The take-home lesson is that wherever gene flow is impeded, no matter how geographically close, population differentiation can be recovered with dense autosomal genotype data.Really fine-scale ancestry analysis is now possible; I suspect that a combination of geography, religion, social class, language, and ethnic identification will be found to be predictive of a person's broad genetic makeup and vice versa. But, to discover these correlations, a large-scale collection of genotypic data is required.
PLoS ONE doi:10.1371/journal.pone.0004654
High Differentiation among Eight Villages in a Secluded Area of Sardinia Revealed by Genome-Wide High Density SNPs Analysis
Giorgio Pistis et al.
Abstract
To better design association studies for complex traits in isolated populations it's important to understand how history and isolation moulded the genetic features of different communities. Population isolates should not “a priori” be considered homogeneous, even if the communities are not distant and part of a small region. We studied a particular area of Sardinia called Ogliastra, characterized by the presence of several distinct villages that display different history, immigration events and population size. Cultural and geographic isolation characterized the history of these communities. We determined LD parameters in 8 villages and defined population structure through high density SNPs (about 360 K) on 360 unrelated people (45 selected samples from each village). These isolates showed differences in LD values and LD map length. Five of these villages show high LD values probably due to their reduced population size and extreme isolation. High genetic differentiation among villages was detected. Moreover population structure analysis revealed a high correlation between genetic and geographic distances. Our study indicates that history, geography and biodemography have influenced the genetic features of Ogliastra communities producing differences in LD and population structure. All these data demonstrate that we can consider each village an isolate with specific characteristics. We suggest that, in order to optimize the study design of complex traits, a thorough characterization of genetic features is useful to identify the presence of sub-populations and stratification within genetic isolates.
Link
14 comments:
The take-home lesson is that wherever gene flow is impeded, no matter how geographically close, population differentiation can be recovered with dense autosomal genotype data.
Yes, you are surely right on this.
And almost certainly the same sort of differentiation has been occurring as long as we have been Homo sapiens. In fact long before then, because it also happens in other species. That's why we can classify them into subspecies.
Just hope you're not advocating for "subspecies" like "Homo sapiens urzuleinse", endemic of the village of Urzulei, etc.
The real problem is that we cannot really divide species into subspecies with any rigor. Even the boundary between species has become blurrier lately.
i must say i didnt know we sardinians were so freaking weird genetically ! what are we? aliens or something LOL! my parents were from quartu sant'elena in cagliari province i wonder why that town isnt listed here?
Sardinians show up as very especial (in the European context) since the first genetic population analysis of the 90s or so. There is some founder effect in the island, surely Neolithic, and low later input, that makes it look different - albeit always in the Euromediterranean context.
Founder effect plus relative isolation make distinct clusters. Sardinia was only colonized in the Neolithic with people arrived from central Italy but with ultimate links to the Western Balcans (Cardium Pottery Neolithic). Seemingly these colonists left a strong founding signal that is still dominant (more surely in the interior than in the coasts).
Anyhow, what this paper is analyzing is genetic clustering within Sardinians. Probably similar results could be obtained in other areas, especially those where geography splits the territory in small rather distinct units.
"Even the boundary between species has become blurrier lately".
Quite. And yet there are still those amoung us who insist that modern humans and Neanderthals, for example, were completely different species.
Well, it seems so. From archaeology it would seem that our evolutionary distance is comparable to that of tigers and lions. We agree that tigers and lions are different species, right?
That is an exaggeration of the distance between humans and Neanderthals as far as I'm aware. Have you got a reference? I was fairly sure the difference between MHs and Ns was less even than that between cattle and bison. These two species interbreed fairly readily.
Can you mention two wild species, please? Domestic expecies can get really confusing.
Anyhow B. primigenius and Bison spp. (don't believe they are two species at all) look extremely alike and have shared enviroment for many many milennia, probably never fully diverging totally.
Instead Neanderthals and AMHs evolved separately in Europe and Africa for maybe as much as a million years (that's what comes from the fossil record, even if geneticists underestimate the distance).
Neanderthals and AMHs are similar more because of convergent evolution towards bigger brains than because of any other particular reason. We share one ancestor: H. erectus and the divergence began some 1 million years ago, when our common ancestor made the first OOA epic voyage. In that thime there was no "globalization" that could stop the divergence: isolated populations evolved separately, albeit somewhat convergently in the main aspect: bigger and more efficient brains. They became different human species through hundred thousand years and, while Asian Homo spp. don't appear to have reached really competitive levels, both H. sapiens and H. neanderthalensis did.
"Can you mention two wild species, please?"
On what possible grounds do you claim that bison and cattle simply became separated when cattle were domesticated? Surely you realise cattle were domesticated long after the separation, and you're aware that wild cattle survived in Eastern Europe until around 300 years ago. Anyway, if you want other wild species that produce fertile hybrid offspring here's a short list of just some of those we're aware of. Red deer and sika, wolves and coyotes, mule deer and white-tailed deer, various species of wild cattle, cats. And that's just mammals. We can arrange a very long list if we include birds, amphibians and fish, all as easily distinguished from each other genetically as are modern and Neanderthal humans.
"Instead Neanderthals and AMHs evolved separately in Europe and Africa for maybe as much as a million years (that's what comes from the fossil record, even if geneticists underestimate the distance)".
Talk about a selective look at the evidence! But you seem able to accept that, like many of the examples above, they diversified as regional varieties of the same, originally widespread species. The process is simply a continuation of that covered in this post of Dienekes: diversification over time through isolation, for whatever reason.
"Neanderthals and AMHs are similar more because of convergent evolution towards bigger brains than because of any other particular reason".
For various reasons convergent evolution is unlikely to be the explanation. They are unlikely to have been exposed to the same ecological conditions for a start. Periodic gene flow is a much more plausible explanation. Especially seeing that, as you point out, "Asian Homo spp. don't appear to have reached really competitive levels". In other words Asian Homo didn't participate in any periodic gene flow, or to a very limited extent.
"isolated populations evolved separately".
Exactly as have all other species.
"albeit somewhat convergently in the main aspect".
Separate evolution leads to diversification, not change in a single direction.
"bigger and more efficient brains".
You seem obsessed with 'bigger brains'. Do you regard yourself as being an intellectual?
I don't say that Neanderthal and Sapiens could not breed succesfully (even if for some reason it seems they did not in fact).
But the examples you mention I can recognize imply species that share same geography. One can perfectly argue that wolf and coyote never fully diverged, as they were all the time introgressing each other because of simlar geography and econiches. Same for Bos primigenius and Bison.
On the other hand tigers and lions, which can breed semi-succesfully only, have diverged for a relatively short time (1 or 2 million years) yet they occupy very different ecological niches and have very different mating behaviours. Hence you never see ligers in the wild, only in zoos.
Similarly Neanderthals and Humans may have not met each other often after divergence, making interspecific breeding less likely.
For various reasons convergent evolution is unlikely to be the explanation. They are unlikely to have been exposed to the same ecological conditions for a start.
We and our Neanderthal cousins are versatile opportunist species (and hence very succesfull, just like seagulls, rats and roaches). Our ability to exploit any sort of opportunity is largery derived from our intelligence, hence, independently of the specifics of each ecological niche, greater inteligence will always offer us greater chances of success.
In Darwinian terms: the dumb tend much more often to die without effective offspring than the smart ones, whatever the circumstances (except maybe in hierarchical conditions, where submissiveness is more valued than intelligence, which can be considered even dangerous for the system). In the long run, in Paleolithic conditions in general, this tends to improve the cognitive abilities of the species overall.
"isolated populations evolved separately".
Exactly as have all other species.
Not all species diverge because of isolation. You cannot probably explain coyotes' and wolves' divergence because of isolation but you may be able to explain because of diverging psychology and social behaviour, as well as some related divergence in the type of prey available to each species. Basically wolves are more rigidly social than coyotes and that's about it. The divergence is not surely complete, much like the divergence between wolf and dog was never really finished.
Separate evolution leads to diversification, not change in a single direction.
Why?
Separate evolution can perfectly be convergent. In fact often totally unrelated species converge into similar models for exploiing similar niches. Like canids and Tasmanian tigers.
It's obvious that one of the crucial assets that the Homo genus had was intelligence and you see Homo spp. evolving everywhere in that same direction: with ever larger brains (and presumably improved cognition). Neanderthal and Sapiens were the summit of such intellectualist trend and hence the only meaningful survivors in the long run (Homo floresensis allowing).
They diverged in other aspects maybe, like body constitution, foraging strategies maybe, specific shape of the head... but in the general trend towards bigger and better brains they evolved in parallel.
You seem obsessed with 'bigger brains'. Do you regard yourself as being an intellectual?
No to the second and no to the first.
In general big heads enclose big brains and this size appears to have a strong correlation with actual intelligence. It is certainly not the only factor but Homo erectus and their descendants tended to have bigger and bigger heads, starting with sizes only slightly larger than those of chimpanzees and ending in sizes apparently somewhat larger than the modern average in H. neanderthalensis and some fossil H. sapiens (as well as some modern humans too). The volume of our heads multiplied by three in 2 mllion years of human evolution, and that is obviously related to a general increase in our cognitive abilities.
Of course it's not just a matter of size but also of proper wiring but still there is a strong correlation between head size and IQ within each gender and, in general, among animals, between brain size in proportion to body size and apparent intelligence as well.
It's not any obsession of mine but an evolutive fact that can't be ignored.
Regarding wolves:
http://www.searchingwolf.com/wevolve.htm
From the chart:
"There was a branching 1 - 2 million years ago from a common ancestor of gray wolves, the eastern North American wolves, and coyote. One of the branches migrated to Eurasia and there gave rise to the gray wolf. The other branch remained in North America and 150,000 - 300,000 years ago branched into the ancestor of the eastern North American wolves and the coyote".
So far so good. But, in the Early Pleistocene "Several kinds of wolves are found in Europe". And the domestic dog descends from one of them. So, it separated from the coyote line around 1.5 million years ago, yet readily hybridises.
"In fact often totally unrelated species converge into similar models for exploiting similar niches. Like canids and Tasmanian tigers".
Exactly. The same ecology. Modern humans and Neanderthals existed in completely different ecologies. Your explanation for their similar brain evolution is completely your own theory adjusted to fit another of your own theories which, in turn has been adjusted to fit yet another of your theories. If your idea is at all correct it would be the only example from all of biology where different ecologies have given rise to convergent evolution.
You may have noticed in the wolf chart how species have moved around almost continuously. Why do you have so much difficulty accepting that ancient humans did so as well? Surely that is the Occam's razor explanation for brain evolution.
The same ecology. Modern humans and Neanderthals existed in completely different ecologies.
Wolves and tasmanian tigers did too. You can't compare Australia with Siberia, can you?
What it seems is tahtr regardles of the specific niche grater inteligence is always beneficial and hence co-evolution happened in that same direction. In fact also happened in other unrelated animals like cetaceans or elephants. Overall mammals are almost always brainier than reptiles... there's a trend and that trend is espeially marked among groups that are opportunistic, that feed on a variety resources and that have relatively little to fear from predators.
You may have noticed in the wolf chart how species have moved around almost continuously. Why do you have so much difficulty accepting that ancient humans did so as well?
I don't see anything clear in that table. They are speculating the same we are with Homo spp.
Anyhow wolves move much more than we humans do normally. And Neanderthals were even less mobile.
Whatever the case I do have a problem with claims based on no evidence, exactly the same I have a problem with the Book of Genesis.
Humans migrated, there is evidence that they did both archaeological and genetical. They did since the time of H. erectus (at least). So I have no problem re. humans migrating and expanding into more or less open niches. What I have a problem is with unlikely claims based on thin air: with migrations that you claim existed but as far as I can see are just a product of your imagination, not of any data.
Why don't you stop asking me generalistic questions and start defending your hypothesis with hard data? Why are you even trying to persuade me, not of anything in particular (because sincerely, at this point I really can't recall anymore which is your hypothesis, if any) but of generalistic blanket affirmations like "humans migrate"?
Yah, I agree: humans sometimes migrate. So what?
"I agree: humans sometimes migrate. So what?"
That has been the main process by which they, and virtually every other species, has evolved.
Post a Comment