...We found that copy number of the salivary amylase gene (AMY1) is correlated positively with salivary amylase protein level and that individuals from populations with high-starch diets have, on average, more AMY1 copies than those with traditionally low-starch diets. Comparisons with other loci in a subset of these populations suggest that the extent of AMY1 copy number differentiation is highly unusual. This example of positive selection on a copy number-variable gene is, to our knowledge, one of the first discovered in the human genome. Higher AMY1 copy numbers and protein levels probably improve the digestion of starchy foods and may buffer against the fitness-reducing effects of intestinal disease.
From the digest in Nature News:
When the researchers ventured beyond university campuses to sample populations in Africa, Asia, Europe and the Arctic, they noticed a trend. Cultures with diets that included high levels of starch tended to have more copies of the amylase gene than cultures that consumed few starches. Starch-loving cultures such as the Hadza of Tanzania who rely heavily on tubers and other root vegetables, have 6.7 copies of amylase, on average. While people like the Mbuti, pygmy rain forest hunter-gatherers from central Africa who eat little starch, have 5.4 copies on average.
This shouldn't be a big surprise. Some of justifications sound like those for lactase persistence, "The ability to digest starch may have had the added benefit of cutting down on diarrhoea." The rest of the commentary just writes itself, so I'll just let you go on with your internal monologue.... Page 2 of the supplemental PDF has some population level data (the N's are small of course for some of the groups). Related:Slow & diverse food.