Humans select fot the traits that are desirable to man and not the animal. For example they select to create a certain kind of dog, say a dwarf dog. After several generations of mating like with like ,ie the shortest legs in the litter and other traits a breed is formed and maintained by "keeping it in the family" so to speak. But these dwarf dogs in being selected for dwarfism have other skeletal disorders I e the long backbone and so they suffer breed specific diseases because The so called desired traits, bred for by man come along with other undesirable traits not specifically bred for by man. In this way many of the animals ( and plants) that man has created by selective breeding are not as man would like them, ideally. It is a vast subject and involves ( or should involve ) a basic understanding of mendelian genetic inheritance. Many creatures suffer horrendously from artificial selection. Take a look at the so called food birds, turkeys, chickens etc all bred for maximum profit and abnormal breasts. They cannot stand on their own legs as their weight is too massive for their little bodies. All intensive breeding is the cause of suffering. Because ALL kinds of ( abnormal traits ) are bred for.