Selective breeding means as its name says that some traits will be selected and only those individuals with the traits will be crossed. Now per se crossing of individuals is not harmful, however how it is done and what traits are selected can be harmful, selective breed must be done in an ethical manner, also the characters selected must be considered carefully as has been seen that many traits have other traits associated that at the long term are harmful for the organism, a quite easy case we can see it with dogs, for example, many dog breeds suffer from hip dysplasia like the german shepherd, on other cases like pugs the animal since birth has serious respiratory problems to the point where can choke himself just by lack of air. Now selective breeding has its benefits, for example, through selective breeding bananas or corn are what they are these days, as the ancestral plants are too small, bitter or with seed excess, so once more selective breed is not bad, but must be done with responsibility.