Who would have thought Stolfi would be an ivory tower Marxist?  Probably everyone?

Race is a real and not imaginary construct.  There were seven main haplogroups in Africa, L0-L6.  L3 split into haplogroup M & N.  N being the precursor to caucasians, and M being the precursor to everything else not African.  The amount of time that haplogroup M+N have been separated from L is something like 100,000 years.  It's not a question of "do races exist", or "are races different", it's only a question of how large of changes can occur in that time span.



That's only a conservative estimate of genetic isolation though.  In America, plenty of Caucasian and Africans live near each other, yet the interbreeding between groups is something miniscule under 5%.  Since white genes are recessive, the white race would not exist if this ratio was higher.  Due to this, whether your Marxist mind acknowledges it or not, if you advocate that everyone on earth interbreed with each other with no regards to race, you are in fact advocating white genocide due to white genes (and Asian) being recessive.

It's pretty easy to verify this with virtually any interracial relationship that exists.  Take Heidi Klum for example.  Here's her with two of her kids, each from different fathers.  She's supposed to be a supermodel, yet has passed along virtually none of those supermodel genes onto one child, while clearly having done so for the other.  This is because white genes are recessive and it's hard to even tell the child on the left is related to her at all.  



When people like Richard Dawkins use terminology like "the selfish gene" to describe how and why organisms reproduce, in the example above, you're effectively throwing your genes out the window, accomplishing nothing, and effectively eliminating yourself from the gene pool due to your genes being recessive.  This is one of the big reasons why intermarriage rate remains low.  Even if they don't understand the science behind it, people subconsciously know this and instinctively avoid it so as to not purposely erase their lineage from history.