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Health

Overplaying "AIDS genes"

Gene ExpressionBy Razib KhanJuly 17, 2008 10:32 PM

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Brandom Keim at Wired says Genes Don't Explain African AIDS Epidemic:

Seen in the wrong light, the numbers could present Africa's AIDS tragedy as a biological inevitability. Several press accounts do exactly that. The New York Times credits the mutation for "explaining why the disease is more common there than expected." Reuters says it could "help explain why AIDS has hit Africa harder than all other parts of the world," as this can't be fully rooted in "sexual behavior and other social factors." The Guardian says it "may go some way" to explaining the African prevalence of AIDS. And the Gene Expression blog titles its coverage, "Evolution, a reason for the African HIV epidemic?"

My first reaction was, "who would be retarded enough to think that one genetic difference entails inevitability?" But on second thought, that's just me being unreflective, many people are that stupid. In any case, I specifically titled my post "a reason" to highlight that the variation on the DARC locus might be able to account for some of the between population differences. It is true that most people aren't used to thinking of a dependent variable which is the outcome of the joint effects of multiple independent variables; that's why the genetic determinism straw-man is so easy to construct. Extrapolating from the study sample and the known nature of the polymorphism on the DARC locus, one could account for around 10% of the difference in HIV infection rates when it came to Duffy negative vs. Duffy positive populations. That's a big impact. In height or IQ the quantitative trait loci, the genes which effect variation, are on the order of 1% or less in effect. Something similar to the polymorphism on the DARC locus would be the skin color genes, the largest few effects being on the order of 10-40%. This brings to light some interesting evolutionary issues. Remember that malaria is probably a relatively recent adaptation; it seems that the mosquitos that transmit malaria find perfect habitat in clearings opened up by humans for the purposes of agriculture. What you see across the world within the last 10,000 years are the emergence of a diverse suite of genetic malaria defenses. Many of these defenses are kludges; quick & dirty evolutionary responses to a new problem which has some side-effects, sickle-cell anemia and glucose-6-phosphate dehydrogenase deficiency being some well known ones. Over evolutionary time given constant selection pressures one could imagine that these responses refine themselves as modifier genes bubble up in the genetic background to mask the deleterious side effects. We just haven't had enough time. Of course, selection pressures may not be constant. It is common among evolutionary psychologists to say that we have a "Pleistocene mind," and many of our contemporary maladaptations are due to latency of natural selection. Whatever you think of that assertion, when it comes to immune responses it seems that we need to bring the time window closer, our ancestors over the past 10,000 years have gone through the roller-coaster of the transition to farming, but now we live in a far different world with new and strange dynamics. I think an analogy to antagonistic pleiotropy can be made; what is good for you in the short term might not be so good in the long term. The evolutionary pressures of today may generate adaptations which are a drag in a new adaptive landscape (note: this obviously dovetails with the Red Queen model).

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