In the case of Amerindians there has long been the model whereby the ancient Beringian population which expanded into the New World had a component of ancestry which was closer to West Eurasia. This is true today among Siberian groups such as Yakuts, but differentiating the more recent introgression of Russian ancestry with an ancient West Eurasian substratum is difficult. But admixture is surely part of the puzzle too. Compare the linear topology of Aboriginals and Amerindians with African Americans.
The PCA plot is putting the focus on between population differences, so these sorts of distributions, so cleanly linear, are indicative of possible recent admixture between two distinct populations. The populations of the New World and Australia were relatively small and thin in terms of distribution, so it should not be too surprising that a substantial uptake of European ancestry has occurred in both cases.
The film Rabbit-Proof Fench was a dramatization of the reality that quite often children of mixed heritage will identify with one culture and parent. This is not an atypical disjunction between genes and culture in terms of their mode of inheritance. Both parents contribute equally genetics to the autosome, but cultural contribution is more of a contingent matter. To further explore the admixture within Aboriginals the authors performed a frappe and Structure analysis.
These two methods differ in the details but perform basically the same operation; they take individuals and assign components of their genome to K putative ancestral groups. Again, proper caution is warranted with these methods because without context we may not be able to interpret the results. But in this case the ends are clear and distinct: what is the extent and range of European ancestry in this Australian Aboriginal sample?
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Both the frappe and Structure programs paralleled each other in outcome; the Aboriginal sample varied quite a bit in ancestral quanta. The Papuans and Melanesians serve as appropriate Oceanian references. The Melanesians have a residual component shaded orange and yellow top to bottom which is similar to East Asians.
This is a marker of the Austronesian expansion into the Pacific. The Papuans and Aboriginals generally lack this, which stands to reason considering their greater cultural isolation from the Austronesians. The frappe and Structure results dovetail perfectly with the PCA plots.
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Both suggest that the Aboriginal population is admixed, with the parental populations being West Eurasian European and Oceanian, and, that that admixture varies from individual to individual. The F st also suggested this, though at a coarser population wide scale, and only with prior knowledge of the possibility of admixture. Not only that, but the mtDNA and Y chromosomal results on these individuals also comes out to the same inferred proportion.
Recall that there were only 8 males, so the Y sample is small. The proportional of autosomal ancestry inferred from these uniparental markers is about what was calculated with their SNP-chip, suggesting the persistence of this sex-biased admixture pattern over the past two centuries. Remember that if something more complex demographically had occurred we may not have been able to infer admixture from uniparental lineages.
Imagine if the Aboriginal tribes in New South Wales which were admixed were decimated by an unadmixed group, so that only the females from the admixed group survived. After that event both mtDNA and Y chromosomal lineages would have been Aboriginal, but the European ancestry would persist in the autosome.
To the left is an estimate of individual-to-individual ancestral quanta. There is clearly a wide range of variation in admixture. The authors do remark that their estimates are probably low balling the Aboriginal ancestral quantum; the SNP-chip was constructed with European genetic variation as a baseline, so it is missing Australian variation on loci where Europeans are monomorphic.
Many Australian whites derive from recent waves of migration and may not have any ancestors with roots back to the 19th century i. But that must be balanced against the fact that Australian Aboriginals are much more European in ancestry than Brazilian Amerindians likely are. Rather, many people of mixed heritage are identifying as indigenous. But as I have observed many a time, one of the primary reasons for this sort of population analysis is to clarify background parameters for medical genetics.
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The life expectancy gap between indigenous and non-indigenous Australians is on the order of 10 years. It also seems plausible that the same disease-driven population crash which occurred in the New World after contact with Europeans was also driving population changes in Australia around the time of European settlement.
Since Aboriginals were generally hunter-gatherer groups many infectious diseases which required higher densities could only be incubated among Europeans. Aboriginals near European settlements, or those who settled within them, would naturally be exposed to these infections and suffer greater morbidity and mortality. As noted in the paper admixed populations present both pitfalls and opportunities when it comes to elucidating risk alleles:.
Whereas the admixture present in the AuR [the Riverina Aboriginals -Razib] sample presents a potential challenge in conducting traditional association methods for disease gene discovery…it opens the possibility of using admixture mapping…Admixture mapping is most suitable for traits, like CKD, that differ in frequency between the two parental populations of an admixture group.
The approach essentially looks for genomic regions with an excess of higher-risk population ancestry relative to other regions or controls.. A set of markers, spread across the genome, that are highly informative as to ancestry ancestry informative markers or AIMs is an essential requirement for admixture mapping. The genomic region which rs is embedded within has been implicated in diabetic nephropathy, a major cause of CKD. Derived here means that the SNP is evolutionarily novel in relation to the ancestral state, which all other human populations exhibit. So why is this derived variant at a high frequency in Aboriginals and other Oceanians , but not other human populations?
It could be random genetic drift. As populations migrated out of Africa they may have gone through bottlenecks and isolations in a step-wise fashion and each group down the spatial and temporal sequence would accumulate their own unique variants. Or, it may have been adaptation, which drive up frequencies through positive selection around that genomic region. But this is where limitations of sample size and representativeness crop up, as these are not quite the coarse questions which we were focusing on earlier.
From the the text:. Differentiation could also be explained by natural selection. The presence of several highly differentiated SNPs in the region, spanning nearly 0. However, it is difficult to distinguish between possible explanations because the sample is small, with extensive admixture hampering phasing and direct investigation of linkage-disequilibrium-based selection signals.
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Note however the presence of the derived allele in Papuans and Aboriginals, but not Melanesians. In the phylogenetic network the Aboriginals are the outgroup, and yet on this character the Melanesians are. This is a locus which will no doubt be explored in the future, because the patterns here will be fascinating to tease apart, and, of possible medical relevance. Overall this paper has confirmed much of what we know, or at least solidified our background assumptions.
The contemporary peoples of Melanesia, New Guinea and Australia have a common ancestral heritage. Coalescence times back to the last common ancestor between these populations and non-Oceanian groups suggest that their residence in their current locations is antique, and possibly back to the first settlement. Parts of southern Australia and Tasmania may have an equitable climate where ancient DNA samples may have been preserved, so that could resolve the issues with more certainty in the future.
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The Australian Aboriginals were aware of agriculture because of contacts with the peoples of the Torres Straits, but they never seem to have adopted it. In contrast to Australia the highlands of New Guinea developed a relatively high population density with the spread of a gardening mode of production. A comparison in allele frequencies between these two populations would then be instructive as to the power of drift and selection to drive evolutionary change over 10, years.
Obviously there needs to be more work done, and extrapolating from one sample will not do. The north coast Aboriginals were certainly in contact with sailors from Southeast Asia before Europeans arrived, and they speak a different group of languages from those in the rest of the continent. The existence of tribes with non-trivial numbers of blonde individuals in the western deserts despite no other apparent European admixture also demands to be explored.
From what Joseph Birdsell documented about the Mendelian inheritance patterns of blondism among these tribes it seems likely that the genetic architecture is very different from that in Europeans. Also see Dreamtime surf quality by season and Dreamtime surf quality by month. Upload your photo of Dreamtime. Use the tab navigation above to view Dreamtime sea temperature, Dreamtime photographs, Dreamtime detailed swell predictions, wind and weather forecasts, Dreamtime webcams, current wind from NSW - North Coast live weather stations and Dreamtime tide predictions.
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