The Y-chromosome haplogroup R1b is extremely common in Western Europe ( > 70%). At the same time, it doesn’t appear to be very old.
Which facts suggest two possibilities. The first is that this particular Y-chromosome haplogroup confers some kind of fitness advantage. It’s moderately improbable, since the Y chromosome doesn’t contain many genes, but it is possible. It’s more likely considering that ways of life have changed a lot in the Holocene, so an old version of the Y might not be very close to optimal, especially in any behavioral effects.
The other possibility is that there was a male lineage that prospered enormously in a way similar to the Golden Family, the direct male descendants of Genghis Khan. We know that reputation has lasted for about 800 years: as late as the 20th century, a huge fraction of the nobles in Mongolia were direct male-line descendants of the Master of Thrones and Crowns. I could see this happening in the big population turnover about 5000 years ago: a leader of a wildly successful invasion might build an enduring reputation, such that even a long time later, people would look to his descendants for leadership. I get the impression than in Mongolia, you’d automatically expect one of these guys to organize your sock hop – nobody else would be worthy.
This could extend even to populations that were not really conquered or replaced by the incoming troublemakers, like the Basque. They too might have been susceptible to R1b glamor: people have certainly borrowed dynasties from other countries.
How to find evidence? Well, it would be difficult, but you might be able to show that graves full of expensive goodies were more likely to hold dead guys with R1b than poor graves, over a large area and for a long time. If there was a fitness advantage, you might be able to demonstrate some kind of difference in men with R1b, a difference that was plausibly beneficial in the past. And no, I don’t mean sperm competition.