"Fat, diabetic, and heart-disease genes may have evolved to survive the cold. "

Well, that certainly explains the propensity for obesity in the people of oceania (Samoa, Hawaii, etc.)

Not really a good example - Polynesians have only been living where they are for the last few thousand years, since outriggers were invented. They were on the mainland prior to that, and may have lived in colder regions.

A better example is African Americans, who are heavier on average than whites, while their ancestors lived in one of the warmest regions.