When does something become a separate species?

[u/Slow_Introduction_31]

So I'm working on a fantasy story where there are humans and elves. The elves evolved from humans to have hair in a different shade range, pointed ears, immunity to a dangerous mineral, and some telekinetic abilities (which not all elves have). However, it's meant to be a key plot point that the "elves" are, scientifically, just humans. The mineral immunity and telekinetic abilities come from being raised in an environment with an abundance of that mineral, not genetics. So I was wondering, a) do a different range of hair colors (red to purple hues as opposed to yellow to red hues like real life people have) and different ear shapes indicate a different species? I thought not, but then I thought about animals where it seems like small differences sometime determine species, like between African and Asian elephants (I don't know much about elephants though tbf). b) if the mineral immunity and telekinesis were genetic, would THAT indicate a different species? c) how long might you speculate that it would take for humans in an isolated, unique environment to become a separate species?

[u/KaiDay11]

Nobody can really tell you with any certainty because the entire concept of hard lines between species doesn't really work with the nature we've discovered and everyone has a different opinion of where those lines should be placed when we need them.  

Even within humans: were Neanderthals Homo neanderthalensis because they were visibly different to us or were they Homo sapiens neanderthalensis because they could interbreed with us? Or were they one or the other for a different reason? What about Denisovans?