Reconstructing human demographic history from IBS segments

07 Jun
Figure 1. An eight base-pair tract of identity by state (IBS).
Identity-by-state (IBS) segments are those located between any two SNPs (polymorphisms, letters that vary among individuals). According to this new paper, they seem to be evolutionarily neutral and therefore their length, modified by recombination events each new generation, is a good trail to reconstruct human demographic history.
Kelley Harris & Rasmus Nielsen, Inferring Demographic History from a Spectrum of Shared Haplotype Lengths. PLoS Genetics 2013. Open accessLINK [doi:10.1371/journal.pgen.1003521]


There has been much recent excitement about the use of genetics to elucidate ancestral history and demography. Whole genome data from humans and other species are revealing complex stories of divergence and admixture that were left undiscovered by previous smaller data sets. A central challenge is to estimate the timing of past admixture and divergence events, for example the time at which Neanderthals exchanged genetic material with humans and the time at which modern humans left Africa. Here, we present a method for using sequence data to jointly estimate the timing and magnitude of past admixture events, along with population divergence times and changes in effective population size. We infer demography from a collection of pairwise sequence alignments by summarizing their length distribution of tracts of identity by state (IBS) and maximizing an analytic composite likelihood derived from a Markovian coalescent approximation. Recent gene flow between populations leaves behind long tracts of identity by descent (IBD), and these tracts give our method power by influencing the distribution of shared IBS tracts. In simulated data, we accurately infer the timing and strength of admixture events, population size changes, and divergence times over a variety of ancient and recent time scales. Using the same technique, we analyze deeply sequenced trio parents from the 1000 Genomes project. The data show evidence of extensive gene flow between Africa and Europe after the time of divergence as well as substructure and gene flow among ancestral hominids. In particular, we infer that recent African-European gene flow and ancient ghost admixture into Europe are both necessary to explain the spectrum of IBS sharing in the trios, rejecting simpler models that contain less population structure.

The most interesting graph, synthesizing the result for standard HapMap European and African proxy samples is figure 7. However I have major issues with the age estimates, which seem to be half what is needed to be realistic according to archaeological and other genetic data (unlineal haplogroup history, for example). Therefore I have annotated it with a revised timeline, so it fits better with the objective data:

Figure 7. A history inferred from IBS sharing in Europeans and Yorubans.
This is the simplest history we found to satisfactorily explain IBS tract sharing in the 1000 Genomes trio data. It includes ancient ancestral population size changes, an out-of-African bottleneck in Europeans, ghost admixture into Europe from an ancestral hominid, and a long period of gene flow between the diverging populations.
(Right margin annotations by Maju).

Indeed the simplest revision of the time-scale was to double it. I guess it can be refined a bit more than that, maybe pushing it a bit further into the past, but the alternative time-scale I propose fits closely enough with known archaeological data like the time of the OoA to Arabia and Palestine or the spread of Acheulean (and therefore H. ergaster, common ancestor of Neanderthals and H. sapiens) out of Africa c. 1 Ma ago to illustrate that the reconstruction seems pretty much correct overall but fails when estimating the dates (because of scholastic-autistic academic biases that are too common in the field of human population genetics).

Update: even Dienekes agrees, on his own well documented reasoning, with a x2 mutation rate being necessary for the above graph.


2 responses to “Reconstructing human demographic history from IBS segments

  1. eurologist

    June 8, 2013 at 11:12 am

    Definitely a factor of ~2 or just slightly larger – fits all dates quite well. I have to cringe at their suggestion of African gene flow into Europe all the way until 13,000 ya (which makes it 23,000 ya using their suggested factor of 1.75, pretty much right at LGM, <26,500 ya). Even a factor of ~2 is just barely tenable, in that respect.

  2. Maju

    June 8, 2013 at 3:55 pm

    Are the blue arrows depicting specific moments of flow? I did not pay much attention to them because, AFAIK, they mean diffuse undated bidirectional flow and nothing else. I paid attention to other details instead: particularly the OoA date and the Neanderthal ("ghost") divergence date.It is notable anyhow that even Dienekes agrees with a x2 mutation rate, and he argues for it much more vehemently and with some strong documented support.


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