> The concept of 'species' only exists within the human mind [1].
That's a theory, anyway. Yes, biologists (and presumably botanists and all those who study the other kingdoms) have made errors in judgment, and yes, the current definition is very poor (often broken, sometimes unprovable in a practical sense).
It does not follow, however, that all life exists on some continuous spectrum of mutations. Members of Equus caballus have distinct genetic traits that do not occur in Equus asinus.
The reality we see about us is that life seems to group itself into channels or pools of limited genetic variation. But that variation doesn't mean the channels/pools aren't real.
The quote you are responding to is a reaction to scientific racism. Forgive me if I'm misinterpreting you, but you said that you find it amusing that this is still controversial.
> The quote you are responding to is a reaction to scientific racism.
No, it is a reaction to treating humans the same as finches. You give the thing he does a label and then attack the label, instead of attacking the thing itself.
Isolated populations "drift" genetically, which is how species form in the first place. Discriminating against people based on those differences sucks, sure, but it's still the ground truth.
Pretty sure there are genetic clustering algorithms which show that perceived race groups approximately correspond to measured race clusters. For example, random East Asians are more closely related to each other than to random West Europeans.