I like the distinction you make with this, maybe I can take it a bit further.
Genes are always changing and adapting across generations, which is exactly what they've been designed to do. 'New combinations' of genetic base pairs which by chance end up working better in a particular environment (i.e. say, a hot, dry environment with mostly red meat and grains as food versus a cool, damp environment with mostly fish and vegetables) will improve some aspect of individual's performance (i.e., energy level, intelligence, functionality, etc.). This will give an individual an advantage, and with this advantage they will be able to choose a better quality mate, acquire greater resources, have more offspring, and have more offspring which survive (at least this is how it tended to work in the past).
With these advantages and the accompanying higher proportion of offspring, such 'highly adaptive' gene sequences, with time, end up being exhibited throughout the majority of the population 'in that particular environment'. This is why I believe that diets may to some degree vary from individual to individual, depending on their genetic makeup and origin.
Genes can also be changed by ones lifestyle. They can be turned on, turned off, ramped up or down, and even destroyed (i.e. oxidative damage due to over-eating, trans fats, radiation, smoking etc.)
The good news is that with a healthy, balanced and calorie-reduced diet, moderate exercise, high-dose pharm. grade fish oil, full-spectrum antioxidants, lots of good quality water, adequate sleep, and some simple stress-reduction techniques, we can not only reset and optimize the functioning of our genes, but we can also dramatically limit the damage they undergo, and therefore dramatically increase the likelihood of a long and healthy life.
But hey, you probably already know this, or you wouldn't be here ;-)