Patterns of genetic differentiation and the footprints of historical migrations in the Iberian Peninsula

Abstract

Genetic differences within or between human populations (population structure) has been studied using a variety of approaches over many years. Recently there has been an increasing focus on studying genetic differentiation at fine geographic scales, such as within countries. Identifying such structure allows the study of recent population history, and identifies the potential for confounding in association studies, particularly when testing rare, often recently arisen variants. The Iberian Peninsula is linguistically diverse, has a complex demographic history, and is unique among European regions in having a centuries-long period of Muslim rule. Previous genetic studies of Spain have examined either a small fraction of the genome or only a few Spanish regions. Thus, the overall pattern of fine-scale population structure within Spain remains uncharacterised. Here we analyse genome-wide genotyping array data for 1,413 Spanish individuals sampled from all regions of Spain. We identify extensive fine-scale structure, down to unprecedented scales, smaller than 10 Km in some places. We observe a major axis of genetic differentiation that runs from east to west of the peninsula. In contrast, we observe remarkable genetic similarity in the north-south direction, and evidence of historical north-south population movement. Finally, without making particular prior assumptions about source populations, we show that modern Spanish people have regionally varying fractions of ancestry from a group most similar to modern north Moroccans. The north African ancestry results from an admixture event, which we date to 860 - 1120 CE, corresponding to the early half of Muslim rule. Our results indicate that it is possible to discern clear genetic impacts of the Muslim conquest and population movements associated with the subsequent Reconquista.

Differences in local population history at the finest level: the case of the Estonian population

“…First, IBD segments are expected to be on average longer and younger and thus have a more localized geographic distribution. This, combined with using total length instead of count and so giving more weight to the longer segments (see a similar approach being applied by Bycroft et al [ 3 ]) allows to focus on a rather recent genetic signal when performing the clustering. See Supplementary text 2 .…”

The genetic architecture of Parkinson disease in Spain: characterizing population-specific risk, differential haplotype structures, and providing etiologic insight

“…The Spanish population has a more diverse haplotypic structure in comparison with other European populations and is somehow isolated within itself in terms of global genetic structure 5 . The long-lasting migratory influences since early centuries and ulterior admixture from a number of civilizations over recent history have left their genetic imprint, thereby creating a particular genome population structure and diversity 6 . Taken together, these observations make Spain an interesting setting to comprehensively study the genetic architecture of PD and other complex diseases.…”

The genomic history of the Iberian Peninsula over the past 8000 years

“…2D and S11). Present-day populations from southern Iberia harbor less North African ancestry (25) than the ancient Muslim burials, plausibly reflecting expulsion of moriscos (former Muslims converted to Christianity) and repopulation from the north, as supported by historical sources and genetic analysis of present-day groups (25). The impact of Muslim rule is also evident in northeast Iberia in seven individuals from Sant Julià de Ramis from the 8–12th centuries CE who, unlike previous ancient individuals from the same region, show North African-related ancestry (Fig.…”