David Reich

It could count and may count and probably should count in some respects.

But it could also be that this population replacement is due to some cultural phenomenon, technology held by one of these groups, not others.

And maybe there's some genetic mutations that are contributing to this.

Who knows?

It's possible.

But what you're seeing is a whole genome shift.

And so what we're looking to see is whether there's one place in the DNA that is driving the change in a way that's different from the rest of the genome.

And really from a statistical point of view, what happens at these times of migration is there's just huge fluctuations in frequencies.

And these are extremely uninformative times for looking and detecting natural selection.

The best moments to detect natural selection is when migrations and population admixtures are not happening for a few hundred years.

And during these times, you can actually see the mutation slowly blowing in one direction as a result.

Really, the way we think about the history of Europe and the Middle East, and the way we think about it for the purpose of this study, is as an archipelago of little populations in space and time, each of which are pretty isolated from each other.

A little population in Britain, isolated for a few hundred years.

A little population in Hungary, isolated for a few hundred years, between big events of migration and mixture.

And in each of those little experiments of nature, we can ask, does this mutation slightly increase in frequency?

Does that same mutation slightly increase in frequency?

And if all the arrows point in the same direction, we win.

And they're telling us that natural selection is occurring.

So for example, 4,500 years ago in Europe, almost all mutations go through huge frequency changes.

And that's not because of natural selection.