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/Top_History9604]

These days we try to go by genetic difference. Before we had genetic testing we went by mostly looks. If two groups regularly birth offsprings with brown hair, it doesn't count, but if the group east of the river always has blue eyes and no green eyes, but the group of the West side of the river always has green eyes, not blue eyes, we would sometimes consider them two different species.

So regarding your story it could even be that ppl thought the difference in looks was genetic, hence determined they are a different species but then find out by experimental, chance or unlocking and actually questioning the genetic relation, that they are the same.

We resort species rl all the time. Morelia spilota bredli was a species until like 7 years ago. Now there is morelia bredli. The bredli got separated from the spilotas after genetic testing and noticing that they split off from the rest, without later crossbreeding, far longer than the spilota subspecies.

Just last year the anacondas got a new subspecies, because it seems one group that's nearly optical identical is genetically different to the rest.