Also why did you implicitly assume that extensive mixed populations are always the result of extensive mixing between groups?
They could simply have higher fitness! Introgression does happen, also things like hybrid vigour or outbreeding depression might make a reasonable size mixed population more or less prominent (and desirable as mates) as the generations go on.
I don’t understand. All of those (except Introgression I suppose) are just additional reasons why the groups would mix. That’s not suggesting that extensive mixed populations are sometimes not the result of mixing between groups.
Actually it is introgression that is an excellent reason to mix if you want to maximise genetic fitness (because of kin selection effects if your group gets some new genes), you just don’t need a whole lot of mixing to acheive it.
And hybrid vigour is a reason in its favour if you want to just make people with neat traits, while outbreeding depression is a reason against.
The effects I mention as colourfully illustrated by Oligopsony change the overall effect of mixing. Under different selective pressures (which may well be caused by dominant cultural preference for visible phenotype!) group A and B may mix at about the same rate in all universes, yet in one universe group A may be 80% of the population (with a few introgressed genes from B) several generations later, while under a different set of pressures it may be 5% of the population, and an AB hybrid could be anything from 1% to 90%. Indeed mixing would not nesecarilly be created equal, it is perfectly possible that 400 years later, even if marriage between the groups was symmetrical 95% of Y chromosomes are variants that group B possessed and 80% of the mDNA is that which was possessed by A. It is even possible that group A and B both receive significant amount of admixture, but differing selective pressures (due to class or mountain people vs. costal people) create two or more coexisting groups say AAB and BBA or BAB.
Also even in the absence of any fitness advantage very small groups can get their genetic imprint wiped out eventually.
If you are doubtful that this has ever happened with humans, you are quite flatly wrong. Modern Tibetans are genetically basically Chinese farmers who swept over the region a few thousand years ago, but they picked up useful altitude adaptations from the pre-existant people who weren’t farmers. Middle Eastern farmers to a large extent replaced Southern and Western European hunter gatherers genetically in several waves in the past few thousand years. Had the Andeans people in America not have been an agricultural people with the demographic numbers that that implies I’m pretty sure they would have shared the ancient Tibetan’s fate and you would have some odd looking white and perhaps black people inhabiting the Andean mountains today. To give another example modern humans swept aside Neanderthals in Eurasia but Eurasian humans picked up from 1 to 4% of their DNA from the Neanderthals, that was most likley adaptive to that environment.
Depends on how you define die out. They didn’t leave a genetic mark much beyond those specific adaptations. Generally speaking waves of farmer expansion seem to be more actual literal expansions than cultural diffusions of farming techniques, despite some mDNA sometimes sticking around from the previous hunter gatherer populations.
Pre-farming people so clearly do or did intermix with farmers, perhaps they culturally assimilated or perhaps their women and men where just enslaved, who knows, but both their small numbers and a probable lack of some adaptations puts them at a disadvantage when it comes to leaving a genetic mark in the long run.
Culturally they are today pretty much extinct and replaced by modern “NeoTibetans”, which ironically may also be culturally swapped by Chinese eventually, but it is hard to say what their culture was several thousand years earlier. Its perfectly possible that the expanding farmers adopted a lot of cultural influence that however got lost in the long march until reliable written records first show up.
Note: It was only later proposed that Tibetan adaptations are “better” (or rather different ) than Andean adaptations because they had more time to evolve, in the previous inhabitants of Tibet, since we know from archaeological evidence peoples have lived there for far longer than just the past few thousand years.
Also Razib Khan on the Gene Expression blog writes a lot about the interplay of history and genetics (genes can document population or cultural shifts and help us build a better picture of history, but they can also be the causes of such shifts—say the expansion of lactose tolerance in Western Eurasia or resistance of Africans/Europeans to old world diseases that killed off many Native Americans)
The key line is “extensive” (and also, of course, “always.”) Imagine that out of a large population of Star-Bellied Sneeches and Sneeches With No Stars Upon Thars, a few marry and have Half-Starred Sneech children. Thereafter (for whatever reason) Half-Starred Sneeches tend to mate with each other, and, in part due to their greater resistance to the Great Whoville Plague, in due time grow to greater numbers than either the Star-Bellied or Blank-Belied populations. But this doesn’t produce a (biologically, rather than culturally, mediated) “reason” to mix except insofar as the original Sneeches who formed multi-ethnic families had genes that made them less ethnocentric (and that this effect continues to be produced by this gene in the new environment etc.)
Also why did you implicitly assume that extensive mixed populations are always the result of extensive mixing between groups?
They could simply have higher fitness! Introgression does happen, also things like hybrid vigour or outbreeding depression might make a reasonable size mixed population more or less prominent (and desirable as mates) as the generations go on.
I don’t understand. All of those (except Introgression I suppose) are just additional reasons why the groups would mix. That’s not suggesting that extensive mixed populations are sometimes not the result of mixing between groups.
How do those things go against my implication?
Actually it is introgression that is an excellent reason to mix if you want to maximise genetic fitness (because of kin selection effects if your group gets some new genes), you just don’t need a whole lot of mixing to acheive it.
And hybrid vigour is a reason in its favour if you want to just make people with neat traits, while outbreeding depression is a reason against.
The effects I mention as colourfully illustrated by Oligopsony change the overall effect of mixing. Under different selective pressures (which may well be caused by dominant cultural preference for visible phenotype!) group A and B may mix at about the same rate in all universes, yet in one universe group A may be 80% of the population (with a few introgressed genes from B) several generations later, while under a different set of pressures it may be 5% of the population, and an AB hybrid could be anything from 1% to 90%. Indeed mixing would not nesecarilly be created equal, it is perfectly possible that 400 years later, even if marriage between the groups was symmetrical 95% of Y chromosomes are variants that group B possessed and 80% of the mDNA is that which was possessed by A. It is even possible that group A and B both receive significant amount of admixture, but differing selective pressures (due to class or mountain people vs. costal people) create two or more coexisting groups say AAB and BBA or BAB.
Also even in the absence of any fitness advantage very small groups can get their genetic imprint wiped out eventually.
If you are doubtful that this has ever happened with humans, you are quite flatly wrong. Modern Tibetans are genetically basically Chinese farmers who swept over the region a few thousand years ago, but they picked up useful altitude adaptations from the pre-existant people who weren’t farmers. Middle Eastern farmers to a large extent replaced Southern and Western European hunter gatherers genetically in several waves in the past few thousand years. Had the Andeans people in America not have been an agricultural people with the demographic numbers that that implies I’m pretty sure they would have shared the ancient Tibetan’s fate and you would have some odd looking white and perhaps black people inhabiting the Andean mountains today. To give another example modern humans swept aside Neanderthals in Eurasia but Eurasian humans picked up from 1 to 4% of their DNA from the Neanderthals, that was most likley adaptive to that environment.
Did the ancient Tibetans die out or did they just get assimilated into the Chinese farmers?
See, now you’re just getting me curious about this stuff.
Nothing wrong with curiosity. :)
Depends on how you define die out. They didn’t leave a genetic mark much beyond those specific adaptations. Generally speaking waves of farmer expansion seem to be more actual literal expansions than cultural diffusions of farming techniques, despite some mDNA sometimes sticking around from the previous hunter gatherer populations.
Pre-farming people so clearly do or did intermix with farmers, perhaps they culturally assimilated or perhaps their women and men where just enslaved, who knows, but both their small numbers and a probable lack of some adaptations puts them at a disadvantage when it comes to leaving a genetic mark in the long run.
Culturally they are today pretty much extinct and replaced by modern “NeoTibetans”, which ironically may also be culturally swapped by Chinese eventually, but it is hard to say what their culture was several thousand years earlier. Its perfectly possible that the expanding farmers adopted a lot of cultural influence that however got lost in the long march until reliable written records first show up.
A somewhat related article in the NYT.
Note: It was only later proposed that Tibetan adaptations are “better” (or rather different ) than Andean adaptations because they had more time to evolve, in the previous inhabitants of Tibet, since we know from archaeological evidence peoples have lived there for far longer than just the past few thousand years.
Also Razib Khan on the Gene Expression blog writes a lot about the interplay of history and genetics (genes can document population or cultural shifts and help us build a better picture of history, but they can also be the causes of such shifts—say the expansion of lactose tolerance in Western Eurasia or resistance of Africans/Europeans to old world diseases that killed off many Native Americans)
The key line is “extensive” (and also, of course, “always.”) Imagine that out of a large population of Star-Bellied Sneeches and Sneeches With No Stars Upon Thars, a few marry and have Half-Starred Sneech children. Thereafter (for whatever reason) Half-Starred Sneeches tend to mate with each other, and, in part due to their greater resistance to the Great Whoville Plague, in due time grow to greater numbers than either the Star-Bellied or Blank-Belied populations. But this doesn’t produce a (biologically, rather than culturally, mediated) “reason” to mix except insofar as the original Sneeches who formed multi-ethnic families had genes that made them less ethnocentric (and that this effect continues to be produced by this gene in the new environment etc.)