Gene Flow

Someone was wrong on the Internet, saying that there had always  gene flow between human groups, which supposedly inhibits people from becoming different.

If you assume that everything is neutral, it doesn’t take a lot of gene flow to keep populations genetically similar  – something like 1 migrant per generation, regardless of population size.  There’s a relationship between average gene flow and Fst:

Fst between CEU ( Mormons) and YRI ( Yoruba, in Nigeria) is 0.165. ( Same as that between timber wolves and coyotes). Going from Fst to number of migrants per generation  :

, where Nm is the number of migrants.

So, the average number of migrants per generation between sub-Saharan Africa and Europe, over the past hundred thousand years or so, was 1.26.  Not a hell of a lot of gene flow.  I blame the Sahara, distance, human cussedness,  and the Ice Age.

Was this enough to keep natural selection from pushing the two populations in different directions?  Not at all. I worked this out in my Ashkenazi paper: for  gene flow to substantially block adaptive divergence of a polygenic trait,  gene flow has to average on the order of  2% or more per generation.  Not two individuals: 2 percent of the population.

Since the actual number for all of sub-Saharan Africa was something like 1 migrant per generation, we don’t have to worry about European admixture damping out local adaptation in Africa, or vice versa.

Now there may well have been significant gene flow between one tribe and a neighboring one in some parts of SSA, which kept them from  becoming too different from each other: but there was nothing keeping them from becoming different from people in the Middle East, or Europe, or East Asia.  In fact, we know some populations in SSA were sufficiently genetically isolated from each other to develop lots of Fst  (Bushmen vs Bantu),  and end up quite different from each other.  Obviously different.

Generally speaking, people seem not to have mixed much in Ice Age times. Fst between the Levant and the Zagros mountains, the west and east ends of the Fertile Crescent, was like that between Germany and China today.

As for the idea that local drift can swamp adaptive evolution — see this.

Lande (1980) argues that, “because of the high mutability of polygenic characters, even small isolated populations of several tens or hundreds…can generate sufficient variation for rapid evolution into a new adaptive zone.”

I knew this, not just by being the sort of nut that reads the literature, but because I knew of many actual examples of rapid adaptive evolution in small populations.

A tip to biologists everywhere: if it happens a lot, it’s probably not theoretically impossible.

{parenthetically, biologists have been known  to say that adaptation ( any adaptation, not some specific adaptation)  is unlikely, because the null in neutral theory is that there isn’t any.  Think about this. }

Eskimos show special adaptations to a high-fat diet, and a body form adapted to cold , consistent with Bergmann’s rule and Allen’s rule. I don’t think there were ever many Eskimos. Tibetans developed their altitude adaptations some time ago, before agriculture: how many hunter-gatherers could there have been in Tibet?

Key deer: need I say more?  An insular populations of possums on an island off Virginia ages more slowly ( lives three years instead of two)  – a consequence of  low predation  risk [ evolutionary theory of senescence].

Drop anybody into a real tropical jungle, wait a few tens of thousands of years (maybe less), and you get Pygmies or Negritos.

Darwin’s Finches on the Galapagos: specialized beaks for different food sources. Specializations that change over a few years of drought [ ENSO cycles].

Every example of island dwarfism or gigantism: tiny Maltese elephant, giant Majorcan dormouse.

Why would someone, not just Dan Graur, say that you need a big population in order for adaptive evolution to occur?  it can help ( generates more favorable mutations) but standing variation is sufficient to allow a lot of selection.