Genetic history of the Iberian Peninsula

Source: Wikipedia, the free encyclopedia.
PCA plot of 17 contemporary Iberian populations[1]

The ancestry of modern Iberians (comprising the

Pontic-Caspian Steppe during the Bronze Age.[2][3]

Modern Iberians' genetic inheritance largely derives from the pre-Roman inhabitants of the Iberian Peninsula:

There are also minor genetic influences from the

Middle Eastern admixture than Italy and Greece, most of which probably arrived to Iberia during historic rather than prehistoric times, especially in the Roman period.[18][19]

Population Genetics: Methods and Limitations

The foremost pioneer of the study of population genetics was Luigi Luca Cavalli-Sforza. Cavalli-Sforza 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 distances 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.[20]

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).[21]

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.[citation needed]

Analyses of nuclear and ancient DNA

(A) Geographic distribution of the inferred proportions in the map. Shadings for proportions are scaled according to the maximum and minimum proportions of each source. (B) Representation of the ADMIXTURE proportions in each target population based on the statistically significant models obtained with qpAdm.

Nuclear DNA analysis shows that Spanish and Portuguese populations are most closely related to other populations of western Europe.[22][23][24] 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.[25]

A study published in 2019 using samples of 271 iberians spanning prehistoric and historic times proposes the following inflexion points in Iberian genomic history:[26]

  1. Mesolithic: hunter-gatherers from the European Steppes of Western Russia, Georgia and Ukraine are the first humans to settle the northwest of the Iberian Peninsula.
  2. neolithic farmers settle the entire Iberian Peninsula from Anatolia
    .
  3. Chalcolithic: Inflow of Central European hunter-gatherers and some gene inflow from sporadic contact with North Africa.
  4. Bronze Age: Steppe inflow from Central Europe.
  5. Basque people
    remains mostly intact from this point on.
  6. Mediterranean
    . Some additional inflow of North African genes detected in Southern Iberia.
  7. Visigothic period: no detectable inflows.
  8. Muslim period: Inflow from Northern Africa. Following the Reconquista, there is further genetic convergence between North and South Iberia.

North African influence

Main article: African admixture in Europe
Distribution of North African Admixture in the Iberian Peninsula

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. Initial studies pointed to the

Straits of Gibraltar acting more as a genetic barrier than a bridge during prehistorical times,[27][28][29] while other studies point to a higher level of recent North African admixture among Iberians than among other European populations,[30][31][32][33][34][35][36]
albeit this is as a result of more recent migratory movements, particularly the Moorish invasion of Iberia in the 8th century.

In terms of autosomal DNA, the most recent study regarding African admixture in Iberian populations was conducted in April 2013 by Botigué et al. using genome-wide SNP data for over 2000 European, Maghreb, Qatar and Sub-Saharan individuals of which 119 were Spaniards and 117 Portuguese, concluding that Spain and Portugal hold significant levels of North African ancestry. Estimates of shared ancestry averaged from 4% in some places to 10% in the general population; the populations of the Canary Islands yielded from 0% to 96% of shared ancestry with north Africans, although the Canary islands are a Spanish exclave located in the African continent, and thus this output is not representative of the Iberian population; these same results did not exceed 2% in other western or southern European populations.[37][38][39][40] 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. Indeed, a prior 2011 autosomal study by Moorjani et al. found Sub-Saharan ancestry in many parts of southern Europe at ranges of between 1-3%, "the highest proportion of African ancestry in Europe is in Iberia (Portugal 4.2±0.3% and Spain 1.4±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 North Africans."[30][34][35]

Recent studies show minor relationships between some Iberian regions and North African populations as a result of the Al-Andalus historical period which in Portugal lasted between the 8th and 12th centuries AD, and in southern Spain continued until the late 15th century AD. Iberia is the European region that has a more prominent presence of haplogroup E3b of the human Y chromosome (E-M81),[41] of haplogroup U (U6) and Haplotype Va, and this may be the result of some original common western Mediterranean population. In Portugal, North African Y-chromosome haplogroups (especially those typically North-West African) are at a frequency of 7.1%.[42] Some studies of mitochondrial DNA also find evidence of the North African haplogroup U6, especially in northern Portugal.[43] Although the frequency of U6 is low (4–6%), it was estimated that approximately 27% of the population of northern Portugal had some North African ancestry, as U6 is also not a common lineage in North Africa.[44] According to some studies, the North African and Arab elements in the ancestry of today's Iberians are more than trivial when compared to the basis of pre-Islamic ancestry, and the Strait of Gibraltar seems to function more as a genetic bridge than a barrier.[45][46][47]

However, a study that has used different genetic markers has reached different conclusions. In an autosomal study by Spínola et al. (2005), which analyzed the

Europeans and North Africans, via many ancient migrations. According to the authors, the North and the South of Portugal show a greater similarity towards North Africans as opposed to the people of the center of the country, who seem closer to other Europeans, since the North of Portugal seems to have concentrated, certainly due to the pressure of Arab expansion, an ancient genetic pole originating from many North Africans and other Europeans, influences through millennia, [clarification needed] while southern Portugal shows a North African genetic influence, probably the result of origins recent from the Amazigh people who accompanied the Arab expansion.[48]

Moura (literally "Moorish"), a small southern town near the Spanish border, known for its Moorish heritage

According to a study published in the American Journal of Human Genetics in December 2008, 30% of modern Portuguese (23.6% in the north and 36.3% in the south) have DNA that shows they have male Sephardic Jewish ancestry and 14% (11.8 in the North and 16.1% in the South) have Moorish ancestry.[49] Despite the possible alternative sources for lineages attributed to a Sephardic Jewish origin, these proportions were testimony to the importance of religious conversion (voluntary or forced), shown by historical episodes of social and religious intolerance.

Five component admixture plots for various contemporary Iberian populations against other European, West Asian, North African and West African populations[50]

In terms of paternal Y-Chromosome DNA, recent studies coincide in that Iberia has the greatest presence of the typically

Northwest African Y-chromosome haplotype marker E-M81 in Europe, with an average of 3%.[31][32] as well as Haplotype Va.[51][33] 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[34][35][36] to the paternal composite of Iberians. A similar 2009 study of Y-chromosome with 659 samples from Southern Portugal, 680 from Northern Spain, 37 samples from Andalusia, 915 samples from mainland Italy, and 93 samples from Sicily found significantly higher levels of North African male ancestry in Portugal, Spain and Sicily (7.1%, 7.7% and 7.5% respectively) than in peninsular Italy (1.7%).[31]

Other studies of the Iberian gene-pool have estimated significantly lower levels of North African Ancestry. According to Bosch et al. 2000 "NW African populations may have contributed 7% of Iberian Y chromosomes".[28] A wide-ranging study by Cruciani et al. 2007, using 6,501 unrelated Y-chromosome samples from 81 populations found that: "Considering both these E-M78 sub-haplogroups (E-V12, E-V22, E-V65) and the E-M81 haplogroup, the contribution of northern African lineages to the entire male gene pool of Iberia (barring Pasiegos), continental Italy and Sicily can be estimated as 5.6 percent, 3.6 percent and 6.6 percent, respectively".[52] A 2007 study estimated the contribution of northern African lineages to the entire male gene pool of Iberia as 5.6%."[53] In general aspects, according to (Bosch et al. 2007) "...the origins of the Iberian Y-chromosome pool may be summarized as follows: 5% recent NW African, 78% Upper Paleolithic and later local derivatives (group IX), and 10% Neolithic" (H58, H71).[54]

Mitochondrial DNA studies of 2003, coincide in that the Iberian Peninsula holds higher levels of typically North African Haplotype U6,

Moorish occupation left a minor Jewish, Saqaliba[60] and some Arab-Berber genetic influence mainly in southern regions of Iberia.[61][34]

The most recent and comprehensive genomic studies establish that

North African genetic ancestry can be identified throughout most of the Iberian Peninsula, ranging from 0% to 11%, but is highest in the south and west, while being absent or almost absent in the Basque Country and northeast.[62][18][19]

Current debates revolve around whether U6 presence is due to Islamic expansion into the Iberian peninsula or prior population movements[34][35][36] 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.[56][58][30][35][63] In 2015, Hernández et al. concluded that "the estimated entrance of the North African U6 lineages into Iberia at 10 ky correlates well with other L African clades, indicating that some U6 and L lineages moved together from Africa to Iberia in the Early Holocene while a majority were introduced during historic times."[64]

Haplogroups

Y-DNA haplogroup frequencies in the Iberian Peninsula[34]

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.[65] 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.

In Portugal as a whole the R1b haplogroups rate 70%, with some areas in the Northwest regions reaching over 90%.[66]

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.

Castille and Leon, 6% in Valencia, and under 1% in Andalusia.[65]
Sephardic Jews
Q
2% [71]

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 surpassing 30% 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.[34]
(E-M81 is widely considered to represent relatively historical migrations from North Africa).

Mitochondrial DNA

MtDNA haplogroup frequencies in the main Iberian regions[72]
Main articles:
Haplogroup U (mtDNA)

There have been a number of studies about the

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.[73]

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:[56]

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.[77]

Subregions

Spain

Frequencies of Y-DNA haplogroups in Spanish regions[34][78]
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%[79] 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%[80] 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%
Málaga 26 31% 4% 0% 15% 0% 8% 43%
Leon 60 10% 7% 3% 5% 2% 7% 62%
Cantabria 70 13% 9% 6% 3% 3% 4% 58%

Portugal

Distribution of the R1b haplogroup in Europe

Excerpts from the Abstract of a study published[81] in 2015:

"[...] 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
(AMH) 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 AMH reaches the highest frequencies in the Iberian Peninsula, in Great Britain and Ireland. In the Iberian Peninsula it reaches 70% in Portugal as a whole, with more than 90% in NW Portugal and nearly 90% in Galicia (NW Spain), while the highest value is to be found among the Basques (NE Spain).

The

subclades
of R1b and references to it can be found in some of the older literature. It corresponds most closely with subclade R1b1a2a1a(1) [L11].

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)

See also

Wikimedia Commons has media related to Genetic studies on Iberian Peninsula.

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Works cited
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