and an elf. Typically these unions are human male/elf female, but in most universes it works the other way around too — the trend towards this is social and not biological, much like how most white/Asian pairings are white male/Asian female. Many universes don’t delve into whether this makes humans and elves the same species, but it can often be discerned using these rules. If the universe has any characters in the canon who are descended from half-elves, this typically means they’re the same species in that universe. If there are no named descendants of half-elves, that probably means they are same-genus and that half-elves are sterile.
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