Thursday, April 19, 2007

Evolution on high   posted by Razib @ 4/19/2007 04:09:00 PM

Ann Gibbons reports on the evolution of high altitude tolerance among Tibetans presented at The American Association of Physical Anthropologists meeting last March:
Beall reported at the meeting that women with high levels of oxygen in their blood had more than twice as many surviving babies as had those with low oxygen levels--a ratio of 1:0.44. This is a startlingly strong selection pressure, she says--even stronger than that on the sickle cell gene, which protects against malaria and has a fitness ratio of 1:0.66.
But exactly how do these women manage to carry extra oxygen in their blood? They do not produce more hemoglobin the way Andeans living at high altitude do. One possibility is that the women with high oxygen have an adaptation that Beall is exploring independently in these same Tibetan villagers. She found that some villagers exhale extra nitric oxide in their breath, a sign of additional amounts of the gas in their blood. In those Tibetans, nitric oxide dilates the blood vessels so they can pump more blood and oxygen to organs and tissues, as measured by images of heart and lung blood vessels. The Tibetans can boost their blood volume--and so pump more oxygen to their tissues--without producing more hemoglobin or raising the blood pressure in their lungs.

From talking to Greg Cochran I am to understand that the Tibetan adaptations result in greater physiological fitness than the strategies in the New World. Certainly it seems that the Andeans utilize a "brute force" technique, just crank out more hemoglobin. The evolutionary context would be that highlands of Peru have been populated by humans influenced by natural selection for only the past few tens of thousands of years (likely closer to the low end), while Eurasia has likely had high altitude living hominids for eons, so the wisdom of selection has had a far longer time to crank out alleles. It is interesting to me that Gibbons uses the sickle cell trait as a point of comparison: this is a recent evolutionary response to a powerful selective pressure, and, a classic case of heterozygote advantage where the population's mean fitness is dragged down by the clumsy and brute force method of this "fix" because of the generation of sicke cell homozygotes. The fact that such genetic variation exists within the Tibetan villages surveyed is somewhat surprising to me insofar as the fitness implications seem so strong that I can not understand why the alleles would have not fixed by now. If it is an overdominant trait, where the heterozygote has greater fitness than the homozygotes, then the variation will be preserved at an equilibrium conditioned by the fitness of the homozygotes. On the other hand, recent migration might have introduced "lowland" alleles into the population. Here is a snip from the author's abstract:
...This paper presents a case study illustrating the human adaptability and quantitative genetic approaches to explaining the unique biological characteristics of Tibetan highlanders that are thought to be adaptations offsetting high altitude hypoxia. There is evidence of strong directional natural selection operating on a major gene for oxygen saturation of hemoglobin in this population, although the genetic locus is not known....

Use of the term "quantitative genetic" implies, to me, a trait with a continuous range. But the impression I got from the quotations above is that the researchers divided women into a few discrete categories (i.e., did they simply compare the fertility of women in category "high oxygen" vs. "low oxygen" or did they did track correlation between oxygen levels and fertility and do a regression to predict the latter from the former?). Finally, reference to a "major gene" means that even if it is a quantitative trait with multiple loci contributing to the total effect there is obviously one locus of large effect. Beall suggests that directional selection is occurring upon this locus, so perhaps the Tibetans are relatively newcomers to their locale if selection is still working to optimize their fitness?

Long time readers of this weblog will know that Cynthia Beall has done work in the past which suggests that different highland populations have developed different adaptations to the same problems. With the recent data that is coming out in regards to skin color this should not surprise. In any case, I notice that the NAS has a interview with Dr. Beall, so check it out if you curious.

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