Seems like dogs have evolved since being domesticated by omnivores, which makes sense...obviously, dogs that can eat our food will produce more offspring than dogs that don't. And yet, they are still very much like carnivores: "The result of these findings, argues Dr. Hendriks, is that the dog is undeniably a true carnivore. The dog just happens to have an adaptive metabolism as a result of living with humans for millennia. That’s why the dog is perfectly capable of eating a grain-based diet, as most commercially fed dogs do."
I recently read an article that humanity evolved recently to adapt to being able to drink milk. Lactose intolerance was an inability to digest milk from other animals, and a small minority of humans had the mutation which allowed them to consume cow milk, for example. But, during hard times, like the plague, the Little Ice Age and such, when large segments of the population were killed off, it was advantageous to humans to be lactose tolerant. Those who were lactose tolerant were more likely to survive (and therefore to produce offspring) because they had an additional food source. Evolution only selects for mutations which enhance reproduction probabilities, because that is how the gene pool changes, and changes in the gene pool are by definition evolution (assuming I remember my studies correctly--mutations occur at the individual level, and evolution occurs at the species level).
Link: http://www.vetstreet.com/our-pet-experts/are-dogs-carnivores-heres-what-new-research-says