Wednesday, December 23, 2015

The Spread of Haplogroups in Europe, Especially R1b

This post is intended to be a general foray into what I call "The Two -Ics" that explain modern haplogroup distributions: demographics and mathematics.  IMO, both are poorly understood.

It's been said, "to be an R1b Fantasist, you have to believe that I2-M26 came to predominate Sardinia by chance (e.g., Founder Effect and Drift) -- but that R1b came to predominate other locales (e.g., Ireland or Spain) by merit (e.g., military superiority or sexual selection)." 

It's also been said, "to be an R1b Fantasist, you have to now believe that R1b marks the spread of the first pastoralists, equestrians, and herders, and that you're now 100% correct that is right -- when just 2-3 years ago, you were 100% that Hg G2 was the mark of the first pastoralists and herders."

With respect to the first saying, I believe that most of the R1b apologists understand the former concepts (of chance as they apply to archaeogenetics), so this post is designed to build upon that knowledge, and add some demographics and mathematics too.

With respect to the second saying, I believe what is most key in a discipline like archaeogenetics is to recognize that theories and findings change from year to year, but the underpinnings of solid scientific method do not.

Let's get into it:

First, it is crucial to outline the possible outcomes.  Every generation, every clade and subclade of every Haplogroup has three "options" (or three outcomes).  Those are:

1.  Mutate (i.e., become something else)
2.  Propagate -- and, in more or less the same form, by having a male child who survives
3.  Die out, by having only daughters, or by having male children who fail to themselves breed

The "stakes" were more pronounced during prehistory than today, because the population sizes were so profoundly lower.  If you don't grasp this and accept it as fact, you can't grasp what I will detail later.

Population of Europe Over Selected Times  
(YBP = Years Before Present)

~50,000 YBP: No more than 10,000 (Neandertals)

~38,000 YBP - 19,000 YBP: No more than 37,000, likely population just 5,000

~12,000 YBP: About 28,000

~2000 YBP: About 35,000,000

-0 YBP: About 743,000,000
You can read more here.

In essence, you must remember that the population of Europe at the beginning of the time we are discussing (the post-glacial-maximum recolonization through the Bronze Age) was about 28,000 and peaked at maybe 100,000.  This is hard for the modern mind to comprehend, I know.  There were less people from Spain to Ukraine then, than there are in one city block in London now.

There are two takeaways:
1.  This made the population more susceptible to chance events, like a plague outbreak, or a famine in an area.

2.  This made the population more susceptible to massive dilution, when population started on its massive upward trajectory, after people started drinking milk, wine, and beer, when they started making cheese, when they started farming cereals and living in one spot, and when they started herding animals and having meat at will.

Going back to our three outcomes for Y Haplogroups, every generation: the first "takeaway" above should inform several likely mechanisms of how R1b spread over time.  If they entered a territory and had different disease resistance, it could have meant that large numbers of a tiny starting population would die off. 

Similarly, because the initial population was so small, when larger populations migrated for whatever reason, indeed possibly even as refugees from other regions, the other haplogroups would seem to have shrunk in size, whereas it really is different population sizes.

All this is just build up.  Our main focus, however, is the simple application of mathematics to Outcome 3 above.

This is what you need to know before we start:

1.  Hunter/gatherer women space babies on average 4.5 years apart, whereas farmers and moderns space them 1.5 years apart.

2.  The average paleolithic woman would have about 3.8 children.

3.  Infant mortality among hunter/gatherers is 30 times higher than among "civilized", and reached approximately 25% at many points during history.

4.  If the average hunter/gatherer family consisted of 3 children to live to adulthood, the odds of each family having just female kids survive was 12.5% each generation.  (.5 x .5 x .5)

Now just these numbers by themselves (HGs having fewer kids than farmers or pastoralists) explain a LOT. 

But the main point is thus: "older" non-mutated Y-chromosome haplogroups are found in lesser numbers simply because they are...older...

Every generation that a Hg exists and doesn't change, there is a 12.5% chance that those bearing it, in any one family, will not pass it along.  To be very clear: if a Hg does not mutate into something else -- or does not die entirely -- its numbers and distribution will decrease over time.  This applies to all except the most recent arrival, which is currently breeding like rabbits.  For example:

Many people believe that C1a was the first Y Hg in Europe.  There were probably just 5000-15,000 of them at any time.  By definition, the Hg C1a are folks that did not go on to mutate into any of the downstream clades.  Over time, the odds will catch up.

Many people believe that I2 was the next Y Hg in Europe.  There were probably just 10,000 - 50,000 of them at any time.  By definition, these are members of the IJ branch, and not members of F or K who mutated.  Over time, the odds will catch up.

These very simple concepts explain much of the modern distribution of haplogroups in Europe.  Is it more complex?  Sure.  Were there other factors?  Absolutely.  But over time, you cannot escape mathematics and demography being the biggest factors.

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