Math. From page 190 of the book:
Recall that the world population has recently spiked. Europe was no exception to this rule. Since a.d. 1400, the number of Europeans has jumped 12-fold. In a.d. 1400, it was 60 million; now it’s 740 million. Or, to turn the equation around, the 740 million Europeans alive today arose from just 60 million ancestors. The branches on the European family tree reflect this math. 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.
In Color Plate 222, this is why existing haplogroups “explain” so much of each population’s males. The vast majority of the branches on the European — and global — family tree have arisen within the last 600 years. The most recent branches alone tell over 90% of their sto-
ries. Less than 10% is all that’s available to tell the story of the earlier millennia of human history.
@John_Harshman said it best:
You understand that Jeanson’s force-fitting of the Y-chromosome tree to his model means that many human populations diverged from each other after 1500, right?
I was going to bring this up separately in my review, but it would be interesting to discuss now, since it’s somewhat related. These are the sentences I thought explained well why the tree has any relevance to population sizes.
From page 218:
Living people are the survivors of the rises and falls in human population sizes over the millennia. Their DNA-based family tree reflects the minimum human population size over the years. The branches from those people who died out or left no descendants won’t be reflected in the Y chromosome DNA of living people. [Emphases his]
From page 96:
Recall that family trees record changes in population size. When populations grow, the number of branches on the tree grows. When populations decline, the number of branches on the family tree declines. This same principle applies to the Y chromosome tree but with a slight modification. The Y chromosome tree that we’re using is based on DNA from the survivors of history. Trees of survivors don’t have branches that lead to peoples who went extinct. They also don’t reveal every instance of population growth. Periods of growth that were wiped out by declines are invisible in these trees. In other words, these trees record effective population sizes. If their ancestors underwent population growth, this shows up in their family trees only if the growth was not cancelled by later population declines. [Emphases his]
This uses ancient DNA, which they date using radiometric dating. My understanding of it (after reading Reich’s book) is they look for genetic fragments that are similar to show admixture has occured. The TLDR version I’d give is that ancient Central Asian people migrated into Europe and all mixed with each other but they were not related to ancient Greeks. I just saw a mention of R1 (along with I and C, IIRC) not R1b.