David Reich

So Europeans and East Asians descend from a common ancestral population 40 or 50,000 years ago that came out of Africa and the Middle East.

This population had a set of gene frequencies, genetic frequencies, and these variants bopped around randomly, a process known as genetic drift, or perhaps under selection in one direction or another.

And the time that's passed since 40 or 50,000 years ago is sufficiently small on an evolutionary timescale that there's just not much genetic differentiation on average between these two groups, Europeans and East Asians.

But however, if there has been natural selection, for example, to help people in one place digest

digest alcohol better, or, for example, digest milk better, or do something else better, what you might expect is that there would be some mutation that would have rocketed up to very high frequency.

And 40 or 50,000 years is a lot of time.

It's maybe 1,500 or 2,000 generations.

And so that might be enough time, easily, to see 100% different in frequency.

And yet you don't see any more compared to what you'd expect by chance.

So this made it seem that just selection has been quiescent.

Maybe a few hundred thousand years ago, the ancestral human population got to some kind of optimum.

And after that, there hasn't been much genetic change in one way or the other.

And there's been small amounts of natural selection or there's been selection to remove bad mutations that are constantly raining down on the genome, but not what we call directional selection, which is newly arising mutations or mutations being pushed in a systematic direction to help the population.

get to a different adaptive set point that's more favorable for the conditions that population is living in.

So we were able to partition how much of the changes in frequencies of all the mutations that we're seeing in the DNA, we're looking at about 10 million positions that vary, is due to directional selection, adaptation, versus other factors, especially genetic drift.

And 98% of it is other factors, especially genetic drift.

So it's overwhelmingly migrations in population structure causing fluctuations in frequency.

And as a result, it's super hard to actually detect the signals of natural selection, in adaptive natural selection, because they're a tiny fraction of the total frequency change.

The vast majority of it are these migrations and mixtures.

Nevertheless, there's so much natural selection, as our study has shown, that in fact it's been rampant in the genome.