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

If they can mate and form viable genetic offspring, what makes them a separate species? Please feel free to apply this same line of logic to all the other separate species that can mate and form viable offspring.
gpt
I don’t believe they are always considered a separate species anymore. IIRC they are often classified as *Homo sapiens neanderthalensis* (with anatomically modern humans being *Homo sapiens sapiens*). 

Part of the confusion stems from how species work. We’ve applied a relatively rigid system of classification to something a lot more fluid. For instance, one of the more common definitions of a species is a population of organisms which can breed together to produce fertile offspring. Therefore, if the offspring of two animals aren’t able to reproduce the parents would be considered separate species. However, hybrids sometimes are fertile, which further muddies the waters.

Species and subspecies are more approximate classifications that solid categories. It’s also worth noting that it was once assumed modern humans and Neanderthals couldn’t interbreed, which is why they were originally classified as a separate species.

Also, looking through the literature, you’ll often see both *H. neanderthalensis* and *H. s. neanderthalensis* referred to, occasionally within the same paper, so it appears they’re quite commonly used interchangeably as a result of the ambiguity over whether they’re a separate species or not.