Evidence of social marginalisation leading to strong genetic differentiation among the Ari of Ethiopia. G. Hellenthal1, L. van Dorp1, S. Myers2, L. Pagani3, C. Tyler-Smith3, E. Bekele4, A. Tarekegn4, M. Thomas1, N. Bradman5, D. Balding1 1) Genetics, Evolution, and Environment, University College London, London, United Kingdom; 2) Department of Statistics, University of Oxford, Oxford, United Kingdom; 3) The Wellcome Trust Sanger Institute, Hinxton, United Kingdom; 4) Addis Ababa University, Addis Ababa, Ethiopia; 5) Henry Stewart Talks Ltd, London, United Kingdom.
A major debate among population geneticists is whether the large amount of observed genetic diversity among human groups (e.g. from Africa) is primarily attributable to ancient substructure or to more recent inter-mixing or drift events. A specific example involves the caste-like occupational groups within the Ari of Ethiopia, which include the Cultivators and socially marginalized Blacksmiths. There are two competing theories about the origins of the Ari Blacksmiths as (i) remnants of an ancient hunter-gatherer population that inhabited Ethiopia prior to the arrival of agriculturists (e.g. Cultivators) during the Neolithic period, versus (ii) relatively recently related to the Cultivators but presently marginalized in the community due to their craft skills. A recent paper by Pagani et al (2012) collected and analysed genome-wide DNA from samples of Ari Blacksmiths and Cultivators, observing genetic differentiation between these two occupational groups (FST = 0.04) at a similar level to that observed across multiple ethnic groups sampled throughout Ethiopia (FST range 0.02 - 0.06). Furthermore, an analysis using the clustering algorithm ADMIXTURE assigned the Ari Blacksmiths almost entirely to a single genetically homogeneous cluster, with other Ethiopian groups -- including the Cultivators -- displaying varying levels of genetic similarity to this unique cluster. The authors noted these patterns were consistent with model (i), with subsequent assimilation of the indigenous hunter-gatherers into the expanding agriculturalist community. We analysed the same samples using a novel haplotype-based approach to test this hypothesis, comparing strings of DNA patterns among different Ethiopian groups and to various groups outside of Ethiopia. Importantly, this new model is able to distinguish genetic structure attributable to allelic drift within a population from that attributable to shared ancestry with outside groups. Using this technique, we provide compelling evidence that the genetic differences among Ari Blacksmiths and Cultivators are entirely attributable to recent social marginalisation, i.e. hypothesis (ii). This finding serves as both a cautionary tale about interpreting results from the widely popular program ADMIXTURE, and a clear demonstration of how social constructions can contribute directly to genetic differentiation among previously similar groups.
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