David Reich

It's because of the steppe migration from the steppe north of the Black and Caspian Sea.

40%, 50%, 80% of the DNA becomes Yamnaya from steppe pastoralists.

And their frequencies of mutations were different, not because of selection necessarily, but just because they had evolved in different places for thousands and tens of thousands of years.

And then if you look at the descendant populations, there's huge changes in frequency.

Yeah.

And it's very, what you need to do is see, oh, is natural selection explaining a shift more than you would expect by chance?

7,200, where we're 50% confident.

Oh, sorry.

So I think we're getting about 7,200 positions in the DNA that have 50% confidence of being real.

Yeah.

So only half of those are real.

We don't know which ones, so 3,600 of them are real.

If you look at the 25 percent probability cutoff, there will be tens of thousands, and there will be many real ones there too.

In fact, multiple analyses we do suggest that the genome is vibrating with natural selection,

And there's all sorts of weaker effects that are there that would be picked up in larger studies even than we've done.

And that, in fact, almost every position in the DNA is correlated to a position and being dragged in one way or the other by natural selection.

Instead of being quiescent, natural selection is everywhere.

Even though it's only 2% of the frequency change, it's tugging the positions in one direction or the other everywhere.

So we analyzed these positions that we had identified, these hundreds of positions, the ones we were super confident about.

And we looked to see whether they were randomly distributed in the DNA or whether they had patterns.