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September 27, 2017

Yes I think about 1% of Afrikaner ancestry is probably Khoikhoi

Filed under: Afrikaner,Afrikaner genotype,Afrikaners,Khoisan — Razib Khan @ 7:48 pm

As a follow-up to my previous two posts on Afrikaners, I wanted to reiterate something that I implied/said earlier: yes, I think about 1% of the ancestry of modern day Afrikaners derives from Khoisan pastoralists of the Cape who were resident there when the Europeans first arrived. These people are often called Khoikoi. Unlike the more famous Bushman the Khoikhoi were not hunter-gatherers. Rather, they herded cattle. Both archaeological and genetic evidence points to the fact that pastoralists arrived in southern Africa through the expansion of East African nomads, who had some Eurasian ancestry (ergo, Khoisan peoples have differing degrees of non-Khoisan African ancestry, as well as Eurasian ancestry).

Today there are no major Khoikhoi groups in South Africa that have not been extensively influenced by other populations (in Namibia the related Nama maintain tribal cohesion and continue the cultural tradition of Khoisan pastoralism). Where did the Khoikhoi go? Many died due to disease, and the privations of slavery. But, some were certainly absorbed into other populations. The Xhosa people have substantial Khoisan ancestry for example.

The plot to the left has various populations, including Dutch, whites from Utah, white South Africans, Nigerians, African Americans, Barbadians, and Bantu populations (click the image for a larger version). As well as Khoisan groups which are a combination of Nama and San Bushmen samples.

If you click the larger image you can see that the South African Bantus are shifted toward the Khoisan. The Kenyan Bantus are skewed in the direction of Eurasians…though only mildly so (no doubt due to Cushitic admixture).

The plot to the right (click to enlarge) is a zoom in. It is clear that the South African samples are very subtly shifted out of the normal Northern European cluster. If you look at the cline from the Nigerians running toward the Northern Europeans, the South African whites look to be perturbed from it. Notably, some of them are clearly shifted in the direction of the Khoisan.

Next, I ran Admixture analysis. I set the reference populations as Esan from Nigeria, Khoisan, and Dutch whites. You can see that African Americans exhibit a cline as you’d expect. A minority of their ancestry is Northern European. But mostly they are African, with the dark blue representing the Esan Nigerian reference population. This is as it should be; most of the slaves who came to America seem to have come from the Congo up the Africa coast all the way to Senegal.

The fraction of African ancestry in the South African samples is low. But observe that many of them have just as high a fraction of the red component, which comes from the Khoisan reference population. These ten mostly white South Africans average 1.4% Khoisan and 2.3% non-Khoisan African.

Finally, I decided to run Treemix and do a three population test.

With two migration edges the results make a lot of sense. The African Americans are placed next to the Nigerians, but there is a migration edge of some significance from the Northern Europeans. The South Africans are in a clade with the Dutch samples, with Utah whites being the outgroup. But, they have a migration edge from between the Esan from Nigeria and the Khoisan.  Recall that there was more Nigerian-like ancestry in the South African whites than Khoisan-like ancestry according to Admixture. The gene flow edge seems to be closer to the Esan by some margin.

Finally, I ran a three population test, which tests gene flow by placing an admixed population as an outgroup to source populations. Negative statistics indicate “complex population history” not accounted for by the tree.

Outgroup Pop 1 Pop 2 f3 Z score
Af_American Netherlands EsanNigeria -0.0103 -89.1922
Af_American UtahWhite EsanNigeria -0.0102 -88.7189
Af_American South_Africa EsanNigeria -0.0099 -87.1784
Af_American Netherlands Khoisan_SA -0.0034 -16.6754
Af_American Khoisan_SA UtahWhite -0.0033 -16.5408
Af_American South_Africa Khoisan_SA -0.0029 -14.6855
South_Africa Netherlands EsanNigeria -0.0015 -10.9174
South_Africa Netherlands Khoisan_SA -0.0015 -10.5677
South_Africa UtahWhite EsanNigeria -0.0014 -9.0416
South_Africa Khoisan_SA UtahWhite -0.0014 -8.7962
South_Africa Netherlands Af_American -0.0011 -10.0158
South_Africa Af_American UtahWhite -0.0010 -8.1132
UtahWhite Netherlands EsanNigeria -0.0001 -1.6344
UtahWhite Netherlands Khoisan_SA -0.0001 -1.6344

The bottom two results can be ignored. What you see is that African Americans have the most negative f3 values with the highest z-scores. There is a drop-off from the Nigerians to the Khoisan as one of the source populations because the Nigerians are a much better fit. The values for South Africans are much lower, which makes sense in light of their lower admixture proportion. But observe that the f3 statistic for using Esan vs. Khoisan is not that different. This suggests neither group is necessarily a better proxy for the other.

As for the ethnographic details of where this ancestry came from, I think it was the proto-Cape Coloured population.

September 23, 2017

No, Afrikaners do not have British or English ancestry

Filed under: Afrikaner,Afrikaner genotype,Ancestry,Boer — Razib Khan @ 9:58 pm

kan

In my post below on the non-European ancestry of Afrikaners, several readers mentioned that friends of Afrikaner background were rather chagrined to have reported British ancestry from genetic tests. The cultural reason for this is well known: many Afrikaners exhibit hostility toward British imperialism due to the deprivation and death which was the consequence of their resistance to the expansion of the Empire during the Second Boer War. This is above and beyond the antipathy which was manifestly made obvious by the fact that with the transfer of the Cape Colony to the British in the early 19th century thousands of white farmers migrated into the hinterlands to escape the new power (in part to preserve their customs, such as slavery).

By the 20th century, this anti-British aspect of Boer identity manifested itself in pro-German sentiments, as can be seen in the film The Power of One.

But the reality is that it is strange for Afrikaners to have British ancestry. Yes, they are not exclusively Dutch, with substantial German and French (Huguenot) components in their background. And there has been some recent intermarriage with English speaking whites. But presumably that’s recent enough that people would know.

Rather, I think what is happening is that genetic tests do not have the power to distinguish well between English and Dutch ancestry. In fact, the minority ancestry from Anglo-Saxons in southeast Britain would have stronger affinities with the Dutch than most of the island.

To figure out what was going on I asked people on Twitter for 23andMe profiles. I got a response from someone whose results I posted above. This individual has Boer ancestry, mostly Dutch, going back to the late 17th century on his mother’s side and late 18th century on his father’s side. And you see 17% “British” ancestry. He also provided his wife’s 23andMe output. Her ancestry dates back to the late 17th century on both paternal and maternal sides, so it is not a surprise she has more non-European ancestry:

She is 18% British. In fact, the European ancestry fractions of both these individuals are rather similar when it comes to “French-German”, British, and Scandinavian. I suspect what we’re seeing here is what the algorithm pops out quanta wise for Dutch.

I took the South African individuals who had some non-European ancestry, and ran them on Admixture and projected a PCA with British and Dutch individuals. You can make your own judgment, but I think these are definitely people who are of mostly Dutch ancestry.

September 22, 2017

The non-European ancestry of Afrikaners

Filed under: Afrikaner,Genetics — Razib Khan @ 12:37 am


A few years ago I got some South African genotypes. Some of the individuals were clearly African. A few mapped perfectly upon Northern Europeans. But many of the samples consistently were European but shifted toward non-European populations.

Based on history of the assimilation of slaves into the European population of Cape Colony in the 18th century, my assumption is that these individuals are Afrikaners.

Recently I realized that Brenna Henn had released some more Khoisan samples, so I decided to look at this question of admixture again. The two Khoisan populations are the Nama and the Khomani. I removed those with lots of Bantu and European admixture and combined them together into one population.

Running unsupervised Admixture shows how distinct the South African whites are.

The average Utah white in this sample (this population is a mix of British, German, and Scandinavian in ancestry) is 99% European modal cluster, and 1% South Asian. The average for the white South Africans in this data set is 94% European modal cluster. The residual is 1% East Asian (Dai modal), 1% Khosian, 1% non-Khoisan African, and 2% South Asian.

I ran Treemix a bunch of times, and every single plot came out like this when I ran it for three migrations:

 

The gene flow from the Utah whites to the Gujuratis is simply an artifact of the fact that the Gujurati sample is mixed caste, and some of the Brahmin or Lohannas have more “Ancestral North Indian.” The gene flow from the Europeans to the Khoisan is probably real, or, might be due to pastoralist admixture via East Africans. The last migration arrow goes from the African populations to the South African whites, with a shift toward the Khoisan.

I also ran a three population test where A is the outgroup, and B and C are a clade. A significantly negative f3-statistic indicates admixture in population A. The negative values are listed below:

A B C f3 f3-error Z-score
Gujrati Dai UtahWhite -0.00121718 0.000140141 -8.68539
South_Africa EsanNigeria UtahWhite -0.00127718 0.000147982 -8.63059
South_Africa Khoisan_SA UtahWhite -0.0012928 0.000151416 -8.53802
Gujrati South_Africa Dai -0.000778791 0.000155656 -5.00329
South_Africa Dai UtahWhite -0.000541974 0.000133262 -4.06699
South_Africa UtahWhite Gujrati -0.000103581 8.46193e-05 -1.22408

This aligns well with the Admixture results. Afrikaners have both African ancestries, and, Asian ancestry.

In James Michener’s The Covenant one of the plot lines alludes to mixed ancestry in one of the Afrikaner families. The results above suggest that mixed ancestry is very common, and perhaps ubiquitous, in this population. True, there are some Afrikaners such as Hendrik Verwoerd who migrated to South Africa from the Netherlands in the past century or so, but these are uncommon to my knowledge.

February 15, 2012

Update on the Afrikaner genotype

Filed under: Afrikaner,Personal genomics — Razib Khan @ 8:50 pm

Since my original post on the Afrikaner genotype, I’ve gotten many responses. No genotypes yet though. At some point I need to organize how to pay for typing many individuals. Currently my intent is to pay for those who will allow their identities to be public so that people can confirm their genealogies. Other people have emailed me to say that Afrikaners with whom they have shared genotypes on 23andMe often have African or Asian ancestral segments. But all hearsay so far.

In other news, I’ve got a Dinka gentoype I’ll be analyzing soon (some minor technical issues with merging datasets has delayed this some).

April 27, 2010

The ancestry of one Afrikaner

Filed under: Afrikaner,Anthroplogy,Genetics,Pedigree,science,South Africa — Razib Khan @ 5:41 am

A few weeks ago I reviewed a paper on the the genetics of the Cape Coloured population. Within it there was a refrence to another paper, Deconstructing Jaco: genetic heritage of an Afrikaner. The title refers to the author himself. It was an analysis of his own pedigree going back to the 17th century, along with his mtDNA, his father’s mtDNA, and his Y lineage. The genetics is a bit thin, but the pedigree information is of Scandinavian quality from what I can tell. Praised the records of the Reformed Church!

The author’s utilizes an inversion of the typical method whereby a survey of a population may give some insight into individuals within that population. Rather, he leverages the thorough church records of his Afrikaner community, and his local roots, to paint a picture of his own ancestry. Then he compares the results to those of the community as a whole. Though an N of 1 certainly has limits it seems that the author concludes that he is relatively representative because some of the statistics that emerge out of pedigree analysis seem to fall in line with what genealogists working with the whole community have found. Additionally, it is clearly that he has deep roots within the historic Afrikaner nation, so assuming random mating and little population substructure, inferences from his pedigree may have some general utility.

Afrikaners apparently have some peculiarities genetically which has made them of some interest to scientists. It turns out that they seem to exhibit high frequencies of classical Mendelian diseases, a hallmark of inbreeding or population bottlenecks. This aligns well with the thesis that Afrikaners are the descendants of a small group of founders who arrived in the 17th century and entered into a long phase of demographic expansion, which culminated with their long Trek into the veld to escape English domination as well as perpetuate their practice of slavery (James Michner’s The Covenant is a fictionalization of this). As I have observed before the primacy of the “first settler” seems to loom large in the minds of demographers.

J. M. Greef, the author of the above paper, seems to refute this simple story in his own genealogy, though not the core aspect of the importance of the first founders. First the abstract:

It is often assumed that Afrikaners stem from a small number of Dutch immigrants. As a result they should be genetically homogeneous, show founder effects and be rather inbred. By disentangling my own South African pedigree, that is on average 12 generations deep, I try to quantify the genetic heritage of an Afrikaner. As much as 6% of my genes have been contributed by slaves from Africa, Madagascar and India, and a woman from China. This figure compares well to other genetic and genealogical estimates. Seventy three percent of my lineages coalesce into common founders, and I am related in excess of 10 times to 20 founder ancestors (30 times to Willem Schalk van der Merwe). Significant founder effects are thus possible. The overrepresentation of certain founder ancestors is in part explained by the fact that they had more children. This is remarkable given that they lived more than 300 years (or 12 generations) ago. DECONSTRUCT, a new program for pedigree analysis, identified 125 common ancestors in my pedigree. However, these common ancestors are so distant from myself, paths of between 16 and 25 steps in length, that my inbreeding coefficient is not unusually high (f approximately 0.0019).

Inbreeding coefficient is the probability that one’s two alleles are identical by descent. That is, they come from the same individual. For example, in the case of Elisabeth Fritzl her children have many genes where the alleles are identical by descent because half of her own genes are from her father, some many of his alleles will come back to reside within the same individual as part of a diploid pair. J. M. Greef notes that his inbreeding coefficient is about twice as high as is the norm for the typical European. Europe is a region of relatively low consanguinity, so this is a stringent reference. In some populations the inbreeding coefficient can be as high as 0.01. In short, he’s not too inbred.

That being said, the data within his pedigree do seem to show disproportionate contribution by some ancestors. This makes sense for two primary reasons. First, some component of reproductive variance is random (often modeled as a poisson distribution). Second, some component of reproductive variance is due to innate fitness (e.g., the Genghis Khan Y haplotype may be a case of this). Equality of contribution just isn’t in the cards.

Figure 2 shows the distribution of relationships within the pedigree:
nf2

Panel a illustrates that one individual is an ancestor of the author 30 times over! Many individuals are ancestors only once. Panel b shows relatedness, and again, some individuals are much closer to the author than others, with a skewed distribution. Panel c shows the number of generations between the ancestor and the author. The median number is well above ten generations, so the author has deep roots in South Africa. Finally, panel d shows the number of steps between his parents for any given ancestor. Because the author’s parents are both Afrikaners they share many common ancestors, but the steps between seem relatively large, and confirms that the author is not particularly inbred (if the parents were first cousins naturally there would be much shorter steps to common ancestors). It is clear disproportionate amount of J. M. Greef’s genes come from early settlers. This makes sense insofar as demographic expansion was likely front loaded, with later settlers having less of a chance to make an impact on an already large population.

The following table shows the contribution by various European and non-European groups to the author’s ancestry, as well as estimates for the total Afrikaner population in earlier studies on the right.

afrikgen

Note one point: only a minority of the ancestry of the author and Afrikaners are ethnically Dutch. This is important, because it shows how culture can spread and overwhelm ancestry. The Dutch imposed their language upon the French Huguenots, and their religion upon the Germans (who I presume were mostly Lutheran if they were from northern Germany, though a minority were Reformed or Catholic surely). Obviously the Reformed Calvinist religion and Afrikaans language both have a unique stamp in South Africa, but the connection of the Afrikaners to the Netherlands remained profound rather late in history. The Prime Minister of South Africa from 1958-1966 was born in the Netherlands. And yet another fact hard to deny is that the Huguenot French component seems to have persevered to a greater extent culturally than the German. The last Afrikaner President was named F. W. de Klerk, his surname being a form of Le Clerc. Another prominent South African head of state was Daniel Francois Malan. The author observes:

It is not clear if my higher estimate of French contribution is because of a systematic mistake in Heese’s (1970) estimate, or if it is because of a quirkiness in my own ancestry. It seemed to be the case that when a lineage hit the French Huguenots it stayed in this group. It will be interesting to compare the degree of inbreeding of the early generations of Huguenots to the other early immigrants. In the light of the calculations of Heyer et al. (2005) there is an interesting possibility that the cultural inheritance of fitness may have led to a systematic bias in Afrikaners, since Huguenots tended to be more educated and trained than German emigrants who tended to be soldiers. We are currently investigating this hypothesis.

There is a joke that the Baltic possessions of the Swedish monarchy were conquered with Finnish soldiers. Similarly, the Dutch overseas colonial possessions were staffed, especially at a lower level, by the rural male population surplus of northern Germany. A great many of these, likely the vast majority, never returned home and died abroad. These men contributed greatly to the census size of the Afrikaner population during much of its history, but it seems plausible that their fitness was far lower than the established Dutch and Huguenot groups because they lacked the resources and capital to flourish in a world which was much closer to the Malthusian edge than today. Many people don’t leave descendants, and it seems plausible that these Germans were fated not to do so to a far greater extent than the Dutch and Huguenots whom they were employed to protect and serve. Because of the genetic closeness of the north German and Dutch populations (in reality, Dutch are really simply another group of north Germans who transformed their regional identity into a national one for various reasons) I doubt that more thorough genetic testing will resolve this, rather, more pedigree analysis needs to be done on other individuals. But it’s an insight into the fact that social parameters have often been crucial to fitness in the human past.

As for the non-white component, the author’s results match those of previous researchers. He confirmed the likely probability of these results by the fact that his father carries mtDNA group M, which is most diverse in India. And in fact his father’s maternal lineage does trace back to a woman who was likely an Indian slave (slave women had particular surnames indicating their origin). My previous posts on the Coloureds highlighted the large Asiatic component to their ancestry, and it looks like previous researchers ignored this and focused on the Khoisan and Bantu. They also attempted to calculate ancestry based on classical markers which were found in African populations, and are present in low frequencies in Afrikaners, but that might ignore Asian signature markers (additionally, I assume that there was some natural selection for G6PD alleles). A survey of the total genomes of Afrikaners should be able to resolve the details of their ancestry, but it seems that the Afrikaners are far more colored than white Americans, by a factor of 5, but far less than white Latin Americans like Argentineans, probably by a factor of 5.

Finally, the author was also able to assess whether his ancestors exhibited a trade between quantity and quality in terms of their optimal number of offspring. In other words, did those who favored an extreme r or K selected strategy suffer vis-a-vis those who produced a more moderate number of offspring, not too low, and not too high? The author did not find any evidence of a tradeoff, and an optimal fitness. He was careful not to generalize too much, especially in light of the fact that Dutch colonial South Africa was an atypical society in many ways. I assume that living on the frontier means not having to say you’re sorry if you breed too much or too little.

Citation: Greeff, J. (2007). Deconstructing Jaco: Genetic Heritage of an Afrikaner Annals of Human Genetics, 71 (5), 674-688 DOI: 10.1111/j.1469-1809.2007.00363.x

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