This isn't remotely true. Look at the wiki article on dog food [https://en.wikipedia.org/wiki/Dog_food#History https://en.wikipedia.org/wiki/Dog_food#History]

This isn't remotely true. Look at the wiki article on dog food [https://en.wikipedia.org/wiki/Dog_food#History 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.