Let me put it this way -
If skin colour is rated 1-10 with 1 being pale and 10 being dark then the Swedes will average around a 3 while the Zulu's will average an 8. The darkest Swede will still be lighter than the lightest Zulu.
Or you could compare Irish and Border collies, both are breeds of collie working dogs but they're clearly differentiated and considered to be separate. Which is not to say they're all that different, but nobody would say that a pure-bred Irish and Border collie were the same breed.
I'm not arguing with the point, that all humans are one species, I'm saying that the argument doesn't really hold up.
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