A new article in Current Biology by George Busby et al. purports to show a lot of comparatively recent genetic mixing among the populations of Europe and West Asia. (This is a completely different phenomenon from the great ancient invasions I have written about before.) Some of the data they present makes sense; for example, you can see a minor flow of genes from northwestern Europe (Franks) into the Basque population around the year 1000, and a significant flow of genes from central Asia into Turkey between 1000 and 1500. They identify the entry of Slavs into southeastern Europe via two clusters of genes, one similar to modern Lithuanians and the other to modern Germans, that entered between 700 and 1200. Everybody invaded the west Caucasus.
But what is one to make of the supposed flow of "southern Italian-like" genes into Britain between 500 and 1000 AD? Or of genes from "Armenian-like sources" into Orkney? Or Mordovian (relatives of the Hungarians) genes into Norway? Of "Cypriot-like" genes into northern Italy?
I think there is something wrong with their statistics, but I don't really understand their methods well enough to say what. One issue is that they rely on randomly sampled modern populations to generate their base lines, rather than trying to identify people who have resided in a particular area for generations. Another is that particular historical events may confuse the notion of where genes "originate." Surely the "southern Italian-like" genes they identify in Britain and Orkney are more likely to be Viking genes carried to southern Italy by Normans in the 11th century and later; I wouldn't have thought that the Norman conquest of southern Italy and Sicily involved enough people to leave a clear genetic signature, but who knows? I can't think of any other explanation.
What I really believe is that there has been so much mixing of populations across west Asia over the past 2000 years that it will be all but impossible to tease out the actual percentages of genes that Group A contributed to Region B at any particular time. But the data do show lots of mixing; there is no population in Europe or West Asia, not even the Basques or Norwegians, that seems to have remained "pure" since classical times.