Amanda Marcotte and Rebecca Watson pointed me towards this absurd article “Why women need fat.” In fairness, I don’t dispute the idea that the recent advent of a diet full of processed sugars hasn’t been good for us, or that different people have different ‘set points’ for what their bodies tell them is ‘normal’ (and part of that set point is no doubt genetically determined). The phrase “evolutionist”, as opposed to “evolutionary biologist” or just biologist, unfortunately, is one of those tells which lets you know there’s going to be a lot of story-telling.

First, I’m tired of people treating the environment in which humans evolved as a constant. Some environments probably favored behaviors that could lead to packing on the pounds when faced with the modern cornucopia of bounty, while others didn’t. Moreover, these environments were likely not directional: some years were ‘fat’ years, others ‘skinny.’ On the Galapagos Islands, there are years that select for larger or smaller beak sizes in finches, but, because the environment varies, there is no constant selection for either larger or smaller beaks. This has the effect of keeping a lot of standing variation in the population. Just something to think about.

Related to the issue of standing variation is that what is good for the goose isn’t necessarily good for the gander. If heavier women are adaptive, that doesn’t mean heavier men are. If you haven’t noticed, we’re not clonal organisms: women don’t undergo binary fission and yield more women (though things might be easier they did…). If ‘heaviness’ is good for women but not good for men, then this is the gender-based equivalent of different environments (in other words, you might produce great women, but the men are pretty lousy).

Another thing that bugs me is the assumption that we will always be insatiable. Yes, if there’s a low-hanging fruit (literally and metaphorically), go grab it and eat it. But if your desire–or need–to pack on weight whenever possible leads you to do stupid things, such as climbing a fifty foot tree, only to be startled, fall out of the tree and bonk your head (I CAN HAZ MADEUP STOREEZ TOO!)…not so adaptive. Some behaviors will be maladaptive in certain environments.

Finally, there’s no reason to think that we are particularly well adapted for any environment (something I’ve touched in in this post, “The Arc of Evolution Is Long, and Rarely Bends Towards Adaptive Alleles“). If weight, obesity, etc. are polygenic (determined by many genes), we would expect some of those genes to be pushing us in the ‘wrong’, maladaptive direction even under selection* for certain traits such as heaviness (I apologize for the anthropomorphization).

Point is, if you want to make statements about human evolution and natural selection, do the heavy lifting: find markers, get some sequence (there’s some floating around…), and start looking for signatures of selection.

Just-so stories aren’t good enough for the 21st century.

*And to the extent human populations have been subdivided at certain points and thus shrank, weakly selected traits would act as if they were not selected at all.