because they die!!!!
. I got to thinking about this when I saw this article titled Leaving the Wild, and Rather Liking the Change, about the emergence of an isolated tribal population into the Columbian mainstream. This caught my eye:
Though it is unclear how big the Nukak population once was, anthropologists believe that what little contact the Nukak have had with outsiders has most likely left them reduced by Western diseases, including influenza and the common cold, to which they have no natural defenses.
Read 1491 by Charles C. Mann, and you'll see that unfortunately the decimation of Amazonia is simply an epitaph of a long process that began with the arrival of Eurasian peoples. In fact, Mann appeals to recent archeological finds to suggest that the Amazon might have been more densely populated than it is today, and that it was plague which reduced the human presence until the region was "pristine." About a month ago I read Land of the Naked People by Madhusree Mukerjee, which chronicles the author's encounters with the natives of the Andaman Islands. It was a difficult read, even discounting the painful patronizing tendencies of the author. The fact is that
Indians, South Asian immigrants, are killing the Andaman Islanders with contact
. The native peoples simply do not have the immunological defenses to deal with a group of people who have spent thousands of years being sifted through the brutal selective cauldron of the Eurasian pathogen pool. Western visitors to India often have to take cocktails just to not become ill, imagine how people who have been relatively isolated for nearly 30,000 years react! The author notes that the natives of Nortn Sentinal Island, which is quarantined by the will of the Indian government and the hostility of the inhabitants seem taller than other residents of the archipelago. I don't think that there is a genetic reason for this, I suspect that they are simply healthier and so develop their full height without their body being swarmed by powerful pathogens. Additionally, the Andaman Islanders, like many traditional peoples, do not take well to the alcohol and carbohydrate diets introduced in regards to their health, though they relish them from an epicurean vantage. I've discussed MHC before, and this is probably an issue for many native peoples. A week ago I was skimming through Evolutionary Genetics: from molecules to morphology, and I stumbled upon this in the chapter on MHC:
...in isolated populations...there has been selection even in the current generations... in the Havasupai, there is a statistically significant deficiency of homozygotes at both the HLA-A and the HLA-B loci, and average of 25.8% over the two loci...other nonselective factors...do not appear to be the cause of this deviation from Hardy-Weinberg proportions.
In short, these populations that are depauperate for HLA polymorphism in the first place and are now placed in the midst of immunologically more fit people are moving through the Neo-Darwinian gauntlet. We aren't talking selection coefficients of a percent or two, we're talking powerful biases which are resulting in massive heterozygote advantage (because most populations are very polymorphic heterozygosity is the norm, only in very homogenous groups is homozygosity a large issue). Reality is known to have a heartless bias.