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Genetic history of the Iberian Peninsula

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The ancestry of modern Iberians (comprising the Spanish and Portuguese) is consistent with the geographical situation of the Iberian Peninsula in the south-west corner of Europe. The large predominance of Y-Chromosome Haplogroup R1b, common throughout Western Europe, is the result of Central European invaders during the Bronze Age, making the Spanish and Portuguese population closely related to others from Western Europe. Similar to Sardinia and unlike the Balkans and Italy, Iberia was shielded from settlement from the Bosporus and Caucasus region by its western geographic location, and its low level of Western Asian admixture probably arrived during the Roman period. Later historical Eastern Mediterranean and Middle Eastern genetic contribution to the Iberia gene-pool was also significant compared to other Western European countries, driven by Phoenicians, Greeks, Carthaginians, Jews and Levantine Arabs.

Like Sicily, and Southern Italy, Iberia has a low but specific level of ancestry, originating both in North Africa and in Sub-Saharan Africa, which is largely ascribed to the long Islamic presence in the Iberian peninsula, and the population of the Canary Islands shows a bigger African admixture than the average Southern Europe due to its location as an African archipelago. Significant genetic differences are found among, and even within, Spain's different regions, which can be explained by the wide divergence in their historical trajectories and Spain's internal geographic boundaries. The Basque region holds the least Eastern Mediterranean ancestry in Iberia. African influence is mainly concentrated in the Southern and Western regions of the peninsula, though the genetic influence is a minor component of the overall mix.

Population Genetics: Methods and Limitations

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.

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).

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.

Main genetic compositions

Y-Chromosome haplogroup distributions in Iberian Peninsula.
File:Percentage of major Y-DNA haplogroups in Europe.png
Y-Chromosome haplogroup distributions in Europe.

DNA analysis shows that Spanish and Portuguese populations are most closely related to other populations of western Europe. There is an axis of significant genetic differentiation along the east-west direction, in contrast to remarkable genetic similarity in the north-south direction. North African admixture, associated with the Islamic conquest, can be dated to the period between c. AD 860–1120.

Y-Chromosome haplogroups

Main article: Haplogroup R-DF27

Like other Western Europeans, among Spaniards and Portuguese the Y-DNA Haplogroup R1b is the most frequent, occurring at over 70% throughout most of Spain. R1b is particularly dominant in the Basque Country and Catalonia, occurring at rate of over 80%. In Iberia, most men with R1b belong to the subclade R-P312 (R1b1a1a2a1a2; as of 2017). The distribution of haplogroups other than R1b varies widely from one region to another.

Although R1b prevails in much of Western Europe, a key difference is found in the prevalence in Iberia of R-DF27 (R1b1a1a2a1a2a). This subclade is found in over 60% of the male population in the Basque Country and 40-48% in Madrid, Alicante, Barcelona, Cantabria, Andalucia, Asturias and Galicia. R-DF27 constitutes much more than the half of the total R1b in the Iberian Peninsula. Subsequent in-migration by members of other haplogroups and subclades of R1b did not affect its overall prevalence, although this falls to only two thirds of the total R1b in Valencia and the coast more generally. R-DF27 is also a significant subclade of R1b in parts of France and Britain. However, it is insignificant in Italy; R-S28/R-U152 (R1b1a1a2a1a2b) is the prevailing subclade of R1b in Italy, Switzerland and parts of France, although it represents less than 5.0% of the male population in Iberia. This underlines the lack of any significant genetic impact on the part of Rome, even though the Latin spoken in the Roman Empire was the most significant, or main, source of the modern Spanish and Portuguese language. R-S28/R-U152 is slightly significant in Seville and Barcelona, at 10-20% of the total population, although it is represented at frequencies of only 3.0% in Cantabria and Santander, 2.0% in Castille and Leon, 6% in Valencia, and under 1% in Andalusia. Sephardic Jews I1 0% I2*/I2a 1% I2 0% Haplogroup R1a 5% R1b 13% G 15% Haplogroup J2 2 25% J*/J1 22% E-M2151b1b 9% T 6% Q 2%

Haplogroup J, mostly subclades of Haplogroup J-M172 (J2), is found at levels of over 20% in some regions, while Haplogroup E has a general frequency of about 10% – albeit with peaks of 19% in certain areas. Overall, E-M78 (E1b1b1a1 in 2017) and E-M81 (E1b1b1b1a in 2017) both constitute about 4.0% each, with a further 1.0% from Haplogroup E-M123 (E1b1b1b2a1) and 1.0% from unknown subclades of E-M96. (E-M81 is widely considered to represent relatively historical migrations from North Africa).

Frequencies of Y-DNA haplogroups in Spanish regions
Region Sample size C E G I J2 JxJ2 R1a R1b Notes
Aragon 34 6% 0% 18% 12% 0% 3% 56%
Andalusia East 95 4% 3% 6% 9% 3% 1% 72%
Andalusia West 73 15% 4% 5% 14% 1% 4% 54%
Asturias 20 15% 5% 10% 15% 0% 0% 50%
Basques 116 1% 0% 8% 3% 1% 0% 87%
Castilla La Mancha 63 4% 10% 2% 6% 2% 2% 72%
Castile North-East 31 9% 3% 3% 3% 0% 0% 77%
Castile North-West 100 19% 5% 3% 8% 1% 2% 60%
Catalonia 80 >0% 3% 6% 3% 6% 0% 0% 81%
Extremadura 52 18% 4% 10% 12% 0% 0% 50%
Galicia 88 17% 6% 10% 7% 1% 0% 57%
Valencia 73 >0% 10% 1% 10% 5% 3% 3% 64%
Majorca 62 9% 6% 8% 8% 2% 0% 66%
Menorca 37 19% 0% 3% 3% 0% 3% 73%
Ibiza 54 8% 13% 2% 4% 0% 0% 57%
Seville 155 7% 4% 12% 8% 3% 1% 60%
Huelva 22 14% 0% 9% 14% 0% 0% 59%
Cadiz 28 4% 0% 14% 14% 4% 0% 51%
Cordoba 27 11% 0% 15% 15% 0% 0% 56%
Malaga 26 19% 4% 0% 18% 0% 8% 51%
Leon 60 10% 7% 3% 5% 2% 7% 62%
Cantabria 70 13% 9% 6% 3% 3% 4% 58%

Mitochondrial DNA

MtDNA haplogroup frequencies in the main Iberian regions.

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 Sami, 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 patrilocality and a lack of polyandry.

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:

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. Its subclade H1b is most common in eastern Europe and NW Siberia. So far, the highest frequency of H1 - 61%- has been found among the Tuareg of the Fezzan region in Libya.

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. Studies have suggested haplogroup H3 is highly protective against AIDS progression.

Autosomal DNA

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.

Portuguese mitochondrial DNA genetic diversity

In a study by Sofia L. Marques, Ana Goios, Ana M. Rocha, Maria João Prata, António Amorim, Leonor Gusmão, Cíntia Alves, and Luis Alvarez. "Portuguese mitochondrial DNA genetic diversity- an update and a phylogenetic revision." FSI Genetics 15 (March 2015): pages 27–32. Excerpts from the Abstract:

" In the case of Portugal, previous population genetics studies have already revealed the general portrait of HVS-I and HVS-II mitochondrial diversity, becoming now important to update and expand the mitochondrial region analysed. Accordingly, a total of 292 complete control region sequences from continental Portugal were obtained, under a stringent experimental design to ensure the quality of data through double sequencing of each target region.* Furthermore, H-specific coding region SNPs were examined to detail haplogroup classification and complete mitogenomes were obtained for all sequences belonging to haplogroups U4 and U5. In general, a typical Western European haplogroup or Atlantic modal haplotype composition was found in mainland Portugal, associated to high level of mitochondrial genetic diversity. Within the country, no signs of substructure were detected. The typing of extra coding region SNPs has provided the refinement or confirmation of the previous classification obtained with EMMA tool in 96% of the cases. Finally, it was also possible to enlarge haplogroup U phylogeny with 28 new U4 and U5 mitogenomes."

The Atlantic modal haplotype (AMH) or haplotype 15 is a Y chromosome haplotype of Y-STR microsatellite variations, associated with the Haplogroup R1b. It was discovered prior to many of the SNPs now used to identify subclades of R1b and references to it can be found in some of the older literature. It corresponds most closely with subclade R1b1a2a1a(1) .

The AMH is the most frequently occurring haplotype amongst human males in Atlantic Europe. It is characterized by the following marker alleles:

  • DYS388 12
  • DYS390 24
  • DYS391 11
  • DYS392 13
  • DYS393 13
  • DYS394 14 (also known as DYS19)

The Atlantic modal haplotype reaches the highest frequencies in Portugal, where it reaches 70% as a whole, with more than 90% in NW Portugal, in Great Britain and Ireland.

See also

Portals:

References

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  11. Nelis, Mari; Esko, Tõnu; Mägi, Reedik; Zimprich, Fritz; Zimprich, Alexander; Toncheva, Draga; Karachanak, Sena; Piskácková, Tereza; Balascák, Ivan; Peltonen, L; Jakkula, E; Rehnström, K; Lathrop, M; Heath, S; Galan, P; Schreiber, S; Meitinger, T; Pfeufer, A; Wichmann, HE; Melegh, B; Polgár, N; Toniolo, D; Gasparini, P; d'Adamo, P; Klovins, J; Nikitina-Zake, L; Kucinskas, V; Kasnauskiene, J; Lubinski, J; Debniak, T (2009). Fleischer, Robert C. (ed.). "Genetic Structure of Europeans: A View from the North–East". PLoS ONE. 4 (5): e5472. Bibcode:2009PLoSO...4.5472N. doi:10.1371/journal.pone.0005472. PMC 2675054. PMID 19424496.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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  13. Novembre, John; Johnson, Toby; Bryc, Katarzyna; Kutalik, Zoltán; Boyko, Adam R.; Auton, Adam; Indap, Amit; King, Karen S.; Bergmann, Sven; Nelson, Matthew R.; Stephens, Matthew; Bustamante, Carlos D. (2008). "Genes mirror geography within Europe". Nature. 456 (7218): 98–101. Bibcode:2008Natur.456...98N. doi:10.1038/nature07331. PMC 2735096. PMID 18758442. {{cite journal}}: Unknown parameter |lay-url= ignored (help); Unknown parameter |laydate= ignored (help); Unknown parameter |laysource= ignored (help)
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