So there was this discussion on non-racialism at this book launch last night ...
and a member of the audience asked that typical question about the obvious genetic basis to race.
I was working up to try and answer there, but found it a bit hard to get my thoughts together in time. So below is working toward how I'd like to answer that in future. I'll correct/refine it with time, hopefully, and corrections and comments from you, the anonymous or not anonymous public, are as welcome as always.
Once our very early, singular human population separated geographically (after what appears to be a series of survival bottlenecks), they evolved separate and particular traits according to the now well understood processes of natural selection and genetic drift.
Even while populations were largely separated, there was still sufficient movement and interaction to allow for genetic information to pass between populations. This, most importantly, would have included the successful spread of immune response adaptations and anti-parasite counter-measures vital to early population survival. [which is why all so called races have within their populations a spread of proteomic pathways for immune function that are common between races]
As populations interacted and genetic information spread, there are a few mechanisms by which certain traits persisted within local populations, despite the relatively thorough statistical mixing of other genetic traits. The mechanisms that ‘preserved’ local ‘race type traits’ include environment specific adaptations (like skin colour) that were continuously selected for within separated populations by survival pressure, and culture-specific sexual selection criteria - that proceeded along with cultural evolution. There are more mechanisms i think, but these two come to mind for now.
Via these mechanisms, an aggregation of certain traits (the sexual selection model accounts for why these are most often just externally visible) would accumulate within populations, while allowing the fortunate mixing of other vital genetic survival strategies without which local populations would most likely have fallen from parasite/pathogen load/stress.
This explains why, with the exception of this small percentage of ‘race type traits’, when looking at particular sequences, we often see more genetic diversity within races than between them — for example, there will be a particular immune system function that is expressed in different ways by several different protein pathways (lets call them A,B,C,D) and each race will have individuals carrying sequences for A,B,C,D. For the larger pathways, two individuals from different ‘races’ carrying the sequences necessary for A will often have more in common genetically with each other than with members of the same race carrying sequences for B, say.
[Immune function has been a big driver for evolution throughout the entire animal line, and so its not for nothing that it gets emphasised when discussing genetic variation.]
Even as our populations experienced civilisational shifts that allowed for more interaction between populations and more and more geographical displacement, patterns of mechanisms like the culture-specific sexual selection mentioned above - now intimately coupled with power, violence, etc. - still worked to keep certain traits prominent within local populations.
Anyway, I think that's the type of traits the dude from yesterday mentioned. That small percentage of genetic traits, often highly visible, that we based the myth of race on.
Of course, the other reason that immune system is so important when discussing us, is that in the very brief evolutionary time since we split up, nothing much else changed.
Sure some of us lightened our skins and straightened our hair, but these were almost insignificant changes when seen against our vast evolutionary history.
The most significant changes to humankind since our ‘forking’ is that the big brains that we evolved before we split led to us being super successful wherever we went - and that success was met in turn by a multitude of parasites rushing in to live off a newly found ecosystem - us.
So, most of our real evolution as humans, neglecting the very very minor surface tweaks, have been in complex immune system responses to those parasites since we became successful. That is why we are closer to members of other races that share our immune pathways than those of our race that don’t - immune complexity makes up most of our evolution since we got here.