system
Given the following post, respond as if you're commenting on Reddit:
human
Why are Neanderthals considered a different species?

From what I remember in science class, two different species can’t produce fertile offspring.

For example, a horse and a donkey can create a mule, but mules are sterile; therefore, horses and Donkeys can’t have fertile offspring and are separate species.

But Many modern humans have Neanderthal DNA, meaning that Neanderthals must have interbred with our ancestors and produced fertile offspring.

Wouldn’t that make Neanderthals the same species, by definition?
gpt
For this exact reason, many researchers don't consider neanderthals to be a separate species, but count them as the subspecies *Homo sapiens neanderthalis*.

However, the whole "same species if they can produce viable offspring" principle is a good guideline, but often hard to apply in practice. There are several examples of closely related populations that will sometimes produce viable hybrids, but do so rarely enough that it doesn't produce much gene transfer and so they retain major anatomical and behavioral differences. And then there are examples of species that will produce viable offspring if made to in a lab but never do so in nature due to behavioral differences. And then there are really weird examples like sexual parasites.

So defining species barriers is to some extent a matter of interpretation, and in the case of neanderthals it appears that though there was some interbreeding, it was never the norm.