Obesity in the genes

Mason Inman has a cute article on Inkling, titled "Do these genes make me look fat?" It's a riff on the thrifty genotype hypothesis:

But why do we have obesity genes in the first place? The most popular theory is that we have "thrifty genes," hand-me-downs from our ancestors, who periodically faced famines and scarcity. The idea is that certain genes allow us to run more efficiently and to get by on scraps when times are lean. The flip side is that when food is a-plenty, we end up carrying around a spare tire. This obesity may or may not have hurt some of our ancestors, but the advantage of being able to survive lean times outweighed the disadvantages - so the theory says.

The reason why this is newsworthy is the profusion of recent papers finding "obesity genes" of one kind or another, including the mysterious FTO:

In the European population studied, around half of adults had at least one "fat" FTO allele. The new gene seems to play a role in diabetes, too - a tantalizing link, since obesity and diabetes are both rising, and the obese are at risk for developing Type II diabetes. Strangely FTO, unlike other diabetes genes, doesn't seem to influence insulin, the hormone that regulates blood sugar levels. Instead, it was directly linked to people's body mass index. And the authors have no clue how this gene works.

That will increasingly be the case; we will know that genes are correlated with phenotypes but we won't know why. It's just a consequence of gene-phenotype correlation studies proceeding faster than our knowledge of gene-gene interactions.

I often ask my students what will happen to obesity in the future. Will it decline under selection? Or will it increase? Or stay the same? It's a great example to use, because even though nobody seems to like it, and everybody thinks it might kill you, there's still not particularly any fitness disadvantage to it. That's because we fat people still reproduce!

Inman's story almost arrives at that conclusion, but sort of wimps out:

Just as natural selection favored fat genes in centuries previous, the stage seems set today for evolution to weed them out. It won't happen overnight, of course. Take the intriguing observation that mildly overweight people show lower overall mortality than normal-weight people. So perhaps, even in this modern world, a predisposition for carrying a few extra pounds might help survival.

Oh yes, surely natural selection will save us from all becoming fat. Except it won't. Moowah-ha-ha-ha!