Genetic mutations are always occurring. Survival of the fittest means that beneficial genetic mutations are retained while deleterious ones die out quickly. When we intervened in dog and cat breeding we influenced genetic pressures. Unwittingly, the Kennel Club and the Cat Fancy, through beauty contests and breeding to written breed standards, accelerated evolutionary genetic changes in dogs and cats in the same way that antibiotics have accelerated evolutionary genetic changes in microbes.
Many of our cat and dog breeds today come from small genetic bases. Any genetic problem that existed in that small genetic base, for example the West Highland White terrier's genetic predisposition to skin allergy, or the Persian cat’s predisposition to cystic kidney disease is magnified through the breed population. Other deleterious genetic changes occurred because of the show success of an individual in a particular breed. His unknown genetic medical defects then were spread to a larger than normal population because of his popularity. We also perpetuate problems, like deafness in Dalmatians or blue-eyed white cats, because they are not critical or life-threatening. Through selective breeding, unwittingly we increase disease risk for some. For example, because of its unnaturally flattened face, a Pekingese or a Persian has a greater risk of eye injuries, choking or facial skin infection than did his ancestors.