Is the Ala12 variant of the PPARG gene an "unthrifty allele"?

BACKGROUND: The thrifty genotype hypothesis proposes that genetic susceptibility to type 2 diabetes results from the positive selection of "thrifty" alleles in the past. A corollary of this hypothesis is that genetic variants protecting against the development of diabetes are "unthrifty" and thus subject to negative selection during human evolution.
METHODS: It was assessed whether age estimates of the diabetes protective PPARG Ala12 allele indicate effects of natural selection. Based on published data from four populations, the date of origin of the diabetes protective PPARG Ala12 variant was estimated using both allele frequency and linkage disequilibrium (LD) with the C1431T single nucleotide polymorphism in exon 6 of the PPARG gene.
RESULTS: The best LD based estimate of the age of the Ala12 allele gave an average of approximately 32,000 years with a maximum upper bound of approximately 58,000 years. Assuming a population with a growth rate of r = 0.01 per generation, the frequency based estimate of the age of the Ala12 variant gave an average of approximately 27,000 years with a maximum upper bound of approximately 42,000 years. DISCUSSION: The similarity of both time estimates is consistent with selective equivalence of the diabetes protective PPARG Ala12 allele and the diabetes susceptible PPARG Pro12 allele.