Most dogs and cat's however are domesticated and have been eating a mostly single-source diet for many many generations... some adaptation has occurred. There is a reason they don't give zoo animals kibble (aside from the herbivores who naturally focus on a single food source, and the pellets are made from that food source). Lions don't get kibble, and they don't even get a single kind of meat, they rotate their food, because if they dont, the lions become depressed and sluggish and sick. We're meant to eat a variety of food - the variety stimulates our internal biome which has more effect over us than just how we digest our food - our gut is basically a second brain and has a direct effect on our mood and our brain chemistry, and can even influence our thinking process (this is how when you eat something that makes you sick, you naturally avoid that thing for a while)
Look at the wiki article on dog food [https://en.wikipedia.org/wiki/Dog_food#History] - diets for dogs have been ad hoc and varied for thousands of years. I'd argue that it is only with the advent of kibble (in the late C19) and canned cat/dog food (in the early C20) that pets started eating single-source diets.
I do not think that this is enough time for adaption, and any selective breeding has been focused on other attributes rather than diet.
