Why humans have so many pulse admixtures

The Blank Slate is one of my favorite books (though I’d say The Language Instinct is unjustly overshadowed by it). There is obviously a substantial biological basis in human behavior which is mediated by genetics. When The Blank Slate came out in the early 2000s one could envisage a situation in 2017 when empirically informed realism dominated the intellectual landscape. But that was not to be. In many ways, for example in sex differences, we’ve gone backward, while there is still undue overemphasis in our society on the environmental impact parents have on children (as opposed to society more broadly).

But genes do not determine everything, obviously. Several years after reading The Blank Slate I read Not by Genes Alone: How Culture Transformed Human Evolution. In this work Peter Richerson and Robert Boyd outline their decades long project of modeling cultural variation and evolution formally in a manner reminiscent of biological evolution. Richerson and Boyd’s program does not start from a “blank slate” assumption. Rather, it is focused on broad macro-social dynamics where cultural variation “swamps” out biological variation.

Recall that in classic population genetic theory a major problem with group level selection is that gene flow between adjacent groups quickly removes between group variation. One migrant between two groups per generation is enough for them not to diverge genetically. For group selection to occur the selective effect has to be very strong or the between group difference has to be very high. Rather than talking about genetics though, where the debate is still live, and the majority consensus is still that biological group selection is not that common (depending on how you define it), let’s talk about human culture.

Here the group level differences are extreme and the boundaries can be sharp. Historically it seems likely that most groups which were adjacent to each other looked rather similar because of gene flow and similar selective pressures. Even though in medieval Spain there was a generality, probably true, that Muslims were swarthier than Christians*, there was a palpable danger in battle of identifying friend from foe because the two groups overlapped too much in appearance.

This brings up how one might delineate differences culturally. In battle opposing armies wear distinct uniforms and colors so that the distinction can be made. But obviously one change uniform surreptitiously (perhaps taking the garb from the enemy dead). This is why physical adornment such as tattoos are useful, as they are “hard to fake.” Perhaps the most clear illustration of this dynamic is the Biblical story for the origin of the term shibboleth. Even slight differences in accent are clear to all, and, often difficult to mimic once in adulthood.

Biological evolution mediated through genes is relatively slow and constrained compared to cultural evolution. Whole regions of central and northern Europe shifted from adherence to Roman Catholicism to forms of Protestantism on the order of 10 years. Of course religion is an aspect of culture where change can happen very rapidly, but even language shifts can occur in only a few generations (e.g., the decline of regional German and Italian dialects in the face of standard forms of the language).

Cultural evolution as a formally modeled neofunctionalism is credibly outlined in works such as Peter Turchin’s Ultrasociety: How 10,000 Years of War Made Humans the Greatest Cooperators on Earth. That’s not what I want to focus on here. Rather, I contend that the reality of massive pulse admixtures evident in the human genome over the past 10,000 years, at minimum, is a function of the fact that human cultural evolutionary processes result in winner-take-all genetic consequences.

A concrete example of what I’m talking about would compare the peoples of the Italian peninsula and the Iberian peninsula around 1500. The two populations are not that different genetically, and up to that point shared many cultural traits (and continue to do so). But, a combination of geography and history resulted in Iberian demographic expansion in the several hundred years after 1500, whereby today there are probably many more descendants of Iberians than Italians. This is not a function of any deep genetic difference between the two groups. There aren’t deep genetic differences in fact. Rather, the social and demographic forces which propelled Iberia to imperial status redounded upon the demographic production of Iberians in the future. In addition, the New World underwent a massive pulse admixture between Iberians, and native Amerindians, as well as Africans, usually brought over as slaves, due the cultural and political history of the period.

The pulse admixture question is rather interesting academically. To some extent current methods are biased toward detection of pulse admixtures, and even fit continuous gene flow as pulse admixtures. A quick rapid exchange of gene flow and then recombination breaking apart associations of markers which are ancestrally informative haplotypes is something you can test for. But I think we can agree that the gene flow triggered by the Columbian Exchange was a pulse admixture, and there’s too much concurrent evidence from uniparental lineage turnover in the ancient DNA to dismiss the non-historically corroborated signatures of pulses as simply artifacts.

Nevertheless continuous gene flow does occur. That is, normal exchange of individuals between neighboring demes as a slow simmer over time. But the idea that we are a clinal ring species or something like that isn’t right in my opinion. Part of the story are strong geographical barriers. But another major part is that cultural revolutions and advantages introduce huge short-term demographic advantages to particular groups, and the shake out of inter-group competition can be dramatic.

Therefore, I make a prediction: the more cultural evolutionary dynamics a species is subject to, the more pulse admixture you’ll be able to detect. For example, pulse admixture should be more important in social insects than their solitary relatives.

* Not only was some of the ancestry of Muslims North African, Muslim rule was longest in the southern and southeastern regions, where people were not as fair as in the north.