Obesity Probably Didn’t Evolve to Protect Us From Famine

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Researchers use genetics to test the “thrifty gene hypothesis” and find it lacking.

By Nathan Collins

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(Photo: Justin Sullivan/Getty Images)

At some time or other, you may have wondered why our bodies take extra calories and store them as fat rather than simply expelling them. You may also have heard that there was, at some point in humanity’s past, an evolutionary advantage to storing all that fat—namely, that it helped our prehistoric ancestors survive famine. It’s called the thrifty gene hypothesis, and it’s probably wrong, according to a new study.

The thrifty gene hypothesis is certainly appealing, like many other evolutionary explanations of our physiology and behavior. At their core, such arguments assume that specific genes evolved to deal with specific challenges we faced in the past, and if those genes are causing problems today, it’s because we haven’t had time to fully adapt to a rapidly changing world. The problem is, evolution doesn’t work like that. It’s a noisy process that produces, as one source calls it, “selection, not perfection.” For a whole variety of reasons, not every gene in our DNA is completely good for us.

In 106 of the genes they examined, there was no evidence at all for positive selection.

So there’s plenty of reason to be skeptical of evolutionary claims such as the thrifty gene hypothesis, but biologists Guanlin Wang and John Speakman wanted a direct test. In particular, they searched for genetic signatures of what’s called “positive selection,”—signs that nature actually favored some degree of obesity over skinnier body types—in 115 genes associated with body-mass index (BMI), the most common, albeit flawed, measure of obesity. Drawing on data from the 1000 Genomes Project, Wang and Speakman looked at how much a gene’s individual nucleotides—the basic molecular building blocks of DNA—vary from person to person. Roughly speaking, the less variation there is in a particular gene, the more evidence there is for positive selection.

Wang and Speakman’s analysis found little reason to believe nature preferred obesity. In 106 of the genes they examined, there was no evidence at all for positive selection. Of the remaining nine, five genes showed signs of positive selection, but in the wrong direction—that is, natural selection pushed those genes toward variants that favored leaner bodies over obesity.

The study has some limitations, of course. For one thing, BMI isn’t a particularly accurate measure of body fat, so the genes Wang and Speakman focused on may not provide the best test of the thrifty gene hypothesis. In addition, the specific methods Wang and Speakman used work best when selection is “hard,” that is, when nature rapidly and near-completely weeds out inferior genes. For the time being, however, “there is little evidence to support the thrifty gene hypothesis that obesity is the consequence of selection under famines or indeed that obesity has been positively selected for any other reason,” Wang and Speakman write.

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