Genetic Clusters and Shared Ancestry

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As someone who knows very little about genetics, I've always wondered how much shared ancestry is needed for genetic cluster to form? With groups like Finns and Ashkenazi Jews we know that those genetic clusters formed due them deriving most of their ancestry from just a few hundred people and presumably have reach identical ancestor points among themselves. Is that level of founder effects needed for clusters to form? For example 23andme has the Scandinavian cluster with Scandinavians on average being about 60-70% Scandinavian if I remember right. Does that imply that Scandinavians get most of their ancestry from a small founder population, or a mixture of a small set of founder effects that combined together form the Scandinavian cluster? I'm wondering if that's how clusters in these genetic tests form or am I misunderstanding something? Any information would be greatly appreciated.
 
A cluster is a group of genetically similar people. This is achieved by generations of mixing.

A level of isolation is needed to mark a clear boundary between clusters. In Europe, this has been achieved by a lack of migration in rural populations from early Middle Age until the industrial age.
 
I get that, but I'm curious how that degree of genetic similarity between that large groups of people emerged if rural areas mostly just mated amongst themselves. For example in the Discovery of France, the author's sources shows how much French peasants didn't want to marry spouses more than a few miles away from them, even considering someone 10 miles away a foreigner to be mistrusted. More defined barriers to mating could be found with some areas forbidding marriages outside of the village or parish. I think, though I may be wrong, that until the last few centuries most of Europe had similar taboos or preferences when it came to marriage partners. To use the Scandinavian example again , is the genetic similarity that all Scandinavians have amongst themselves due to a continuous mating market so to speak with little to taboos on the geographic origin of a spouse, unlike in the French examples? Or is the genetic similarity due to population histories more like the Finns or Ashkenazi Jews where the population derives most of it's ancestry from one or just a few few founding population events that had enough population growth to spread throughout all of Scandinavia?
 
Both clusters and homogeneity can be detected at many levels. To follow your examples, Finns stand out as a cluster in relation to other countries, but there are also clusters within the country, depending on how closely you look.

See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5633394/

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The auhors detect even finer structure but contrast this to the findings on the UK:

”When we follow the more detailed tree to its 52 leaves (Figure S3 in File S1), these four regions (Kainuu, SE Finland, N Finland, and SW Finland) split into 11 (178 individuals), 18 (427), 8 (123), and 15 (314) populations, respectively. Hence, we observe fine-scale population structure across the whole of Finland, which is in contrast to an FS analysis of the UK of the late 1800s (Leslie et al. 2015) where a large unstructured population covered a major part of the country. However, we note that in TVD-tree, the southwest corner of Finland, which has been permanently inhabited the longest (Jutikkala 1933), is the last region to split into smaller parts both in Figure 4B and in the TVD-tree of all 52 populations (Figure S3 in File S1).”

So, one can make conclusions about the population history of a region by looking at which level there is any detectable clustering. When you zoom out, the clusters would merge and stand out from other regions, based on more ancient migrations. Finland and Eastern Baltic are more separate due to an Uralic compenent.

People belonging to groups like Jews or Roma have been isolated on different terms, of course.

I think that your question on small founder populations should be true as well, considering that populations in most places have been growing more through breeding than migration, and at the same time diseases have regularly wiped out whole villages and cities (i.e. ancestral lines).
 
I get that, but I'm curious how that degree of genetic similarity between that large groups of people emerged if rural areas mostly just mated amongst themselves. For example in the Discovery of France, the author's sources shows how much French peasants didn't want to marry spouses more than a few miles away from them, even considering someone 10 miles away a foreigner to be mistrusted. More defined barriers to mating could be found with some areas forbidding marriages outside of the village or parish. I think, though I may be wrong, that until the last few centuries most of Europe had similar taboos or preferences when it came to marriage partners. To use the Scandinavian example again , is the genetic similarity that all Scandinavians have amongst themselves due to a continuous mating market so to speak with little to taboos on the geographic origin of a spouse, unlike in the French examples? Or is the genetic similarity due to population histories more like the Finns or Ashkenazi Jews where the population derives most of it's ancestry from one or just a few few founding population events that had enough population growth to spread throughout all of Scandinavia?

I think traveler addresses the general topic very well.

If I may interject, mating patterns will vary in many ways, meaning more or less structure, depending on location, i.e. geography, land use, economic structure, religion, social mores etc.

Rural areas are going to have less variation than urban areas. In rural Italy, a common saying was "moglie e buoi dai paesi tuoi". Wives and oxen from your own village. That way you won't get fooled; you know their history and that of their families going back generations. There's something to be said for that.

If your country is very mountainous, as is the case with Italy and Greece, for example, you're going to get variation geographically. Isolated mountain villages in the Emilian Appennines in Italy formed the basis for Luigi Cavalli Sforza's monumental work on genetic drift.

Different migration waves hitting different areas even if only by degree will have the same effect. I think that applies to different areas of Finland as well as countries like Italy and Spain and Greece. For Italy, even if you zoom out to the level of most PCAs, there's so much genetic variation in Italy that it spans a distance almost as large as Europe as a whole.

The coming of the industrial revolution further scrambled European genes and global commerce continues that trend today.

England is so unstructured party because it was the first country to intensely industrialize, which usually results in the movement of people to different parts of the country. Of course, some countries resist it anyway, because familial bonds and bonds to one's own area are extremely strong. That's true at least in my part of Italy. I know many people who maintain a one room studio apartment in Milano, or Genova, or Torino, or even Rome for work purposes, try to get flex hours where they work long hours on Mon-Thurs, and then come "home" late Thursday evening or early Friday morning. So, it depends partly on the cultural milieu.

I'm a perfect example. I'm still homesick decades after we came here. Had I been the one making the decision we would never have left, no matter the economic consequences. I would have stayed right where we were, right in our ancestral area of Italy, like the mussels clinging to the rocks in the Bay of La Spezia. :)
 
I suppose what I'm asking is if founder effects can inferred in, for a lack of a better term, "normal" European populations. I.E. not unique linguistic regions like Finns or island populations such as the Sardinians. I'm talking about populations like Mainland Italians, Scandinavians, and Greeks. As you noted Italians have a lot of genetic structure due their geographic and cultural history, but from what I understand most ancestry calculators can still find a general "Italian cluster". I'm less interested about clusters involving small regions that I am in how large clusters like the general Italian cluster, general all encompassing Scandinavian cluster, or the Northwest European cluster formed. In the case of the Northwest Euro cluster, do geneticists infer that Northwest Euro descendant peoples descend from shared founder effects, maybe not as severe as the Finns, but comparable in size. For example if this idea of mine is right, I would be imagining that the lBK culture and Michelsberg culture, from which most of the Farmer dna in North/Central Europe comes from, originated from one or two founder effects of just a few thousand individuals each, just a little larger in size compared to the founding population of the Finns that would grow to fill the area and split into different tribes/clans that would mostly marry amongst themselves. And that similar founder effects could perhaps have been the origin of the Corded Ware/Central Euro Bell Beaker cultures, with that being the explanation for why a Northwest Euro Cluster can be detected, since Northwest Europeans would derive 90 plus percent of their ancestry from the same 5 or so founder effects? Of course I could be completely wrong and there was more mixing between tribes, clans, rural areas, and chiefdoms in pre-modern Europe than I am imagining or that populations couldn't have grown as fast as I am speculating.
 
I suppose what I'm asking is if founder effects can inferred in, for a lack of a better term, "normal" European populations. I.E. not unique linguistic regions like Finns or island populations such as the Sardinians. I'm talking about populations like Mainland Italians, Scandinavians, and Greeks. As you noted Italians have a lot of genetic structure due their geographic and cultural history, but from what I understand most ancestry calculators can still find a general "Italian cluster". I'm less interested about clusters involving small regions that I am in how large clusters like the general Italian cluster, general all encompassing Scandinavian cluster, or the Northwest European cluster formed. In the case of the Northwest Euro cluster, do geneticists infer that Northwest Euro descendant peoples descend from shared founder effects, maybe not as severe as the Finns, but comparable in size. For example if this idea of mine is right, I would be imagining that the lBK culture and Michelsberg culture, from which most of the Farmer dna in North/Central Europe comes from, originated from one or two founder effects of just a few thousand individuals each, just a little larger in size compared to the founding population of the Finns that would grow to fill the area and split into different tribes/clans that would mostly marry amongst themselves. And that similar founder effects could perhaps have been the origin of the Corded Ware/Central Euro Bell Beaker cultures, with that being the explanation for why a Northwest Euro Cluster can be detected, since Northwest Europeans would derive 90 plus percent of their ancestry from the same 5 or so founder effects? Of course I could be completely wrong and there was more mixing between tribes, clans, rural areas, and chiefdoms in pre-modern Europe than I am imagining or that populations couldn't have grown as fast as I am speculating.

Sorry, but imo you're wrong.

For one thing, I'm leaning towards the idea that Corded Ware and the Beakers, for example, formed as a mixture of people from the steppe with Copper Age farmers from cultures like Globular Amphora and Cucuteni-Tripolye, with minimal input from farming cultures further west and south.

One group of Beakers, from my interpretation of the relevant papers, especially those by Casey, moved rather quickly from North/Central Europe to Britain. The resulting inhabitants were at most about 10% local farmer partly because there had been a major climate change caused population crash in Britain shortly before the arrival of the Beaker people.

The Celtic speaking population was then invaded by people from the areas of the Netherlands, northern Germany, etc., the "Anglo-Saxons". These people were heavily Corded Ware descended I would think, with perhaps some Beaker, and with perhaps local admixture, yet although Northern European like the Celtic speakers of Britain, they can be distinguished with no difficulty from the prior inhabitants of Britain to the extent that the admixture can be measured rather precisely going east to west, with the majority percentage being about 1/3 if I recall correctly.

Next we have the "Northmen", both as settlers and Vikings, to the extent that almost a majority of England was called the "Danelaw". Whether they were all that different from the Anglo-Saxons I don't know, as I don't specialize in British genetic history.

What I do know is that a recent paper indicated that there was a "southern" shift in the English population since that time, with the trail leading to France. A similar thing happened to the Scandinavian population, and doubtless to Germans, as can be seen in the difference between Southern Germans/Austrians and Northern Germans, for example.

It was only with the Industrial Revolution, imo, that the regional differences in Britain were somewhat blurred, creating more of a distinct "cluster". At the same time, any decent PCA will show you that people from eastern England and people from the Netherlands are very close genetically, as are Yorkshiremen to Danes.

All countries are a mixture of different strains, more or less amalgamated depending on the location and history of an area.

Italy is far less "amalgamated" than any northern European country. Yes, it's true that you can still recognize the cluster formed by Italians, largely because, imo, there is a large Neolithic farmer cluster which covers the entire mainland of Italy and also because the later migrations to Italian affected the whole mainland, although with the differing intensity which creates the long genetic space held by Italians.

That difference between North and South, with almost a break in the cline south of Rome, is because of the differing effect of the migrations to the north and south of that line. The migration from the southeast starting in the Bronze Age hit southern Italy first and more intensely, although that ancestry definitely diffused northward with significant impact. The Italic and Celtic migrations moved into Italy from the north and diffused southward, with no Celtic influence of any importance south of Rome to my knowledge. As a result, after all the northern and central Italian regions, I am closer autosomally to northern Balkan populations like Albanians and Bulgarians and Romanians, and then to Spanish populations than I am to far southern Italians.

That has all started to change, of course, with the migration from south to north starting in the 1960s, a change to which I have contributed, as I married someone of Neapolitan and Calabrian descent. :)

I would suggest that you zoom out when looking at autosomal differences in Europe and indeed the world. Everything depends on the lens you use. If you look at a PCA of the world's populations, all Europeans merge into one cluster. The differences at that scale are inconsequential.
 
If I'm understanding you correctly, are you suggesting that most large clusters withing Europe such as the general Italian or Northwest Europe clusters exist simply due to people in those regions having similar levels of ancestry from older populations and not due to any unique founder effect that distinguishes these populations from the rest of Europe?
 
If I'm understanding you correctly, are you suggesting that most large clusters withing Europe such as the general Italian or Northwest Europe clusters exist simply due to people in those regions having similar levels of ancestry from older populations and not due to any unique founder effect that distinguishes these populations from the rest of Europe?

Yes, that's exactly what the papers show, imo.
 

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