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I've read that there are several SNPs associated with increased risk of clinical or morbid obesity. I was wondering if there is any evidence that these are under positive selection. Would you expect them to be so?

I guess I'm interested to know if they would previously offered a selective advantage and only recently, say in the post-war period in the West morbid obesity become a problem due to differences in the diet.

Related question: obesity risk and single gene polymorphisms

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There are reasons beyond aesthetics why obesity is dangerous. Do not confuse obesity with "chubbiness", the latter can indeed be quite healthy and confer a selective advantage. –  terdon Nov 16 '12 at 16:08
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This is called the "Thrifty Gene Hypothesis" which was first used to explain why diabetes is so common. Basically it suggests that these alleles would have provided some kind of advantage, over the other possible alleles at that loci, until the environment changed. Then the environment changed and the allele became harmful. Environments are always changing and therefore favour different alleles (variant of a loci).

I suggest the genes linked to obesity in the modern world would, if tested on the old world populations would have been linked to being of a healthy weight. Further, those without it may have been skinnier and more likely to suffer from the disadvantages of being in such condition.

However, this paper present some counter arguments to the thrifty gene hypothesis. Speakman suggests that the genes responsible spread via genetic (allelic) drift rather than by positive selection. It is worth remembering that drift is very poor at spreading new mutations through large populations so there are also weaknesses to his arguments.

One example of a change in the favoured allele caused by an environmental shift is the DDT resistance gene in Drosophila which was present in the population at very low levels for a long period of time. As an aside, it possibly remained this way because it was subject to sexually antagonistic selection - beneficial to females whilst detrimental to males. It then spread through the population when DDT pesticides were used because it obviously became highly beneficial. Many other examples of environmental change leading to different selection are around, this is just one off the top of my head. Another is the Galapagos Finches in a drought in the late 20th century.

HTH, if not please ask more and if it does then please accept it!

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Hi, thanks for the answer and the explanation of the general theory explaining how this would happen. –  niallhaslam Nov 19 '12 at 10:42
    
I would think that genes favouring obesity nowadays in contexts of constant food abundance were in the past useful to store extra nutrients, when the food availability was irregular. One day you get something, you eat as much as you can of it and store the extra in the form of fat, and then you starve for some days, living on the fat you stored on the good day. I think I heard or read about such theories, but I have no reference for it. –  bli Nov 27 '12 at 14:23
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