You are confusing genetics with evolution. There can be many causes of genetic mutations and predispositions isolated to certain areas. Cleft palate, certain genetic blood disorders such as sickle cell, and so on are found more in one area than others.
I'm not discounting that there are quite a few overweight people who can't keep thamselves away from the Snickers bars, but I think if you look into it more closely, you will find that there are far more people who put on weight with just a moderate increase in calories.
Many people, in order to meet the "ideal weight", must go to extrordinary measures such as supplemental exercise and a dedication to dieting. But if they don't perform these actions on a consistent basis (actions many without such a predisposition to weight gain never have to consider), then they are subject to merciless ridicule and are deemed "fat lazy slobs" by society. Folks imagine them to stuffing their faces with Cinnbons when in all fact they may eat just as healthy a diet as anyone else (equal caloric intake), but they don't restrict themselves to a 1500 calorie regimen.
For those who still want to berate all fat people and blame them for the high cost of health care, consider that the life expectancy for a heavy person is far less and you don't pay any health care expenses for a dead person. How much of a burden on society is the 99 year-old slim guy getting a knee replacement on medicare?
The point is, folks should not feel they can get away with ridiculing a heavy person, regardless of the reason for their overweight. No one prefers carrying around 100 extra pounds and buying all their clothes on-line and trying to pretend that there is anything else going through the mind of someone they're talking to, than how horrible they look. Many try to fight the battle, some have given up, but it's not up to us to try and make their world any crummier than it already is.