Despite the popularity and simplicity of the idea, researchers have continually poked holes in its thesis for years. If these genes were so advantageous, why did only some people and not others inherit them? Shouldn’t they have, by now, spread to the entire population? To figure it out, researchers in this study looked at 115 genes known to be associated with obesity and tried to identify evidence that showed the genes to be under positive selection, also called Darwinian selection, where a gene that provides an advantage to the individual sweeps through the entire population. Typically, if a gene has very little variation, then it is more likely to be under positive selection.