David Reich: evaluation

Transcript context

…Can I ask a clarifying question here? Why are we discounting population admixture or replacement as selection? If you think about it at a group level, if one population replaces another population, isn’t that selection? I remember from the last episode you were explaining how there have been huge changes in what kinds of people are in a specific area. One population came in and replaced the previous one, and then a new population came in and replaced that one. To the extent that the genetics are relevant to why that population replaced the other one, why should that not count towards what we understand to be selection over the last 10,000 years? 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 and not others. And maybe there are some genetic mutations that are contributing to this. Who knows? It’s possible. But what you’re seeing is a whole-genome shift. 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. From a statistical point of view, what happens at these times of migration is there are just huge fluctuations in frequencies. These are extremely uninformative times for detecting natural selection. The best moments to detect natural selection are when migrations and population admixtures are not happening for a few hundred years. During these times, you can actually see the mutation slowly blowing in one direction as a result. The way we think about the history of Europe and the Middle East for the purpose of this study is as an archipelago of little populations in space and time, each pretty isolated from each other. You have a little population in Britain isolated for a few hundred years, or a little population in Hungary isolated for a few hundred years, between big events of migration and mixture. 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? If all the arrows point in the same direction, we win. They’re telling us that natural selection is occurring. For example, 4,500 years ago in Europe, almost all mutations went through huge frequency changes. That’s not because of natural selection. It’s because of the steppe migration from north of the Black and Caspian Sea. 40-80% of the DNA becomes Yamnaya from steppe pastoralists. 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. When you look at the descendant populations, there are huge changes in frequency. What you need to do is see if natural selection is explaining a shift more than you would expect by chance. So you found these locations that seem to be under selection. I have another clarifying question. You say you found 3,800 locations which you’re 50% confident have been under selection in the last 10,000 years.…

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