Originally Posted by
Orion Arm
No disagreement on that there have always been contact zones where R1b and R1a mixed.
Even today there are in-between zones — Central Europe, Scandinavia — where both exist in significant quantities.
But your premise that R1b and R1a were fully interbred since EHG times is incorrect, and I will explain why: Certain CWC groups (Battle Axe, Fatyanovo, Sintashta/Andronovo) are all R1a, no R1b. Bell Beakers groups are all R1b, maybe exceptionally rare R1a exist, but they're just that — exceptions. And even if you look at modern situation, after several thousand years of intermixing between Europeans, you will still find lopsided ratios among modern descendants of BB and Eastern CWC. The ratio of R1b-to-R1a in modern Western Europe (British Isles, France, Iberia, Low Countries, West Germany, Switzerland, West Austria, Italy) is completely lopsided, just as the ratio of R1a-to-R1b in modern Eastern Europe (Belarus, Lithuania, Ukraine, Russia, to a lesser extent Poland, Estonia, Latvia) is also completely lopsided. In these countries the ratio between these two haplogroups varies from lopsided 5:1 to absurd 100:1, 200:1. This suggests two disparate waves, not one wave of R1a-R1b mixed soup.