This is called the "Thrifty Gene Hypothesis" which was first
used to explain why diabetes is so common. Basically it suggests
that these alleles would have provided some kind of advantage,
over the other possible alleles at that loci, until the
environment changed. Then the environment changed and the allele
became harmful. Environments are always changing and therefore
favour different alleles (variant of a loci). I suggest the
genes linked to obesity in the modern world would, if tested on
the old world populations would have been linked to being of a
healthy weight. Further, those without it may have been skinnier
and more likely to suffer from the disadvantages of being in
such condition. However, this paper present some counter
arguments to the thrifty gene hypothesis. Speakman suggests that
the genes responsible spread via genetic (allelic) drift rather
than by positive selection. It is worth remembering that drift
is very poor at spreading new mutations through large
populations so there are also weakn