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naraburns  nihil supernum  1yr ago (text post)   5896 thread views

Small-Scale Question Sunday for January 22, 2023

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the popular factoid that there's more "genetic diversity" in Africa than elsewhere

Is that not true?

It's true inasmuch as we literally talk about "genetic diversity" in the context of anthropological research, but implications that people try to smuggle in with these words are false. I'll just quote Cochran again.

On African genetic variation:

Occasionally I hear people talk about Africa’s great genetic variety. It exists: the genetic difference between Bushmen and Bantu is bigger than the difference between Bantu and Finns. A couple of thousand years ago, before the Bantu had arrived in South Africa & mixed with the Bushmen, it was even bigger: looking at ancient DNA from those unmixed Bushmen, looks as if they split off from the rest of the human race at least a quarter of a million years ago. Well before anything that looks like behavioral modernity, by any definition. Half as divergent as Neanderthals.

But the most divergent populations are small. There are fewer than 100,000 Bushmen, on the order of a million Pygmies, around 1000 Hadza. Most people in Africa are Bantu or related populations: next after that are Nilotic peoples.

And the venomously sarcastic Economists and biology:

Naturally, economists know a lot about human biology and evolution, just as civil engineers have to know about the properties of timber, concrete and steel. They have a good grounding in psychometrics, behavioral genetics, and quantitative genetics – how else could they do their job? Populations vary in traits that play key roles in economic activity and growth – in intelligence, asabiya, savings propensity, etc – you have to be aware of that variation, else whole continents would be economic mysteries. In the same way they know that those observed differences are a product of selection – which means economic historians think seriously about psychometric changes over time and their consequences, such as the Industrial Revolution. That kind of analysis helps predict where modern economic institutions can be successfully introduced, and where they cannot. [...]

Deirdre McCloskey has a new book out: Bourgeois Equality: How Ideas, Not Capital or Institutions, Enriched the World. I’m sure that there are many good things in it. But McCloskey makes a significant error in talking about genetics.

“Know also a remarkable likelihood in our future. Begin with the sober scientific fact that sub-Saharan Africa has great genetic diversity, at any rate by the standard of the narrow genetic endowment of the ancestors of the rest of us, the small part of of the race of Homo sapiens that left Mother Africa in dribs and drabs after about 70,000 BCE…. Any gene-influenced activity is therefore going to have more African extremes. The naturally tallest people and the naturally shortest people, for example, are in sub-Saharan Africa. The naturally quickest long-distance runners are in East Africa. The best basketball players descend from West Africans. In other words, below the Sahara the top end of the distribution of human abilities – physical and intellectual and artistic – is unusually thick. …

The upshot? Genetic diversity in a rich Africa will yield a crop of geniuses unprecedented in world history. In a century or so the leading scientists and artists in the world will be black – at any rate if the diversity is as large in gene expression and social relevance as it is in, say, height or running ability. ”

So by this argument that the most cold-tolerant Africans must be more cold-tolerant than Eskimos: but they’re not. The most altitude-tolerant Africans must be more altitude-tolerant than Tibetans – but they’re not. McCloskey is thinking that a turn to free markets will make Africa rich, and that will give educational opportunities to Africans now denied them – but a fair-sized mostly-African population already lives in the United States, a population that is already much more prosperous than sub-Saharan Africans. How are they doing? How many geniuses are they producing?

The whole argument is flawed. Overall genetic variation is mostly in neutral loci. By itself it tells you nothing about any particular trait. Europeans do have less overall genetic variation than sub-Saharan Africans (~20% less), but they show more variation in hair color and eye color than Africans.

Essentially every domesticated species has less genetic variation than its wild progenitor. Dogs have less genetic variation than wolves. So, does this mean that the tallest wolf is taller than any dog? No – the tallest Great Danes are taller than any wolf. The heaviest mastiffs are heavier than any wolf. Chihuahua are the smallest. Greyhounds are faster than wolves (by a little). [...]

What matters is the frequency of alleles that influence a trait, not overall genetic diversity. If, for example, the variants that tend to boost educational achievement (some of which were found in the just-released Nature study) were on average less common in sub-Saharan Africans than in Europeans – says 5% less common – Africans would tend to do less well in school. Like they actually do. Now Africa is a big place, and some groups are genetically quite distinct from others. Bushmen are genetically more distant from the Bantu than the Bantu are from Chinese. Some African populations might have experienced selective pressures that were more (or less) favorable for intelligence than others. Is there evidence, either in test scores or cultural accomplishment (better than any test), that some African populations may have smarts comparable with, or better than, people in Switzerland or Holland or Scotland?

TL;DR: Africans do have greater genetic diversity, but "genetic variation is mostly in neutral loci", i.e., greater genetic diversity does not imply greater phenotypic diversity, and even if there is greater variation in one phenotypic trait (e.g. height), this does not imply that there is greater variation in all traits. Correct?

Pretty much. Actually it's four points.

  1. Most of the asserted "genetic diversity" of Africa as a whole is due to peoples from ancient population clusters, which make up less than 1% of African population and are not associated with colloquial "black people". They are anthropological marvels, but frankly they're not the guys anyone is talking about. Related Razib Khan post.

  2. "Genetic diversity" for a given population is overwhelmingly a measure of ancestral population size and diversity of interchangeable variants that have spread by genetic drift. Out-of-Africa populations have passed through a series of severe bottlenecks, so they've shed some genetic markers we can find in African Bantus. Doesn't have much to do with anything consequential. It's like a JPG that's gone through lossy compression: there are color bands now, and less diversity of color patches or raw pixel values, but it doesn't mean the semantics of the image is altered, or that the range of any color channel is diminished, in fact the opposite can be true.

  3. Local adaptation can very quickly act on non-neutral loci, a few thousand years are more than enough to change a typical polygenic trait by 1 SD. We have no grounds to presume that ancestral (>50KYA) diversity explains modern diversity for groups that have evolved in different environments.

  4. By the same logic, yes, traits are independent products of selection. Just like all Sub-Saharan Africans are pretty dark relative to other races, they can be pretty ... fast runners, putting aside their internal ranking and their variation in toxin resistance or deep-diving talents or cricket skills.

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