Identity by descent & the Völkerwanderung

By Razib Khan | July 17, 2012 9:07 pm

Early this year I received an email from Dr. Peter Ralph, inquiring if I might discuss some interesting statistical genetic results from analyses of the POPRES data set which might have historical relevance. I’ve been excitingly waiting for the preprint to be made public so it could trigger some wider discussion. I believe that the methods outlined in the paper perhaps show us a path into the near future, where we might gain a much sharper perspective upon the recent past. So it’s finally out, and you can read it in full. Ralph and Dr. Graham Coop have posted put it up at arXiv, The geography of recent genetic ancestry across Europe. The paper uses ~500,000 SNPs from the POPRES data set individuals, and looks at patterns of identity by descent as a function of geography. By identity by descent, we’re talking about segments of the genome which are derived from a common ancestor. Because of recombination the length of the segments can give us a sense of the date of the last common ancestor; long segments indicate more recent ancestry because fewer recombination events have chopped up sequence.

Here’s the big takeaway of the paper: …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….


When I first saw the panels above, which illustrate the proportion of IBD sharing between populations, with the starred population being the focal one, I immediately thought of the early medieval Slavic expansion. There is already evidence of this in the genetic data. Dienekes noted that modern Greeks seem to have a significant component of “northern” ancestry, which is attenuated in Turks, and nearly absent in Greek Cypriots. These results suggest that the peoples of eastern Europe share a very large number of common ancestors within the past 1,500 years, irrespective of geographic distance (note that it is difficult to observe a decay in the size of the circles, which is more evident in other panels).

The other surprising pattern, at last to me, is the deep structure of the Italian population. These results imply that Italian relatedness has a notably deeper time depth than that of other European nation-states. I’ll quote the authors here: This suggests signi cant substructure and large population sizes within Italy, strong enough that di erent groups within Italy, share as little recent common ancestry as other distinct, modern-day countries, substructure that was not homogenized during the migration period. These patterns could also reflect in part a history of settlement of Italy from various sources, including: settlement of Greeks in southern Italy, settlement of Illyrians in eastern Italy, and an influx of people from across the Roman empire, including gene flow from Africa…but is unlikely to be entirely due to these eff ect. Spain seems to exhibit the same distinctness from the rest of Europe as Italy, but has a much more normal pattern of IBD, with shallower time depth to common ancestry.

There are plenty of other possible inferences one could make. For example, is the negative correlation between IBD tracts in individuals of UK origin affiliated with Germany and Ireland a function of a difference in Celtic and Germanic ancestry dating to the Dark Ages, or is it simply due to the fact that the United Kingdom has had a recent wave of Irish ancestry in the 19th century, or perhaps just a natural result of a geographic continuum and isolation by distance? The last is an issue which will need addressing in the future. The authors make the case that because of the power of the IBD method one can make inferences without a finer geographic granularity, but what is sufficient for statistical genetics is not sufficient for historical-demographic inference. The POPRES data set was collected in London and Lausanne, and there are limits to how much geographic information you can squeeze out of this. I assume in the near future these sorts of methods which infer IBD tracts will be applied extensively, so this is just here to whet out appetites.

This paper has a wealth of results. You can create many stories. But to create credible stories you need “thick” and “deep” knowledge. So I invite readers to dig through the results and see what jumps out at them. It’s no cost to you, and I don’t think the time spent pursuing this material is going to be time wasted.

CATEGORIZED UNDER: Demographics, History
  • http://www.astraean.com/borderwars/ Christopher@BorderWars

    From visits to both Spain and Italy, I was surprised at the deep degree of regionalism in both countries. Of course some of my surprise comes from a simplistic view that these countries are monolithic cultures and the common misconception that they are even old political entities.

    But as much as one would find it genetically difficult, if not impossible, to identify “American” based upon a genetic cluster, and yet there is overwhelming cultural allegiance to “being American” in this country, I was struck by how much the sentiment in Spain and Italy was the opposite.

    “I am not Spanish, I am Basque,” or Catalan were common refrains and this was emphasized in the local signage. In Italy there is still a mournful appreciation to the damage done by fascist Italianization and the subsequent marginalization of dialects and languages that corresponded to historical demes.

  • Raimo Kangasniemi

    Anyone who isn’t an über-patriotic Greek and has knowledge of the history of Greece in the late antiquity and early Middle Ages could have guessed the heavy Slavic imprint in the current population; the Slavs overran much of Greece in in the late 6th and early 7th centuries and it took the Eastern Roman Empire until late 9th century to fully regain control. The number of Slavic place names alone testify to this era’s strong legacy to Greece.

    When it comes to population structures in Italy, look at the geography. One might think that Italy as a peninsula would be united as a result of it’s geography, but the mountains of Italy have been effective barriers inside it. The sea has been able to unite some of the coasts, but it hasn’t often reached deep inland and through foot, mule or horse, it was a very difficult journey from the eastern to the western coast of Italy. Long standing political borders played their role too.

    The division of Italy was well shown in the richness of it’s local pre-modern dialects and languages. In mid-17th century Friuli a local peasant couldn’t understand what a clergyman from Umbria spoke without the help of a translator.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    When it comes to population structures in Italy, look at the geography.

    yes, but this parameter is similar in iberia and the balkans. and yet these patterns aren’t replicated there. why? you probably answered it later: . Long standing political borders played their role too. the barbarian penetration of italy seems to have been demographically less significant that the full on sweep you allude to in mainland greece after the 6th century and before the 10th century byzantine revival.

  • pconroy

    In terms of Ireland, UK and Germany – it’s a pity that they did NOT collect location information on grandparent for those samples collected in London.

    Also, Northern Ireland is part of the UK, so they are building confounding factors into their data – which is silly.

    My take would be that there could have beeen a massive undocumented arrival of Britons to Ireland, resulting from the displacement of Britons around 0 AD or so by the Romans Empire. Then you have large numbers of English moving into Leinster (East) and to a lesser extent Ulster (North) and Munster (South), during the period of colonization. Then you have massive immigration of Irish during the famine years and during the industrial revolution to the UK, and continuing strong emigration till the 1980’s or so. It’s been estimated that 25% of the English population is of recent Irish ancestry.

    So for this piece of the paper, they would have been much better off to compare “England”, NOT the UK, to Ireland and Germany.

  • Wim Van Dijk

    IIRC, it had been found earlier that the Europeans that were genetically the furthest from the “European average”/norm were Finns and south Italians (the latter being much closer to west asians than the other Europeans IIRC).

  • Paul Ó Duḃṫaiġ

    Regarding Irish migration to Britain, it’s been recored since at least the 16th century. Obviously it picked up during the industrial revolution before reaching all time high in period after 1840. Here is some stats on number of irish born living in Britain in period 1841 to 1921

    1841 415,725
    1851 727,326
    1861 805,717
    1871 774,310
    1881 781,119
    1891 653,122
    1901 631,629
    1911 550,040
    1921 523,767

    I don’t have figures for 1931-1950, but here are those from 1951 to 2001, obviously the UK census for 2011 hasn’t released the relevant figures let. Given the economic situation there has been a noticable increase in migration to Britain. During the dark days of the 1980’s over 330,000 went to Britain in period 1982-1993.

    1951: 716,028
    1961: 948,320
    1971: 957,830
    1981: 850,397
    1991: 837,464
    2001: 869,093

    There has been constant Irish migration to Britain which is born up by the fact that the Irish-born population has been relatively stable over the last 170 years. At the moment it’s reckoned that about 6 million people in Britain have at least one Irish grandparent and are thus eligible for Irish citizenship (10% of the population). When you factor in the bigger picture and migration during the 19th century you end up with up to 14million people having some Irish ancestry (nearly 25% of population).

  • pconroy

    @6 (Paul),

    Nice data support there!

    I’ve read that the percentage of recent Irish ancestry in places like Central Scotland (Glascow area) is close to 50%.

  • Paul Ó Duḃṫaiġ

    @7

    I saw the following figures on the Glasgow forum regarding % Irish in Coatbridge (16km east of Glasgow City center) in 1851: 35.8% Irish born!

    There are figures available in “The Irish in Britain, 1815-1939″ (By Roger Swift, Sheridan Gilley) which is partially available on Google Books

    See the map taken from this book here which shows the percentage Irish born across Britain in 1841, note several districts with over 30% particulary concentrated in around Glasgow and Liverpool, and this is before the Great Famine even started!
    http://books.google.ie/books?id=q6PwHF6FUYUC&lpg=PP1&pg=PA47#v=onepage&q&f=false

    The Irish born population of Liverpool city peaked at 22.5% in 1851 not surprising given that it was main entrance point for refugees from the famine.

  • http://dispatchesfromturtleisland.blogspot.com ohwilleke

    FWIW, Italy was not unified as a single country (following is disintegration after the fall of the Roman empire and its aftermath which was complete by the Middle Ages) until the 1870s. It’s unification at all was a close thing historically. There continue to be N. Italian secessionist factions that are a meaningful force in Italian politics even today (e.g. the “Northern League”).

    Modern era migration patterns have also been very different. There way a major exodus of Southern Italians to the U.S. in the late 19th century and early 20th century; this rivaled Irish migration to the U.S. and was similar in motivation and character. There was no a comparable scale exodus of Northern Italians.

  • pconroy

    @9,

    It’s funny I had a similar discussion with my wife, who is 1/2 Sicilian on either side, about 3 weeks ago. When my daughter asked her about her ancestry she said 1/2 Italian, and was miffed when I corrected her with “Sicilian”. Her argument was that Sicily is a part of Italy and so the correct term is Italian, but my argument is that Sicilian is the correct ethnic term, and Italian is a recent political term. When some of her Sicilian ancestors left Europe, they did NOT leave Italy, rather left the “Kingdom of the Two Sicilies”:
    http://en.wikipedia.org/wiki/Kingdom_of_the_Two_Sicilies

    This paper has proven me correct!

    In terms of Northern Italian descent, a lot of Italians in South America are of Northern descent, especially those in Uruguay, Argentina and Chile – or the Southern Cone.

    My eldest daughter’s mother is French, and her mother is 1/2 Northern Italian, 1/2 French ancestry, but if you ask her about her ethnicity, she prefers to say 1/2 French, 1/2 Lombard…

    ;)

  • Sandgroper

    #6, #7 & #8 – That time I said “We are all Irish!” I wasn’t just pulling Paul Conroy’s leg :)

  • Eurologist

    I have commented on the overwhelming ancestral Swedish/Baltic/Polish/German/Northern Balkan connection apparent in the data @ Dienekes’.

    One has to be careful about how important migrations are (vs. continuity), and what they really influence, and how they can be sorted out. For example, on the autosomal side, all Germans are rather poorly related to each other: so, some early subgroup migrating elsewhere, unlike y-DNA, will generally not display much relation to extant Germans. Conversely, Eastern Germanic tribes in conjunction with Slavic expansion have left both y-DNA (I and ancient R1a) and strong autosomal signatures that unite the regions I am mentioning in the first sentence – mostly because of climatically restricted population sizes but also ensuing homogeneity.

  • peter ralph

    @paul, #4: Good point about Northern Ireland: it would be great to use “England” here, but there are only 22 “English” in the sample, compared to 358 from the “UK”. This project: http://www.peopleofthebritishisles.org/ should clear this aspect of things up, at least.

  • Mr. Guy

    @ Raimo Kangasniemi

    “Anyone who isn’t an über-patriotic Greek and has knowledge of the history of Greece in the late antiquity and early Middle Ages could have guessed the heavy Slavic imprint in the current population; the Slavs overran much of Greece in in the late 6th and early 7th centuries and it took the Eastern Roman Empire until late 9th century to fully regain control. ”

    No one has to be an “über-patriotic Greek” (stereotype alert!) to know that the impact of the Slavic invasions in Greece is quite exaggerated and that the supposed “heavy Slavic imprint” in the current Greek population is historically dubious as Greece’s geography/climate did not favor any form of en masse Slavic settlement (much to the chagrin of both Peter Charanis and the author/authors of the Chronicle of Monemvasia).

    But let’s play devil’s advocate and argue that the Slavs did settle in large numbers and stayed for a while. Why haven’t archaeologists found piles of buried Slavic wares, weapons, hearths, and Zbruch idols throughout Greece? Now I don’t mean to burst anyone’s “gene bubble”, but “Traces of Slavic culture in Greece are rare.” (I too can’t believe that the well-esteemed editor of the “Oxford Dictionary of Byzantium”, Alexander P. Kazhdan, would turn out to be such an “über-patriotic Greek”.)

    “The number of Slavic place names alone testify to this era’s strong legacy to Greece.”

    Toponyms alone are not reliable indicators of migration/settlement patterns unless you believe that the town of Sparta in New York was first established and settled by ancient Spartans. And last time I checked, most of the Slavic place names collected by Max Vasmer seem to be absent in the Byzantine literature (including “Stolos”, which interestingly is not a Slavic word given that it means “fleet/equipment” in ancient Greek).

    As for the IBD study, where in all the “gene jargon” is there proof that the alleles shared by different European groups are in fact identical and ancestral? Also, do the authors provide any information on gene flow directionality between the groups analyzed? And if not, then wouldn’t the absence of gene flow directionality serve as a problem in terms of determining the exact age of the “ancestral alleles” shared by the studied groups?

    To make a long story short, it appears that this study is basing its historical inferences on one too many genetic uncertainties and/or false positives (though I could be mistaken). All I can say with certainty is that more research is needed.

    Cheers

    Mr. Guy

    P.S. @Razib Khan: Your blog is very informative sir. Keep up the good work.

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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