[2] Speakman's critique of the thrifty gene hypothesis is based on an analysis of the pattern and level of mortality during famines.
[3][4] Moreover, there is some confusion among proponents of the thrifty gene hypothesis about how long famines have played a role in evolution.
[dubious – discuss] It is argued instead that the modern distribution of the obese phenotype likely comes about because of genetic drift in the genes encoding the regulation system controlling an upper limit on our body fatness.
Such drift may have started because around 2 million years ago when ancestral humans effectively removed the risk of predation, which was probably a key factor maintaining the upper boundary of the regulation system.
It is argued by Prentice that while famine may actually have only been a force driving evolution of thrifty genes for the past 15,000 years or so, because famines exert effects on both survival and fertility the selection pressure may have been sufficient even over such a short timescale to generate the current phenotype distribution of BMI.