more on ibd and historic mating patterns in europe

t (thanks, t!) points me to this article (this story seems to be making the rounds this a.m.):

“All Europeans are related if you go back just 1,000 years, scientists say”

“A genetic survey concludes that all Europeans living today are related to the same set of ancestors who lived 1,000 years ago….

“The researchers were surprised to find that even individuals living as far apart as Britain and Turkey shared a chunk of genetic material 20 percent of the time. To explain that degree of genetic commonality, the researchers say those pairs of individuals would have to have a huge number of common genealogical ancestors 1,000 years ago — a number that takes in everyone who was alive in Europe back then….”

the results of the survey being discussed here have just been published on plos biology: The Geography of Recent Genetic Ancestry across Europe.

before i go on to discuss the bits i’m interested in (the identity by descent, or ibd, rates that they found), i just want to quote something from the plos article related to this business that all europeans share the same set of ancestors that lived 1,000 years ago. yes, we do, but keep in mind that:

“[S]omeone in Spain may be related to an ancestor in the Iberian peninsula through perhaps 1,000 different routes back through the pedigree, but to an ancestor in the Baltic region by only 10 different routes, so that the probability that this Spanish individual inherited genetic material from the Iberian ancestor is roughly 100 times higher. This allows the amount of genetic material shared by pairs of extant individuals to vary even if the set of ancestors is constant.”

in other words, some europeans are more related to one another than to others. but we all knew that already.

anyway…

this is the same (really awesome!) study done by ralph and coop that i posted about last year here and here. (oh, and here, too.) some of the data were available online back then after the researchers had given a presentation somewhere or other [pdf].

i’m interested in ibd data since they, like runs of homozygosity (roh), can give us some clues about how inbred or outbred populations are. it’s not a clear-cut interpretation, though, because both ibd and roh can be affected by other population genetic processes like bottlenecks and migration and simply population size (and probably other things, too, about which i am blissfully ignorant), so one has to make some educated inferences and guesses.

unfortunately, the authors don’t seem to have included in the plos publication the following illustration from their earlier presentation (unless it’s buried in the supplemental data — i didn’t see it there, but there’s a LOT of supplemental data files). that’s a shame, because it’s one of the most interesting:

coop et al - mean within-country ibd rates

the map shows the mean ibd rates for each of the european populations studied (the mean length of the blocks was >1 cM). individuals in the populations with higher mean ibd rates (bigger circles) share more identical stretches of their dna with their fellow countrymen than those in populations with low mean ibd rates. lots of outbreeding can lower the amount and lengths of ibd blocks in a population. as i posted previously, i think you can see the historic (since the early medieval period) outbreeding patterns of western europeans in the low mean ibd rates in western europe. this pattern is even clearer when you add the hajnal line to the map (the hajnal line being a good indicator of the geographical limits of the roman catholic church’s/secular authorities’ push to, amongst other things, ban cousin marriage in the medieval period).

now, here from the plos paper is a table indicating “mean number of IBD blocks shared by a pair of individuals from that population (‘self’), and mean IBD rate averaged across all other populations (‘other’)”:

ralph and coop - mean number of ibd blocks

i put the mean ibd “self” (i.e. within a population) numbers on a map and added the hajnal line. (note that the “mean length of these blocks was 2.5 cM, the median was 2.1 cM, and the 25th and 75th quantiles are 1.5 cM and 2.9 cM, respectively”.) [click on map for LARGER view.]:

europe map - ralph & coop ibd rates + hajnal line

ralph and coop suggest that the rates are so high in eastern europe, and particularly the balkans, because of the fairly recent slavic migration into the area and the fact that the slavs settled in relatively uninhabited areas. they further suggest that the germanic migrations into western europe are not so apparent in the ibd rates since these were already heavily populated areas and maybe even that the germanics were an heterogeneous group to start off with. those are really good theories (especially the one about the slavs), and i think that — yeah — we are probably seeing signals of those migrations in these data. however, once again, i think you can also see the long-term historic inbreeding/outbreeding (greater cousin marriage vs. little cousin marriage) mating patterns of european populations reflected in the ibd rates. (see “mating patterns in europe series” below ↓ in left-hand column for more details on all the mating patterns which i mention in the next few paragraphs.)

my “core europeans” — the english, the french, the belgians, the dutch, the germans, the north italians (not so much the ones in the alps, though), and to some extent the swiss and scandinavians — have the longest history of outbreeding (i.e. avoiding cousin marriage) in europe beginning in the early medieval period — and they have the lowest ibd rates. the rates are a bit higher for scandinavia since they converted to christianity later and, thus, didn’t adopt the cousin marriage bans until later. same with the irish and the scots (in fact, i think that highland scotland should be indicated as being outside the hajnal line, but that’s a discussion for another day). that the netherlands has a higher ibd rate than neighboring belgium and germany also makes sense if you know about the (probable) late adoption of the cousin marriage bans by those living in the marshes like the ditmarsians.

the ibd rates are higher east of the hajnal line and that, too, makes sense if you know that the eastern orthodox church was both later at instituting and less consistent in enforcing cousin marriage bans. the very high rates in albania and kosovo are probably related to the fact that these populations include a majority of muslims and that muslims typically have no bans on marrying cousins (while the albanians, and likely the kosovans [or whatever you want to call them!], have probably avoided paternal cousin marriage, maternal cousin marriage seems to have been an option, possibly even preferred).

the very low rate in italy is puzzling and, as i have said elsewhere, may have to do with the fact that, as the authors suggest, italy has experienced so many influxes of different populations. alternatively, it may have to do with a sampling bias (i.e. where did the italian samples come from? the more outbred north, or the more inbred south?).

the authors also broke down the ibd rates by several european regions of their own devising: “These five groupings are defined as: Europe ‘E,’ lying to the east of Germany and Austria; Europe ‘N,’ lying to the north of Germany and Poland; Europe ‘W,’ to the west of Germany and Austria (inclusive); the Iberian and Italian peninsulas ‘I’; and Turkey/Cyprus ‘TC.'” here is their table:

ralph and coop - mean number of ibd blocks by region

i made a map — and added the hajnal line (of course!):

europe map - ralph & coop regional ibd rates + hajnal line

again, there’s the east-west divide that i’ve pointed out before and which, i think, corresponds to the edge of the hajnal line. there also seems to be a north-south divide, which is apparent on both sides of the east-west (fuzzy) border, and which may have to do with long-standing lower population densities in northern europe. (that does make sense if you think about it — smaller populations inevitably experience closer matings or greater “inbreeding.”)

mating patterns matter! particularly long-term mating patterns. i think so anyway.

previously: ibd and historic mating patterns in europe and ibd rates for europe and the hajnal line and ibd rates and kindreds in germanic populations and russians, eastern europeans, runs of homozygosity (roh), and inbreeding and western europeans, runs of homozygosity (roh), and outbreeding and runs of homozygosity and inbreeding (and outbreeding) and runs of homozygosity again

(note: comments do not require an email. whatcha doin’ there?)

Advertisements

14 Comments

  1. Just a minor quibbling about the latter table, the one that organizes the data into macro-regions. In my opinion to lump together the Iberian and the Italian peninsulas is arbitrary, according to the data. It’s like a touristic drive, something like “let’s put together the southern sunny beaches I use to travel to”.

    Iberia would have values like, let’s say, 1.6 – 0.42 , while Italy is 0.6 – 0.5 . It’s doesn’t make sense to lump them together when France, for example, has 0.7 – 0.5. Why not to lump together the italians and french into some “Frataly” region? Completely arbitrary.

    Reply

  2. I wonder if the Italian score is related to not comparing like with like i.e. if you look at a relief map of Italy it’s divided into lots of “bowls” by all the mountains. In that context i’m not surprised Italians *as a whole* don’t share a lot of Identity By Descent but i would expect the sub-populations within each “bowl” i.e. Tuscans, Umbrians, Calabrians etc to share a lot of IBD with other Tuscans, Umbrians and Calabrians?

    Reply

  3. The distinction here being between populations who live “in” mountainous regions (who you might expect to have a lot of IBD) and populations who live in regions which are *split up* into sub-regions – the relatively flat bits – by lots of mountains where you might expect high IBD *within* each sub-population but low IBD *between* the sub-populations.

    Reply

  4. @grey – “if you look at a relief map of Italy it’s divided into lots of ‘bowls’ by all the mountains. In that context i’m not surprised Italians *as a whole* don’t share a lot of Identity By Descent but i would expect the sub-populations within each ‘bowl’ i.e. Tuscans, Umbrians, Calabrians etc to share a lot of IBD with other Tuscans, Umbrians and Calabrians?”

    i wonder. dunno.

    it’s going to be so hard (if not impossible) to sort this out until they (teh scientists) start collecting dna samples with at least some provenance info attached to them (i.e. not just “italian” – *facepalm*). i know everyone’s been trying to protect those giving up the samples and making sure that their studies conform to ethical standards, yada, yada, yada … but i don’t see the problem with the subjects just signing off on sharing more info. if someone doesn’t agree to sharing their dna AND their family geneaology, that’s ok — move on to the next subject and ask them. what’s the big deal?

    Reply

  5. @tomás – “In my opinion to lump together the Iberian and the Italian peninsulas is arbitrary, according to the data.”

    yeah, i don’t know why they did it that way either. -?- (i’ll have to go back and have another look at the paper.)

    Reply

  6. I’d previously read that pretty much all Europeans are descendants of Charlemagne, and that pretty much all “caucasians” (Europeans, Middle-easterners, Iranians, and subcontinental Indians) are descended from Mohammed.

    Reply

  7. About 1200 years ago real seafaring ships arrived. Not only could you travel long distances over open seas in relative safety, a whole new source of food from the high seas became available. Over a hundred of these “viking” ships have been found all over Europe, and by looking at the wood, they were being built from Russia to Ireland.

    Before real seafaring ships, you had to land hop, with all the danger this involved, from the locals overpowering the crew. Once seafaring ships arrived, you could go straight to a secure harbor in a town where you were protected, because the town lived from trade.

    Once the locals got to know these seafaring people, they could invite some of them to make a fishing village, so that the locals could trade food and goods for fish and imported goods.

    One such fishing town is Povoa de Vazim in Portugal.
    http://en.wikipedia.org/wiki/Culture_of_P%C3%B3voa_de_Varzim

    Most people in academia are landcrabs, and are so stuck in the anti-Nordic paradigm, that they refuse to even think about the Vikings and their ships as an explanation for the Europeans having a common ancestor ca 1000 years ago.

    Reply

  8. “I’d previously read that pretty much all Europeans are descendants of Charlemagne,”

    Is that very likely, compared to them being related to the ones who traveled the whole of Europe by ship around the same time, and continued to do it until railroads came along?

    Dutch, Norwegians and Danes, have almost the same DNA from this trade, but there was no Charlemagne in Norway and Denmark.

    Norway is by the way, named after one of these waterways, as the sheltered shipping route from Denmark to the Russian border was called the North-way. The East-way was through Russia to Constantinople, while the West-way went from Denmark along the Atlantic coast.

    Reply

  9. @greying wanderer – “I wonder if the Italian score is related to not comparing like with like i.e. if you look at a relief map of Italy it’s divided into lots of ‘bowls’ by all the mountains. In that context i’m not surprised Italians *as a whole* don’t share a lot of Identity By Descent but i would expect the sub-populations within each ‘bowl’ i.e. Tuscans, Umbrians, Calabrians etc to share a lot of IBD with other Tuscans, Umbrians and Calabrians?”

    hey! check out steve sailer’s latest article in takimag:

    The Italian Invasion of American Culture

    … A new genetic study helps illuminate how nurture and nature worked together to make Italy a place where you definitely want to visit although you might not want to live there. Geneticists Peter Ralph and Graham Coop’s paper The geography of recent genetic ancestry across Europe reveals that Italians stand out for their genealogical “deep structure.” Razib Khan called attention to Ralph and Coop’s statement that “different groups within Italy share as little recent common ancestry as other distinct, modern-day countries.…”

    While much of northern Europe, especially the Slavic regions, has been swept repeatedly by vast invasions, prehistoric and historic, modern Italians are largely living in the lands of their ancestors. The deep structure of Italian culture—the sense that today’s Italians are exquisitely adapted to their particular cities and villages—is palpable to the visitor. Perhaps the finest cinematic portrayal of Italian culture’s rootedness is the Sicily chapter in The Godfather.

    Or consider Florence. Not only was Tuscany the center of the Renaissance six centuries ago, it was home to the artistically advanced Etruscan civilization when Rome was a minor-league city-state.

    The bulk of today’s Italian population appears to have family ties in their regions going back before Imperial Rome at least to the Greek settlements in the Age of Archimedes and to the Gauls crossing the Alps around 400 BC Ralph and Coop note:

    ‘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 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.…’

    Not only haven’t Italians been intermarrying much with non-Italians, they haven’t been intermarrying much even with other Italians beyond their own valleys. My wife’s great-grandfather, for instance, was considered a moonstruck romantic for wooing a girl not from his own village high in the Apennines, but from the town down in the valley, requiring a 500-meter climb back up after each date.

    But this deep structure also means that Italian families have been stabbing each other in the back for centuries, which may help explain why Italy doesn’t work as well as, say, Finland….

    bowls=deep structure. (^_^)

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s