Natural selection has varied traits as it never decides on one or few preferred traits. The more these traits pass on through natural selection, the more it will help the individual's chances of survival. This happens in the wild, or in an environment where it is not controlled by mankind.
Artificial selection, however, has less variations in traits and focuses on (a) specific trait(s). This makes the individual less fit for survival as only one or two traits are being enhanced while other traits eventually die out unless selected from an individual that has that trait, and the individuals can often become inbred. This is purely only done by mankind and is not done in the wild under normal circumstances.
The only time this isn't caused by mankind is when the population in a habitat dwindles down until they're given no choice but to inbreed for the sake of giving offspring to try and keep their population up.
