Sensitive Detection of gene flow

One problem with arguing with ignorant people is that they don’t usually have indicator lights that tell you exactly how ignorant they are.  This matters when you’re trying to explain something: it’s not always clear  how much goes without saying.

About the imaginary high levels of gene flow between distant populations in the past: if that had happened, Fst between those populations would be close to zero.  It’s not. And when an inevitable consequence of a certain proposition is false, the proposition is false.

But here’s another example: falciparum malaria probably originated in Africa ( it’s closely related to a kind of malaria that infects gorillas). At some point it spread to humans, and it’s a bear – in terms of evolutionary pressures, probably the worst disease in the world.

Many different expensive genetic defenses have arisen in populations exposed to malaria. But those seen in southeast Asia/PNG are, at the molecular level, entirely different from the ones seen in  Africa. As far as I know, there is no overlap at all. You see mutations of the same gene showing up in those far-separated populations ( convergence) , but the SNPs are never the same.

The sickle-cell allele would have been advantageous in, say, PNG lowlands, and if even a few copies had ever arrived there, it would have become common rather rapidly. A smidgen of gene flow from sub-Saharan africa , even 20 individuals total, would have left a genetic signature in PNG. [ just as a few copies of Denisovan altitude-tolerance alleles were enough to transform Tibetans]  That smidgen would have grown geometrically with time: it would function as a very sensitive detector of African gene flow.  But it never happened. Contrariwise, some of the malaria defenses in PNG ( like Melanesian ovalcytosis)  would have spread widely in Africa with even a little gene flow.  But that never happened either.



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56 Responses to Sensitive Detection of gene flow

  1. catte says:

    Suppose you had a mother with the African anti-malaria allele and a father with the PNG one. Does the child get both effects, and is super-resistant to malaria?

  2. mobiuswolf says:

    Assuming everybody had it at the time?
    We’re all ignorant. ;o)

  3. Steve Sailer says:

    Here’s a map of where falciparum malaria was found in 2007:

    There are a lot of places around the periphery of the Indian Ocean where it is endemic. Looking at this map it wouldn’t seem implausible for resistance to it to have spread. Are there places outside of sub-Saharan Africa where the sickle cell mutation has spread due to positive selection for it because falciparum malaria is endemic?

    For example, in 1981 Barack Obama met the black slave of one of his college buddies on his family’s plantation in Pakistan, a country that has a modest amount of this kind of malaria. Is this mutation spreading South Asia, or is the social discrimination against the estimated several hundred thousand sub-Saharans in South Asia enough to keep it down?

    What about in the New World in places with some degree of this kind of malaria? Are people like Hugo Chavez, George Zimmerman, Albert Pujols, or Neymar Jr. more or less likely to have the mutation than by random chance?

    • gcochran9 says:
      Here’s a gene-frequency map of the HbS allele – basically for 1500. Before blacks were imported to the Americas. Even then, much of that spread was due to the [Arab] slave trade, but in malarious regions, HbS protected against falciparum malaria, was net beneficial, and spread further after introduction. In non-malarious regions, its frequency shrank with time.

      Just to complicate life, the incidence of kids with severe illness goes as the square of the gene frequency.

    • dearieme says:

      The Arab slave trade involved a lot of castrating of male slaves. Are there any genetic mutations that would behave differently if they spread only through mothers?

      • Janet says:

        Yes, of course. Anything based on mitochondrial mutations can only be spread by females, for one thing. Anything X-linked would spread differentially among females vs. males– a mutation which was beneficial in heterozygotes but dangerous in homozygotes would spread more rapidly if only females were brought in. G6PD jumps to mind (another anti-malarial trait). Another factor is age discrepancy– this is mostly young girls (early teens, sometimes less than that) being sold as concubines to older males, so traits which took time to develop would not be selected against as sharply as in “normal” societies. (There would be no equivalent to extended families or towns being known for, and avoided in marriage because of, “bad blood”, such as happened for Huntington’s Disease.)

        In general, anything that was less severe in females than males would tend to spread faster under these conditions (and the reverse, as well, of course). But you’d also have to take into account the high rates of inbreeding in traditional Arab countries– mutations are going to spread much faster anyhow in those societies.

        • dearieme says:

          ” Anything based on mitochondrial mutations can only be spread by females, for one thing. Anything X-linked would spread differentially ….”

          I had hoped someone might say something that wasn’t bleedin’ obvious.

  4. A DARC question says:

    If it were an old variant that would be the case. Out of genuine curiousity, how old is this variant? West African populations have spread across the continent fairly recently. Whats the frequency like in pops within Africa like the Khoi that are not West African but have West African gene flow fairly recently? Same, different, higher than expected on admix fraction, etc.

  5. James Thompson says:

    Could one individual have left a signature? I think you had argued that in a previous post, but cannot remember which.

    • gcochran9 says:

      Yes, although the probability is fairly low. The probability depends on how advantageous the allele in question is.

      • TB says:

        But since the advantage is all in the heterozygote, it would spread quite easily if it made it past the first generation.

        • gcochran9 says:

          It can also be lost by chance – likely, when the copy number is very small. Generally, the chance of success is 2s, when s is the selective advantage. Result from the theory of branching processes. Haldane worked this out over a hundred years ago.

  6. David Chamberlin says:

    The Reich team has shown how the Yamnaya replaced surrounding populations up to 100% all the way down to the Mediterranean back in the Bronze age. But after that partial population replacement another one occurred moving back in the opposite direction as malaria spread and genes that helped protect against it’s worst effects spread northward. Falciparium malaria as shown in the above map linked to by Steve Sailor used to be devastating in Europe all the way north well beyond Rome back before modern medicine beat it back to it’s present locations. The older history books always talked about the decline of Rome as if these killing diseases had no effect. They clearly did.

    • BB753 says:

      Do Southern Europeans still carry genes that protected against malaria to a significant extent? And were those genes imported from Africa or local? Malaria was still around in Southernmost Europe well into the last century (Spain until 1964, Greece 1974, in Italy it was never fully erradicated. As David Chamberlain has commented, malaria coincided with the fall of the Western Roman Empire in the V century AD.)

      • David Chamberlin says:

        A much clearer picture will emerge in the decades to come as to why northern populations simply died out when they tried to move back to the south where multiple diseases such as faciparium malaria existed, the crusaders for example who tried to take back Jerusalum. The tools created by the Reich group are fantastic for telling us detailed pictures of far more than population replacements. We will soon see why there are parts of ancient graveyards in Egypt no less that are comprised of blonds and redheads where none survive today. Nothing pushes human evolution harder and faster than pathogen resistance. When these deadly diseases moved into the Mediterranean from farther south those people that survived in southern Europe were given a tremendous advantage if they carried genes that protected them from the worst effects of the newly arrived diseases. Same thing happened in the Americas, those that survived the new diseases post 1492 greatly benefited from had some European decent. Those that survived in southern Europe where newly arrived diseases ravaged the local populations benefited from African ancestors who carried better genes for disease resistance.

        A blond Brad Pitt as Achilles or a blue eyed fair haired Jesus strikes one as preposterous if they represent the population that lives in those areas today. But things have changed, they could have been back then.

      • Henry Scrope says:

        I remember reading about British conscripts from the Fens of East Anglia being unfit for service in 1915 due to the endemic malaria there, can’t find a reference though.

        • Michael Kennedy says:

          The Mayo Clinic is in Minnesota because old Dr Mayo kept moving north to avoid malaria in places like St Louis.

      • Glengarry says:

        There was malaria in the Nordic countries as well. For example, last known case in Finland was in 1954.

    • Folderol says:

      “The Reich team has shown how the Yamnaya replaced surrounding populations up to 100% all the way down to the Mediterranean back in the Bronze age.”

      So far, they have shown the exact opposite with Yamnaya-related ancestry reaching only up to 50-70% even in the early steppe-related horizons of central and northern Europe. Lower in subsequent cultures due to increased admixture.

      Much less than that in Mediterranean Europe with both Bronze Age Iberia and Greece showing <20% Yamnaya-related ancestry. Similarly low levels in Iron Age Iberia (small increase) and Iron Age Greece (about the same as Bronze Age).

      • David Chamberlin says:

        The Yamnaya did replace 100% of the population living nearest to their source point in the centuries after they began expanding, there is no argument from anybody on this point. The graves of these early Yamnaya expantionists show no admixing with the first farmers. However you are correct there was partial replacement as they progressed further south and there was considerable intermixing between Yamnaya and surrounding populations later in time.

        • Folderol says:

          You should actually read these studies instead of trying to interpret whatever summaries you find wherever.

          The early steppe-derived horizons of northern Europe – Corded Ware and Bell Beaker – had substantially less than 100% of Yamnaya-related ancestry. Later Bronze and Iron Age northern cultures even less due to increased admixture with local pre-IE groups. Even the steppe itself sees a drop in Yamnaya-related ancestry as early as Potapovka, due to farmer ancestry being brought back east.

          Everything you wrote is basically wrong and your theory about malaria effecting a significant drop in Yamnaya-related ancestry just seems crazy.

          • David Chamberlin says:

            You are putting words in my mouth i didn’t come close to saying and then calling me crazy. I enjoy the internet, but it is what it is. It won’t change. Good luck Folderol. I can yammer this and you can yammer that and all we are doing in reality is petting our bloated egos. Meanwhile back at the ranch real science is rolling forward in a very interesting area we all know very little about, human prehistory. I proposed an interesting hypothesis, that isn’t crazy, but it is far from proven as well. That African ancestry increased in southern Europe after diseases that originated from Africa and possibly other equatorial regions spread north for one simple reason. The Africans had better genes that allowed them to better survive these diseases.

            We shall soon see, because new tools in science are being developed that can actually tell us what happened.


            • Its The Ships says:

              Disease may have had a helpful effect on increasing African ancestry proportions but….

              Say we proposed that disease increased the proportion (and to a far greater extent) of European ancestry in the Americans. In a sense, you’d be right. But in another deeper and more distal sense, the real explanation is simply that Europeans were able to get to the Americas. Ocean going ships.

              In the same way, African ancestry in the ME probably increases mainly simply because more Africans are able to get there (as some mix of slaves, traders, mercenaries, raiders, that depends on time and place), and that’s because of the camel and the rough “infrastructure” that supported desert caravans, and again because of ships.

            • David Chamberlin says:

              “0 to 1 percent African genes in Greeks and Southern Italians” as per Hoverica elsewhere in the thread, so the Africanization of southern Europe was relatively minor. It will be interesting to see when the data set becomes available how the genes in warmer Mediterranean Europe changed in response to the influx of diseases that came from the south. The African genes may have spread but it didn’t effect the gene pool very much. Sardinia is a very interesting place genetically being that it changed very little genetically since the first farmers arrived. Sardinians had to retreat to the mountains and leave the coast for two reasons. Once Phoenicians brought Malaria to the Island the Sardinians had to vacate the coast to the dryer highlands. Mussolini may be a villain to the rest of the world but he is a hero in Sardinia as he is credited with successfully combating Malaria. The other plague that hit Sardinia in ancient times was slave raiders coming from the coast of Africa. Sardinia has the weirdest national flag in the world in direct response to the slave raiders. It has four severed heads of Africans on it called the flag of four Moors. When slave raiders were caught their heads were cut off and placed on sticks as a warning to other slave raiders.

            • David Chamberlin says:

              Evidence implies the following. African genes are below 1 percent in southern Italy and Greece. That is not much genetic impact however there appears to be a huge impact by genes that gave even slight disease resistance advantages over many generations. Near pure bred Yamnaya that moved to the south after the arrival of Falciparum Malaria and other nasty diseases had arrived simply died out.

              What Cochran has repeatedly stressed regarding population sinks and slow but steady growth of disease resistant genes is a far more accurate picture than simply an Africanized general population increasing. When the crusaders moved to Palestine, when blonds and redheads in classical times moved to Egypt, they simply fell below replacement level because of decreased disease resistance and winked out of existence .

        • Yamnaya and the Farmers says:

          The Yamnaya have about 10-15% ancestry from EEF –

          “Ancient human genome-wide data from a 3000-year interval in the Caucasus corresponds with eco-geographic regions”.

          Using qpAdm with Globular Amphora as a proximate surrogate population, we estimated the contribution of AF ancestry into Yamnaya and other steppe groups. We find that Yamnaya Samara individuals have 13.2 ± 2.7% and Ukraine or Caucasus Yamnaya individuals 16.6 ± 2.9% AF ancestry (Fig. 4; Supplementary Table 17)—statistically indistinguishable proportions. Substituting Globular Amphora with Iberia Chalcolithic does not alter the results profoundly (Supplementary Table 18).

          The rest of Yamnaya’s ancestry is probably largely from a steppe zone somewhere north of the Caucasus mountains but largely southeast of Ukraine, but there may be a small percentage from the northern EHGs from further north at places like Samara.

          Yamnaya may have a greater overall percentage of ancestry from cultures like Sredny Stog from Ukraine which were intermediate Yamnaya and EEF/Ukrainian Hunter Gatherers.

        • epoch says:

          “The graves of these early Yamnaya expantionists show no admixing with the first farmers.”

          The Hungarian Kurgans have yielded samples which were 100% farmer, there is a CWC burial found with a 100% farmer. Bulgarian Yamnaya burials show large farmer admixture.

  7. J says:

    Brad Pitt as Achilleus is historically correct. The Achaians of the Illiad left no posterity.

    • Sardinian Achilles says:

      Achilles would probably be best represented today by a Sardinian; we find Greek Mycenaean era samples (1700–1200 BCE) to present about 13% Yamnaya ancestry, 12% “CHG” ancestry and 75% EEF ancestry – This includes samples from elite tombs, which are pretty much indistinguishable or at any rate don’t have more steppe ancestry.

      That’s rather more EEF than any Greek person today and rather less Yamnaya ancestry. Less than Southern Italians to boot (though they have more Near Eastern ancestry to boot), certainly less than Northern Italians, or Spanish or Basques. The Sardinians are fairly comparable in EEF (maybe slightly more EEF, but closer to Mycenaeans than any other modern people), but with less CHG and more Western Hunter Gatherer ancestry (and probably more of WHG’s associated Northern European skeletal traits, albeit the Sards seem to have some island dwarfing changes in their genetics that are unique to them).

      This is highly likely to be a continuous ancient Greek profile (including for the Ionians and all the rest), up until the rise of the massive Hellenistic and Roman Empires and the Slavic migrations and all the changes that came with this. I say this as we find that Greek colonists at Empuries in Northeast Spain at 500 BCE (other than some obvious Iberian locals and Gauls) to be exactly genetically the same as the Mycenaeans –

      Hirisplex prediction on the ancient Mycenaean samples matched with them being generally dark eyed and dark haired. (One of the Minoan samples, without Yamnaya ancestry, had a Hirisplex prediction that they may have been blonde, but none of the Mycenaeans was blonde, and all were predicted as dark haired).

      • J says:

        Sard: Those Archaeogenetic grave-robbers you quote are all the time coming up with new fancy theories while the Poet is immortal. Homer’s Hektor and the Trojans are black haired and the Achaian leaders and their gods are blond and bright-eyed. gcochran9 (June7, 2014) wrote: If people from village X in Sardinia look amazingly like Otzi or an LBK farmer, and almost completely lack a genetic component common in other modern Europeans, that’s the way it is.

        • Sardinian Achilles says:

          J, I daresay you will insist that, whatever genetics finds, those Japanese depicted in those manga cartoons really did have blue hair, and Goliath really was ten foot tall, and human women really did mate with fallen angels to produce the Nephilim…

          • J says:

            Sard, I do see Japanese youth with blue hair, yes, whatever genetics finds. Really. Regarding Sardinian Achilles (you?), more research is required.

            • Sardinian Achilles says:

              You’re naive if you believe that to be, as in the mangas, a product of nature…

              On the Mycenaeans, we have Mycenaean frescos which more purportedly than Homer and with no hurdles of translation and poetic flourish depict the Mycenaeans themselves (no blonde gents or ladies), and we have the archaeogenetics as discussed, and we have a classical period in which no one much talks about how the blonde barbarians they met really looked just like Achaean heroes. I would suggest there is a fairly clear most likely hypothesis here.

              (For clarity’s purposes Sardinian Achilles (you?) not a statement on my personal phenotype or ancestry, simply a random name given as “anon” creates confusing discussions.)

              • J says:

                I insist that Brad Pitt as Homer’s Achilles is correct. The history was composed generations after the events, that is, after successive inflows of Northern peoples and mixing in South West Asia. Actual Mycenaeans may have been as you say, agreed, but Homer’s Achaian heroes are as he would have liked them to be. Like Brad Pitt.

              • Sardinian Achilles says:

                If you look to argue that Homer, for whatever reason, described the appearance of the Achaeans in somewhat idealized terms as blondes, etc. (perhaps he was taken with the majesty of some Scythian warrior or other?), then I suppose that is one thing (presuming that this read of described hair colour is even right). Perhaps Brad is, then artistically accurate to Achilles.

                But it is not quite the same thing at all, as what is implied from Dave posting that “A blond Brad Pitt as Achilles or a blue eyed fair haired Jesus strikes one as preposterous if they represent the population that lives in those areas today. But things have changed, they could have been back then.” and a response that Brad Pitt as Achilleus is historically correct. The Achaians of the Illiad left no posterity.. And the genetics in no way agrees with this implication.

              • J says:

                Since the conversation has turned towards malaria, let me point out the fact that after the fall of the Ancient world, the formerly well drained and irrigated areas around the Mediterranean were abandoned and reverted to swamps. That was followed by the spread of Anopheles and depopulation and rapid spread of African genes protecting from malaria. The Fulani did not become Danish, and Mediterranean populations did not become Bantu, but they became a bit darker. Southern Italians, Greeks, Palestinians all carry African genes and most have black hair. It seems to me reasonable to assume that the frequency of blonds after thousand years of malaria would decline severely.

                Regarding Achilleus was not a historical personage; or, rather, the figure in the poem might or might not be distantly connected to a real figure, but that isn’t the point. Achilles, as we have him and as the Greeks had him, is a mythical figure and a poetic creation. So the question is not ‘What did Achilles look like?’ but ‘How does Homer portray him?’ We have only one thing to go on here: Achilles is said in the Iliad to have xanthos hair. This word is translated as ‘blond’ or “golden”.

                The BBC’s Troy: Fall of a City (2018-) which cast black actors in the roles of Achilles, Patroclus, Zeus, Aeneas … I thought Brad Pitt believable, but I do not insist, the BBC may be artistically (and politically) more correct. Let them have it.

              • Hoverica says:

                Original human hair was black. Blondes are recent and only common in a few places. There’s just 0-1% Sub-Saharan African genes in Greeks and Southern Italians. Northern and Eastern Europeans have more East Asian genes than that, so they should have more black hair than Meds according to your “reasonable assumption”.

              • Sardinian Achilles says:

                @J, genetics thus far indicates that European populations were universally darker in the past than they are today, and that detectable pigmentation variants associated with lighter eyes, hair and skin, are at higher frequency today than in the past, pretty much universally across Europe.

                Most dramatically and best studied in the north, but probably also in the south and even in the likes of Lebanon, e.g. as Khan puts it based on Lebanese dna –“Selection has continued, so that alleles for lactose tolerance and lighter skin have changed in frequency even since that period. The derived allele for SLC45A2 is found at about 2/3 frequency in modern Lebanon, but was absent in these five Sidonians. Though the sample size is small, this was somewhat surprising, and suggests that they were a swarthier people than modern Lebanese.”. Despite African admixture into Muslim Lebanese….

                Inasmuch as this is not totally speculative (selection for malaria would lead to individuals having darker hair for some reason that is unexplained and is unconnected to the introgression of African ancestry overall?), and perhaps the genetics is presenting incomplete models of phenotype, it does not back that blonde hair or any sort of light pigmentation was more frequent in populations on the northern shore of the Mediterranean in the past, or even likely its eastern or southern shores. Ancient people probably darker (excepting “poetic creations” who may well have been lighter, like the angels of Medieval art.)

      • Folderol says:

        The genetically closest populations to these guys are modern southeast Europeans – Greeks, Italians, European Jews (Ashkenazi, Sephardi etc.) who have more European than actual Jewish ancestry.

        • Sardinian Achilles says:

          Hmmm…. may be true in Fst due to stronger genetic drift in Ogliastrian Sardinians, I’m less sure of it being true net of that (Italians have much more Slavic, central European and Near Eastern ancestry than Mycenaean).

          • Hoverica says:

            Italians don’t have Slavic or Central European ancestry, only the few non-Italian-speaking minorities from the northern border regions do.

            And I don’t know what you mean by “Near Eastern”. EEF is already Near Eastern, so is the CHG/Iran that makes up half of Yamnaya. All Europeans are more Near Eastern than anything else.

            But if you mean something more recent, Italians don’t have that either, except a few % in Sicily from the Moors.

            • Sardinian Achilles says:

              North and Central Italians certainly (and Southern) have ancestry likely from Central European origin cultures like Bell Beaker which is lacking in Mycenaeans (well before we think about any Germanic related input which is unclear and low). Greeks have clear Slavic input in the mainland or else more Near Eastern input in Crete / Anatolian Greek speakers etc.

              EEF is a different kind of Near Eastern than Levant related ancestry which comes in later to some degree in Southern Italian populations in a fairly reasonable proportion and increases distance to heavily EEF populations like the Mycenaeans.

              Don’t get me wrong, Southern Italians and Greeks, net of drift in Sardinians, are still probably a close second in relationship to the Mycenaeans. But there are clearly changes and differences in their populations.

              • Hoverica says:

                Yeah, I though you meant Germanic input. Not sure I’d call Bell Beaker “Central European ancestry” though. And you originally said Italians had Slavic ancestry, not Greeks. That’s what I was claiming was false.

                Levant_N is very similar to the Anatolian_N of EEF (like Iran_N is very similar to CHG). It wouldn’t increase distance to EEF populations that much. Besides, Southern Italians aren’t shifted toward the Levant; they’re shifted toward Yamnaya (especially the CHG/Iran part).

                Sazzini (2016) modeled all Italians as “Sardinian” + “Iranian/Caucasus” + “Russian” – so EEF/Anatolian_N/Levant_N + CHG/Iran_N + EHG/ANE.

              • Sardinian Achilles says:

                Apologies, meant to write “Italian+Greek” there and got lost along the way.

                Levant_N has some Fst from Anatolia_N, PPNB to Barcin farmers about 0.022 vs CHG/Iran Neolithic to Barcin about 0.074 or WHG to Barcin about 0.103, so it’s not nothing although it is relatively compared to between other ancient groups. S Italians are shifted towards both Levant and Steppe relative to EEF groups.

              • Hoverica says:

                Southern Italians cluster with Mycenaeans between Minoans and Modern Greeks:

              • Sardinian Achilles says:

                2d dimensions only, heavily compressed. (Take the graphic that literally and you’d think Latvians were closer to EHG than Yamnaya and yet…)

              • Hoverica says:

                Well, Yamnaya are EHG + CHG, so they look about in the right place. And Latvians have other admixtures too, so they’re shifted away from Yamnaya.

                Here’s what the Mycenaean study says:

                “Mycenaeans were least differentiated from populations from Greece, Cyprus, Albania, and Italy, part of a general pattern in which Bronze Age populations broadly resembled present-day inhabitants from the same region.”

                South Italians/Sicilians also cluster with Peloponnesian Greeks, the closest descendants of Mycenaeans:

              • Sardinian Achilles says:

                Hov, I’ve already granted that, in Fst terms, Sicilians / mainland Greeks may be marginally closer than Sardinians, so we’re kind of circling at this point. But they don’t overlap them and there is still some pretty serious difference in ancestral proportions, and some population differentiation.

                Btw, for some direct data on how Sicilian folk today differ from inhabitants of the island, check out: – “The Arrival of Steppe and Iranian Related Ancestry in the Islands of the Western Mediterranean” dated March 21, 2019. This has actual ancient dna from Sicily itself… Check out Figure 4 and differences between their models for present day and ancient Sicilians from the Late Bronze Age…. (teaser – “Finally, when we model modern Sicilians, we find that they require not only Steppe and Iranian related ancestries” (in addition to EEF) “but also North African ancestry, confirming the ample historical and archaeological evidence of major cultural impacts on the island from North Africa after the Bronze Age”).

                ( history from the Neolithic to present on the Mediterranean island of Sardinia: An ancient DNA perspective dated March 21, 2019, also good on what really happened on Sardinia).

              • Hoverica says:

                Modern Sicilians have ~5% North African admixture from the Phoenicians and Moors. Other Southern Italians have almost zero. All the other components are found in the ancient individuals as well.

  8. Rev. Right says:

    ” [ just as a few copies of Denisovan altitude-tolerance alleles were enough to transform Tibetans] ”

  9. Erik Sieven says:

    I know that in South India sickle cell disease prevalence is higher among “tribal communities”. I’ve always thought that those tribal communities are the descendants of the people who had lived there long before the Indo-Aryans or even the Dravidians came. But now I think the higher sickle cell anemia prevalence implies rather recent African influx?

    • gcochran9 says:

      Not much: it implies at least a little African ancestry (if current ideas are correct and the HbS mutation happened only once) but it mainly implies that those tribals are exposed to a lot of malaria. It doesn’t take much gene flow to transmit a strongly advantageous allele: the Fulani have a fair amount of the European lactose-tolerance allele, but that’s not because they’re mostly Danish.

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