Everybody agrees that the genome has not changed. But the environment certainly has. We have radically increased the quantity of calories available to every man, woman and child in the United States, food has become more cheaper, more calorie-laden, and more easily accessible. Genes generally require an environmental stimulus to be expressed, and it is possible that the weight gain brought on by this surfeit of calories could trigger genes.
I'm not talking about cases of purely genetic obesity like Prada-Willi syndrome, where people continuously eat and eat and eat and eat without stopping. I'm talking about genes which may subtly affect appetite, leading to increases in weight. Once the weight is on, the body unleashes all manner of endocrinological defenses to protect that weight as best it can.
It makes sense that these genes would be widely distributed through the population because evolutionarily, people with the ability to store large energy reserves would have a higher chance for survival in lean times.
I don't quite follow your drunk driving analogy, but I do agree that the environment is the trigger. But simply because the environment is a trigger does not mean that genetics have "nothing" to do with it. The two are not mutually exclusive - although that theory is simpler and thus preferred by many - it's easier to explain the situation by loading all the culpability on the victim, rather than examining the circumstances which brought about the condition or lead to increased susceptibility.
