Dog genetic plasticity is a result of their selective breeding by man, not a reason for it. "Repeated events of population size reduction, such as the bottleneck at the time of divergence between dog and wolf and the founder effects leading to the establishment of the different dog breeds (...) have facilitated the significant expansion of dog SINEs." - Cordaux and Batzer, taken from the link above Bottleneck any genome sufficiently and repeatedly over the course of 10 or 15 thousand years and you will most certainly get a brachycephalic animal, so to speak. Nobody has ever bred a cow for that purpose, of course we won't have brachycephalic cows running around until we decide to breed for them.