The tendency of human bodies to store fat as a survival mechanism against times of famine. Thought to be a contributing force in regard to obesity, Type II Diabetes, and heart disease in modern human populations.
Native American Pima Tribe, Arizona, USA
This Native American tribe has a rate of obesity that is more than twice that of caucasian-americans. Members of this tribe posess a genetic makeup different than that of other populaitons that genetically predisposes them to obesity. Functions of more than 200 "thrifty genes" working to would have allowed their ancestors to survive the harsh desert climates in prehistoric times but leads to obesity in the presence of a stable food source
PBS.org, 2013: Fat and Happy?
An alternative theory suggests that obesity in modern humans is more likely due to an absence of selection pressure (i.e. a lack of predation on humans by other animals) and genetic drift could be the reason behind so much obesity in modern times.
Speakman, J.R. (2008). Thrifty genes for obesity, an attractive but flawed idea, and an alternative perspective: the "drifty gene" hypothesis. Int. J. Obes., 32(11): 1611-7. doi: 10.1038/ijo.2008.161