The Iberian Peninsula is often regarded as a source for northward postglacial expansions2,3,5
and a sphere of Neolithic influence from the Near East.38
Our study suggests that its recent history has also had a profound influence on its diversity of Y-chromosomal lineages. Historical accounts should allow us to account for this, but they are sometimes written long after the incidents they describe, are usually scarce, and are always recorded with a particular audience in mind (and, therefore, are subject to bias).17
The marked genetic differentiation between the contributing populations in this case allows an attempt to disentangle their influence; such recognition may be more difficult when source populations for migrations or invasions are only slightly differentiated from recipient populations, as in the case of the Anglo-Saxon60
contributions to the British Isles, for example.
Our admixture approach has identified high mean levels of North African and Sephardic Jewish patrilineal ancestry in modern populations of the Iberian Peninsula and Balearic Islands. We find a mean of 10.6% North African ancestry, somewhat higher than previous ad hoc estimates,38
and a mean of 19.8% Sephardic Jewish ancestry, a figure that cannot be readily compared with any other study. These findings attest to a high level of religious conversion (whether voluntary or enforced) driven by historical episodes of religious intolerance, which ultimately led to the integration of descendants.
It has been claimed that there is some archaeological evidence to support prehistoric African influence in the Iberian Peninsula,62
and a single mitochondrial DNA (mtDNA) haplotype of North African origin found among ancient DNA samples of Iberian Bronze Age cattle from northern Spain63
has been taken as support of this claim. However, we observe low diversity of the prominent North African lineage hgE3b2 in Iberian populations, which argues against a prehistoric origin for the majority of chromosomes in this lineage, the low diversity being more compatible with their arrival in more recent times.
North Africans entered the Iberian Peninsula from the south, and after a rapid northward expansion soon retreated southwards, being finally expelled from Andalusia over 700 years after their arrival. Thus, they apparently spent the least amount of time in the north, and we might therefore expect a south-north gradient of North African ancestry proportions. However (and in agreement with studies of independent samples36,41,64
), we find no evidence of this. Indeed, the highest mainland proportions of North African ancestry (>20%) are found in Galicia and Northwest Castile, with much lower proportions in Andalusia. The most striking division in North African ancestry proportions is between the western half of the peninsula, where the proportion is relatively high, to the eastern half, where it is relatively low (). This distribution could reflect genetic drift, as well as the history of enforced relocations and expulsion of moriscos.
The entire large community of moriscos
in Granada was relocated northward and westward following the war of 1567–1571.23
In addition, the final expulsion of moriscos
, ordered by Philip III and beginning in1609, was highly effective in some regions of Spain, including Valencia and Western Andalucia, but less so in Galicia and Extremadura, where the population was more dispersed and integrated. Jewish communities were already widespread and long-established by 711 CE, so we might expect the level of Sephardic ancestry to also be widespread and undifferentiated. With the exception of the far northeast (NE Castile, Gascony, and Catalonia), this is indeed true for the mainland.
It is important to consider factors that might act to elevate the apparent proportions of Sephardic Jewish ancestry that we estimate, because these values are surprisingly high. Choice of parental populations in admixture analysis can have a major effect on the outcome, and among the parental populations in our analysis, the Sephardic Jewish population has a different status compared to the two others: whereas Basque and Moroccan samples are drawn from sizeable populations that have maintained their existence in situ, with a probable low level of admixture with the other parentals, the Sephardic Jewish sample is taken from a comparatively small group of self-defined individuals whose ancestors have lived in various parts of the Iberian Peninsula and were themselves probably subject to some degree of admixture with Iberians. This potential past admixture would have the effect of increasing the perceived level of Sephardic Jewish ancestry compared to the actual proportion. The presence of the typically western European lineage hgR1b3 at a frequency of 11% in the Sephardic Jewish sample might be a signal of such introgression. To examine this, we constructed a network of hgR1b3 Y-STR haplotypes in Iberian, Sephardic Jewish, and Moroccan samples (). Twelve of the 20 Sephardic Jewish R1b3 haplotypes are shared with Iberian examples, suggesting that they will indeed affect the admixture proportions. However, eight of the 20 are unique, and five of these are peripheral in the network. They will have little impact on the admixture proportions, and they probably reflect R1b3 chromosomes of Middle Eastern origin. It therefore seems that, overall, the ancestry proportions are likely to be only slightly affected by Iberian admixture into the Sephardic Jewish sample.
Diversity of Y-STR Haplotypes Belonging to Haplogroup R1b3
An additional factor that could lead to overestimation of Sephardic Jewish ancestry proportions is the effect of other influences on the Iberian Peninsula from eastern Mediterranean populations that might have imported lineages such as G, K
, and J. These influences fall into two different time periods: the Neolithic era, beginning in 10 KYA, the demographic effects of which are a matter for heated debate;1
and the last three millennia, the time period of Greek and Phoenician colonization.65
Effects in the second case are expected to be most marked in the eastern part of our sample area, but despite this, the apparent Sephardic Jewish ancestry proportions remain substantial in the west (). The confounding effects of earlier population movement are likely to be particularly strong for Ibiza, Majorca, and Minorca, whose island natures make them more susceptible to influence by immigration and subsequent drift than inland sites. For example, history records that Ibiza, found to have a high apparent Sephardic Jewish ancestry proportion in our study, had an insignificant Jewish population compared to its neighbors66
yet had previously been an important Phoenician colony. Likewise, Minorca is recorded as having a substantial Jewish population,66
yet here, it shows no Sephardic Jewish ancestry.
Our study has focused on the Y chromosome, but can we say anything about whether admixture has been predominantly male-mediated? Some mtDNA studies32,33
find evidence of the characteristic North African haplogroup U6 within the Iberian Peninsula. Although the overall absolute frequency of U6 is low (2.4%33
), this signals a possible current North African ancestry proportion of 8%–9%, because U6 is not a common lineage in North Africa itself. If this figure is reliable, it is not dissimilar from the level of paternal ancestry that we find. This might suggest that initial admixture involved movement of approximately equal numbers of males and females. However, because of drift through the differential reproductive success of males and females carrying different lineages, current relative proportions are an unreliable guide to proportions of the past. Comparable mtDNA data reflecting Sephardic Jewish contributions to the various areas of Iberia are not available, but sequence data on hypervariable regions I and II in a sample of 31 Sephardic Jews from Turkey has shown that their sequences and haplogroup frequencies are similar to those of Iberian populations,67
suggesting that admixture might be difficult to detect. Interestingly, analysis of European genome-wide SNP data68
shows the western half of the Iberian Peninsula to display the highest mean heterozygosity values in the continent, an observation that might reflect its history of population admixture from very different sources.
In this study, we have demonstrated the dramatic impact of recent events on the genetic landscape of an important part of the European continent. Immigration events from the Middle East and North Africa over the last two millennia, followed by introgression driven by religious conversion and intermarriage, seem likely to have contributed a substantial proportion of the patrilineal ancestry of modern populations of Spain, Portugal, and the Balearic Islands. In studies that seek to trace the imprint of key events in the earlier prehistory of Europe, the impacts of such recent episodes of gene flow and integration must be taken into account.