Beneficial mutation
From RationalWiki
A Beneficial mutation is exactly what it says it is: a mutation in an organism's genome that produces a beneficial effect. Specifically, it affects the organism in some way as to increase its chances of reproductive success, and therefore the chance of the mutation in question being passed along. It is safe to say that the bast majority of mutations in an organism are not beneficial. Often a mutation is neutral, producing neither a beneficial effect nor a negative effect. Many are negative mutations; producing a disorder in the organism that significantly lowers its survival and reproductive capabilities. In the case of sexual reproduction in animals, a negative mutation can prevent the embryo being correctly conceived or even the gametes being fully functioning enough.
Despite the odds against an individual mutation being beneficial, the driving force of natural selection means that when one does appear, it can quickly dominate a population and make its presence well known.
Them silly evil-utionists! Mutations can never be beneficial!
