Then this really doesn’t have anything to do with genetics.
It “shows” no such thing. In fact that it is unclear that it even claims this. The original quote was:
Today, 740 million branches exist. In a.d. 1400, only 60 million existed. To reduce 740 million to 60 million, you have to connect a whole bunch of branches. In terms of percentages, 60 million is just 8% of 740 million. Consequently, by a.d. 1400, you have to connect 92% of today’s branches. Prior to a.d. 1400 is when the remaining 8% of the branches connect.
You will see that there is no mention that “92% of people in Europe have to share a common ancestor 600 years in the past”, just because the population is now 1/(1-0.92) times what it was. This in fact would have been a very odd claim. You can have population growth in isolated populations without any intermixing, you can also have intermixing without population growth. There is no reason to expect there to be a hard-wired relationship between the two.
I’m not sure what Jeanson is attempting to claim, but I’m fairly sure it is not this.
Addendum:
I would also note that, quite apart from any of the above issues, nothing in the quoted passages even mentions R1b, let alone how R1b was meant to have teleported past Eastern and Central Europe (which have lower R1b rates) into Western Europe, in the last two millennia.
Why would we want to do that?
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I was able to get piecemeal access to Traced through Google Books. As far as I can ascertain Jeanson does not cite any sources for his “facts”.
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As far as I can see from his bio, and his very short list of peer-reviewed co-authored publications before he absconded to AiG, he appears to have no substantive training or background in genetics.
This means that I’ve (i) no reason to believe that Jeanson knows what he’s talking about, and (ii) no ability to trace back what he’s saying to what the research he’s supposedly basing it on actually says.