Selection can either be positive or negative. Certain traits are maintained through generations that either facilitate an organism's survival or its fitness. A simple example would be development of antibiotic resistance in bacteria - beta lactamase producing bacteria are resistant to penicillin. A complex example would be natural selection that controls evolution of species.Artificial selection is the deliberate breeding (or direct genetic manipulation) of a particular species in order to produce or enhance a desired trait. This could be anything from milk production in cows to the ability of a yeast cell to overexpress a particular protein. Although there is no distinction between the underlying principles that guide selection, natural selection can be thought to drive populations towards improved "survivability" or "fitness" within their environment, while artificial selection is intentionally performed out of human interest and does not necessarily contribute to an organism's fitness.