Just to clear this up for those who find this sounds sort of ridiculous (which honestly is understandable): Nobody's saying skin colour is a social construct, only the transition between races is so smooth, that any hard distinction would be arbitrary.
It's not like with different animal species where you can clearly say "this is that species, this is that species, there's no in-between or transition".
Imagine all of your male ancestors standing in one line, back to the fish: Sure, you could put them into categories, and say "this here is a fish and this is a human". And there'd be genetic differences and commonalities, and you could clearly say there's a difference.
But it's completely arbitrary how we divide these ancestors up. We divide them up by how their appearance has changed, and that is, well: All very subjective. Scientifically, we can only say that there's a line of animals which have slowly transitioning differences, but there is no scientific way to clearly divide them up.
So you could say "This person is clearly of an African race, this person clearly of a European race", but what about the in-betweens? "Well", you might say, "the transition is the Arabs". 'But the Turks don't look very different from Eastern Europeans' I say, "So there's also a Balkanise race". And so on, and so on. But it's an infinitely smooth transition, and our DIVISION of races is arbitrary. Or in other words: Based on social concepts, such as the history of different groups (Wonder if Muhamad may be led to the idea of the "Arab" race).
RE: There Is No Biological Basis for Different Races in Modern Humans