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Author Topic: Out of post-LGM Iberia...again  (Read 592 times)
Mkk
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« on: August 27, 2012, 03:13:07 PM »

Abstract:

Cavalli-Sforza and colleagues (1963) initiated the representation of genetic relationships among human populations with principal component analysis (PCA).Their study revealed the presence of a southeast–northwest (SE-NW) gradient of genetic variation in current European populations, which was interpreted as the result of the demic diffusion of early Neolithic farmers during their expansion from the Near East. However, this interpretation has been questioned, as PCA gradients can occur even when there is no expansion, and because the first PC axis is often orthogonal to the expansion axis. Here, we revisit PCA patterns obtained under realistic scenarios of the settlement of Europe, focusing on the effects of various levels of admixture between Paleolithic and Neolithic populations, and of range contractions during the Last Glacial Maximum (LGM). Using extensive simulations, we find that the first PC (PC1) gradients are orthogonal to the expansion axis, but only when the expansion is recent (Neolithic). More ancient (Paleolithic) expansions alter the orientation of the PC1 gradient due to a spatial homogenization of genetic diversity over time, and to the exact location of LGM refugia from which re-expansions proceeded. Overall we find that PC1 gradients consistently follow a SE-NW orientation if there is a large Paleolithic contribution to the current European gene pool, and if the main refuge area during the last ice age was in the Iberian Peninsula. Our study suggests that a SE-NW PC1 gradient is compatible with little genetic impact of Neolithic populations on the current European gene pool, and that range contractions have affected observed genetic patterns.

http://mbe.oxfordjournals.org/content/early/2012/08/25/molbev.mss203.abstract

Dienekes: http://dienekes.blogspot.co.uk/2012/08/out-of-iberia-arenas-et-al-2012.html



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avalon
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« Reply #1 on: August 30, 2012, 04:10:23 AM »

What are we to make of this? I won't pretend to completely understand the abstract here, like what is meant by "range contractions" but I guess that in the fast-paced and complicated world of population genetics nothing is completely settled.


So... 5 to 10 years ago the consensus (or was it just Oppenheimer?) seemed to be that modern Europeans were largely descended from Stone Age Hunters. Now, the consensus is that R1b is much younger and that its large presence in Western Europe might be attributable to Bronze Age migrants.

I must admit, to a person like me, with a general interest in Archaeology and Genetics, it's probably best to keep an open mind. DNA is obviously extremely complex, as are population movements in the distant past, and even the experts probably only understand a fraction of the whole story.

I wonder what the consensus will be in 10 years?
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Mkk
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« Reply #2 on: August 30, 2012, 04:51:51 AM »

Quote
So... 5 to 10 years ago the consensus (or was it just Oppenheimer?)
From looking at old papers, seemed to be that modern Europeans were largely descended from Stone Age Hunters.
Yes, this is what most papers from this time proposed.

Basically, it was thought that there were three or four LGM refugees:

-R1b in Pyrenees region.

-R1a in Ukraine

-I in Balkans

-Some papers also included N somewhere in Siberia)

The other lineages were assumed to have been brought largely by Middle Eastern immigrants during the Neolithic...

Thus you have your 80 percent Paleolithic. But now we have reasons to suspect only I is truly Paleolithic, so the true balance would be the other way round, aproximately 20 percent Paleolithic and 80 percent Neolithic or after.

That's just on the Y-DNA side. But there are simmiliar indications from mtDNA. The U lineage dominates in Paleolithic Europeans, and is the only one (per Jean M) that has been identified with certainty in ancient DNA prior to agriculture.

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Jean M
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« Reply #3 on: August 30, 2012, 05:14:44 AM »

The U lineage dominates in Paleolithic Europeans, and is the only one (per Jean M) that has been identified with certainty in ancient DNA prior to agriculture.

My list of Ancient Western Eurasian DNA shows some other possibilities, but mainly from studies that were not taking modern precautions against contamination, and/or which did not test enough of the mtDNA genome to allocate a haplogroup with certainty (or did not report the results of same anyway.)
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Mike Walsh
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« Reply #4 on: August 30, 2012, 10:29:55 PM »

Quote
So... 5 to 10 years ago the consensus (or was it just Oppenheimer?)
From looking at old papers, seemed to be that modern Europeans were largely descended from Stone Age Hunters.
Yes, this is what most papers from this time proposed.

Basically, it was thought that there were three or four LGM refugees:

-R1b in Pyrenees region.

-R1a in Ukraine

-I in Balkans

-Some papers also included N somewhere in Siberia)

Mkk, is this paper talking about Y DNA or mt DNA or autosomal DNA?  

What are you citing that says R1b spent the LGM in the Franco-Cantabrian refugium?  Do you or they really mean M343 (R1b)? On the Y DNA side, most of Western Europe is descended from downstream SNPs under L11/S127 (R1b1a2a1a1).
« Last Edit: August 30, 2012, 10:31:31 PM by Mikewww » Logged

R1b-L21>L513(DF1)>S6365>L705.2(&CTS11744,CTS6621)
Mkk
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« Reply #5 on: August 31, 2012, 03:17:14 AM »

Quote
So... 5 to 10 years ago the consensus (or was it just Oppenheimer?)
From looking at old papers, seemed to be that modern Europeans were largely descended from Stone Age Hunters.
Yes, this is what most papers from this time proposed.

Basically, it was thought that there were three or four LGM refugees:

-R1b in Pyrenees region.

-R1a in Ukraine

-I in Balkans

-Some papers also included N somewhere in Siberia)

Mkk, is this paper talking about Y DNA or mt DNA or autosomal DNA?  

What are you citing that says R1b spent the LGM in the Franco-Cantabrian refugium?  Do you or they really mean M343 (R1b)? On the Y DNA side, most of Western Europe is descended from downstream SNPs under L11/S127 (R1b1a2a1a1).
The paper is about principal components analysis . in 1997, Cavalli-Sforza found that south-east-north-west cline explained 28 percent of the variation in Europe, the spread looking very simmiliar to the expansion of agriculture:

http://racialreality.110mb.com/genetic_variation_files/pc1.jpg

This paper says that this cline can easily be explained by most Europeans being descended from Paleolithic hunter-gatherers if their main refuge was Iberia.

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