Friday, March 1, 2013

Lack of significant Northern geneflow into the Italian and Iberian peninsulas over the last 1500 years.

Everyone who is a student of history knows that after the fall of the Roman empire, there was a "migration period" where Germanic groups migrated all over Europe, including the Italian and Iberian peninsulas, setting up and reigning over Kingdoms there. However, it appears that after the fall of those Kingdoms they were quickly absorbed into the local population and have had a minimal genetic impact (probably because they were very outnumbered as a whole). A recent genome wide IBD (Identity-By-Descent) study shows that Italian [which includes Northern, Central, and Southern Italians] and Iberians have very minimal IBD sharing with Northern Europeans over the last 1,500 years. 


The geography of recent genetic ancestry across Europe

Peter Ralph, Graham Coop
(Submitted on 16 Jul 2012)

"The recent genealogical history of human populations is a complex mosaic formed by individual migration, large-scale population movements, and other demographic events. Population genomics datasets can provide a window into this recent history, as rare traces of recent shared genetic ancestry are detectable due to long segments of shared genomic material. We make use of genomic data for 2,257 Europeans (the POPRES dataset) to conduct one of the first surveys of recent genealogical ancestry over the past three thousand years at a continental scale. We detected 1.9 million shared genomic segments, and used the lengths of these to infer the distribution of shared ancestors across time and geography. We find that a pair of modern Europeans living in neighboring populations share around 10-50 genetic common ancestors from the last 1500 years, and upwards of 500 genetic ancestors from the previous 1000 years. These numbers drop off exponentially with geographic distance, but since genetic ancestry is rare, individuals from opposite ends of Europe are still expected to share millions of common genealogical ancestors over the last 1000 years. There is substantial regional variation in the number of shared genetic ancestors: especially high numbers of common ancestors between many eastern populations likely date to the Slavic and/or Hunnic expansions, while much lower levels of common ancestry in the Italian and Iberian peninsulas may indicate weaker demographic effects of Germanic expansions into these areas and/or more stably structured populations. Recent shared ancestry in modern Europeans is ubiquitous, and clearly shows the impact of both small-scale migration and large historical events. Population genomic datasets have considerable power to uncover recent demographic history, and will allow a much fuller picture of the close genealogical kinship of individuals across the world." 

Notice in the figure below, appreciable IBD sharing between Italy and Iberia with Northern Europe is only from the 1515-2535 years ago period. It's interesting that Italy does share some noticeable IBD sharing with the Balkans from the 555-1500 years ago period. Perhaps genetic exchange happen between the two groups during population movements in the Byzantine era.


SOURCEPeter Ralph, Graham Coop, The geography of recent genetic ancestry across Europe Populations and Evolution, arXiv:1207.3815v2 [q-bio.PE]

LINK

4 comments:

  1. This is not correct, because almost two thirds of the Italians in the POPRES dataset are from South (see Moorjani et al.), while in reality Southerners make just 20% of Italian population.

    Actually Northerners share a lot more IBD blocks with both the French and the Brits.

    http://i.imgur.com/PmiqF8C.png

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    1. Yes it is correct. This post is talking about *recent* gene flow. The IBD blocks shared in your link (Figure 2) are not within the last 1500 years, but much older.

      "In addition to the very few genetic common ancestors that Italians share both with each other and with other Europeans, we have seen significant modern substructure within Italy (i.e., Figure 2) that predates most of this common ancestry, and estimate that most of the common ancestry shared between Italy and other populations is older than about 2,300 years (Figure S16)."

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    2. Indeed, I certainly never denied northern geneflow into Italy and Spain and much of Southern Europe in the Bronze or Iron ages. The 20-30% Yamnaya component for instance found in Southern Europe didn't canoe it's way in from North Africa. I'm mostly debunking claims that Italians or Iberians have all this 'Germanic' admixture because of the great migration period. These were migrations of 50 to 100 thousand people heading into peninsulas filled with at least several million. Absorbing these groups isn't going to make a significant impact.

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    3. BTW, Moorjani et al. 2011 did use the PORPES dataset and used 90 Northern Italian samples and 121 Southern Italian ones. 90/211 = 42.6%.

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