t (thanks, t!) points me to this article (this story seems to be making the rounds this a.m.):
“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.
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:
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’)”:
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.]:
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:
i made a map — and added the hajnal line (of course!):
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
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