Genetic history of the Iberian Peninsula

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The ancestry of modern Iberians(Spanish and Portuguese) is consistent with the geographical situation of the Iberian Peninsula in the south-west corner of Europe:There is a strong prehistoric connexion particularly with Atlantic Europe but also with the wider Mediterranean region, albeit the latter is lesser than regions to the East of the peninsula (particularly the Balkans) due to Spain being the furthest away from the Bosphorous region considered the main bridge of population expansions into Europe during the Neolithic. On the other hand, Iberia has the strongest genetic proximity in Europe to North Africa, purportedly resulting largely from northward population movements during the seven centuries of Muslim rule within the peninsula. A significant part of the Mediterranean/Near Eastern component of Iberia's genetic make up is also speculated by geneticists to be of relatively recent (historical) origin. The Basque Region in Northern Spain is genetically distinct as well as typically Atlantic European, holding the least Neolithic/Mediterranean and North African ancestry in Iberia. Modern day Basques are likely to be genetically similar to the Iberian peninsula's paleolithic inhabitants who repopulated Western Europe after the last Ice Age.

Population Genetics: Methods and Limitations[edit]

One of the first scholars to perform genetic studies was Luigi Luca Cavalli-Sforza. He used classical genetic markers to analyse DNA by proxy. This method studies differences in the frequencies of particular allelic traits, namely polymorphisms from proteins found within human blood (such as the ABO blood groups, Rhesus blood antigens, HLA loci, immunoglobulins, G-6-P-D isoenzymes, among others). Subsequently his team calculated genetic distance between populations, based on the principle that two populations that share similar frequencies of a trait are more closely related than populations that have more divergent frequencies of the trait.[1]

Since then, population genetics has progressed significantly and studies using direct DNA analysis are now abundant and may use mitochondrial DNA (mtDNA), the non-recombining portion of the Y chromosome (NRY) or autosomal DNA. MtDNA and NRY DNA share some similar features which have made them particularly useful in genetic anthropology. These properties include the direct, unaltered inheritance of mtDNA and NRY DNA from mother to offspring and father to son, respectively, without the 'scrambling' effects of genetic recombination. We also presume that these genetic loci are not affected by natural selection and that the major process responsible for changes in base pairs has been mutation (which can be calculated).[2]

Whereas Y-DNA and mtDNA haplogroups represent but a small component of a person’s DNA pool, autosomal DNA has the advantage of containing hundreds and thousands of examinable genetic loci, thus giving a more complete picture of genetic composition. Descent relationships can only to be determined on a statistical basis, because autosomal DNA undergoes recombination. A single chromosome can record a history for each gene. Autosomal studies are much more reliable for showing the relationships between existing populations but do not offer the possibilities for unraveling their histories in the same way as mtDNA and NRY DNA studies promise, despite their many complications.

Genetic studies operate on numerous assumptions and suffer from methodological limitations such as selection bias and confounding. Phenomenon like genetic drift, foundation and bottleneck effects cause large errors, particularly in haplogroup studies. No matter how accurate the methodology, conclusions derived from such studies are compiled on the basis of how the author envisages their data fits with established archaeological or linguistic theories.

Main genetic compositions[edit]

DNA analysis shows that Spanish and Portuguese populations are most closely related to other populations of western Europe.[3][4][5]

According to Dupanloup et al. (2004) the main components in the European genomes appear to derive from ancestors whose features were similar to those of modern Basques (Paleolithic) and Near Easterners (Neolithic), with average values greater than 35% for both these parental populations, regardless of whether or not molecular information is taken into account. The lowest degree of both Basque and Near Eastern admixture is found in Finland, whereas the highest values are, respectively, 70% ("Basque") in Spain and roughly 60% ("Near Eastern") in the Balkans.[6]

However, a controversial study, published in January 2010, claimed that despite this apparent East-West cline, Western Europeans (including Iberians) are of a nearly equal combination of the European hunter gatherers of the earlier Paleolithic period and more recent Neolithic period ancestry from the Near East, that likely came from Anatolia.[7][8][9]

Autosomal DNA[edit]

A 2007 European-wide study including Spanish Basques and Valencian Spaniards, found Iberian populations to cluster the furthest from other continental groups, implying that Iberia holds the most ancient European ancestry. In this study, the most prominent genetic stratification in Europe was found to run from the north to the south-east, while another important axis of differentiation runs east-west across the continent. It also found, despite the differences, that all Europeans are closely related.[10]

Y-Chromosome DNA[edit]

Y-haplogroups in Europe. Iberia is together with France, England, Northern Italy and Switzerland mainly R1b.

Y-chromosome analysis[11] had suggested Paleolithic ancestry among populations in the Iberian Peninsula and that Iberia may have played a role in the re-population of western Europe after the last glaciation.[12] This shows an ancestral bond between Iberia and the rest of western Europe, and in particular with Atlantic Europe, which share high frequencies of these haplogroups. R1b1a2, the most common western European haplogroup, arose 4,000 to 8,000 years ago in southwest Asia and later spread to Europe.[citation needed]

Haplogroup composition of the ancient Iberians was very similar to that found in the modern Iberian Peninsula populations, suggesting a long-term genetic continuity since pre-Roman times.[13][14]

Mitochondrial DNA[edit]

There have been a number of studies about the mitochondrial DNA haplogroups (mtDNA) in Europe. In contrast to Y DNA haplogroups, mtDNA haplogroups did not show as much geographical patterning, but were more evenly ubiquitous. Apart from the outlying Saami, all Europeans are characterized by the predominance of haplogroups H, U and T. The lack of observable geographic structuring of mtDNA may be due to socio-cultural factors, namely the phenomena of polygyny and patrilocality.[15]

The subhaplogroups H1 and H3 have been subject to a more detailed study and would be associated to the Magdalenian expansion from Iberia c. 13,000 years ago:[16]

H1 encompasses an important fraction of Western European mtDNA, reaching its local peak among contemporary Basques (27.8%) and appearing at a high frequency among other Iberians and North Africans. Its frequency is above 10% in many other parts of Europe (France, Sardinia, British Isles, Alps, large portions of Eastern Europe), and above 5% in nearly all the continent.[17] Its subclade H1b is most common in eastern Europe and NW Siberia.[18] So far, the highest frequency of H1 - 61%- has been found among the Tuareg of the Fezzan region in Libya.[19][20]

H3 represents a smaller fraction of European genome than H1 but has a somewhat similar distribution with peak among Basques (13.9%), Galicians (8.3%) and Sardinians (8.5%). Its frequency decreases towards the northeast of the continent, though.[17] Studies have suggested haplogroup H3 is highly protective against AIDS progression.[21]

Links with North African Populations[edit]

A number of studies have focused on ascertaining the genetic impact of historical North African population movements into Iberia on the genetic composition of modern Spanish and Portuguese populations. Although initial studies pointed to the Gibraltar straights acting more as a genetic barrier than a bridge,[6][22][23] more recent studies point to a substantially higher level of recent North African admixture among Iberians than among other European populations.[24] [25][26][27][28][29][29][30][31]

In terms of autosomal DNA, the most recent study regarding African admixture in Iberian populations was conducted in April 2013 using genome-wide SNP data for over 2000 individuals, concluding that Spain and Portugal hold significantly higher levels of North African than the rest of the European continent. Estimates of shared ancestry averaged between 4 and 20% whereas these did not exceed 2% in other western or southern European populations. However, contrary to past autosomal studies and to what is inferred from Y-Chromosome and Mitochondrial Haplotype frequencies(see below), it does not detect significant levels of Sub-Saharan ancestry in any European population outside the Canary Islands.[32] Indeed, a prior 2011 autosomal study by Moorjani et al. found Sub-Saharan ancestry throughout Europe at ranges of between 1-3%, "the highest proportion of African ancestry in Europe is in Iberia (Portugal 3.2±0.3% and Spain 2.4±0.3%), consistent with inferences based on mitochondrial DNA and Y chromosomes and the observation by Auton et al. that within Europe, the Southwestern Europeans have the highest haplotype-sharing with Africans."[33][29][30]

In terms of paternal Y-Chromosome DNA, recent studies coincide in that Iberia has the greatest the greatest presence of the typically Northwest African Y-chromosome haplotype marker E-M81 in Europe.[26][34] as well as Haplotype Va. [35][36] Estimates of Y-Chromosome ancestry vary, with a 2008 study published in the American Journal of Human Genetics using 1140 samples from throughout the Iberian peninsula, giving a proportion of 10.6% North African ancestry.[29][29][30][31] A similar 2009 study of Y-chromosome study with 659 samples from Portugal, 680 from Northern Spain, 37 samples from Andalusia, 915 samples from mainland Italy, and 93 samples from Sicily found significally higher levels of North African male ancestry in Spain, Portugal and Sicily (7.7%, 7.1% and 7.5% respectively) than in Italy (1.7%).[37]

Recent Mitochondrial DNA studies coincide in that the Iberian Peninsula holds higher levels of typically North African Haplotype U6,[29][30][38][39] as well as higher frequencies of Sub-Saharan African Haplogroup L.[40][41][42][42][43][44] Mean frequency of Haplogroup L lineages reaches 3.83% and the frequency is higher in Portugal (5.83%) than in Spain (2.9%). Furthermore, there is considerable geographic divergence across the peninsula with high frequencies observed for Southern Portugal (10.80%), Central Portugal (9.70%), Western Andalusia (14.6%)[43] and Córdoba (8.30%).[44]

Current debates revolve around whether U6 presence is due to Islamic expansion into the Iberian peninsula or prior population movements[29][29][30][31] and whether Haplogroup L is linked to the slave trade or prior population movements linked to Islamic expansion. A majority of Haplogroup L lineages in Iberia being North African in origin points to the latter.[41][42][45][46] [47][48][49]

See also[edit]

References[edit]

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  45. ^ However, contrary to past autosomal studies and to what is inferred from Y-Chromosome and Mitochondrial Haplotype frequencies(see below), it does not detect significant levels of Sub-Saharan ancestry in any European population outside the Canary Islands. [3]
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