Why are human's defying narural evolution with mammals?

Answer 1

The question is a little obscure. Strictly answering the question as posed would result in the answer "Humans are not defying the natural evolution of mammals." Now, if your question speaks to animal husbandry, then the answer is because humans are breeding mammals with enhanced favourable traits and reduced undesirable traits. For example, a good cowherd will interbreed those milch cows which give good quality milk and prevent breeding those who are inferior producers. The hope is that the offspring will similarly be good producers. But if not done properly, the genetic line can be degraded and the phenotype may carry over undesirable traits, (such as susceptibility to disease).