Evolution!

People from societies with diets that have historically been rich in starch are twice as likely to have at least six copies of the Amylase gene.

Amylase is one of very few genes that exhibits significant differences in copy number between individual humans. It encodes for the protein in saliva that begins the digestion of starches, and is necessary to get nutrition out of grain. What's more, contrary to what happens in some other places, the "extra" copies of the amylase gene are fully functional and do lead to more amylase protein being produced, which in turn leads to faster and more effective digestion of starches. Also, since the differences in sequence between the three copies of the amylase gene in the reference human genome sequence are so small, it seems that this duplication might have happened quite recently; even perhaps since the birth of modern humans. Which means things like the development of agriculture and therefore the increased levels of starch in some societies' diets might very well have effected the evolutionary path of those societies. Which is cool -- we are effecting our own evolution. And not just by killing genetically diseased babies, like certain geneticists would want us to.