[gomaaa]

Mutations happen all the time. Individuals within a species can vary wildly in appearance, abilities and other characteristics. It doesn't have to be anything so drastic as suddenly growing a third arm. Such a mutation is almost always harmful to the individual and might well make it harder for it to survive. Little differences, though, can give certain organisms an advantage. Individuals with that sort of trait are more likely to survive and over time this can shift the original distribution. The rule is not "chosen" by anyone, certainly not by the organism. Survival is the only criteria. If it works, it keeps propogating. If it doesn't it goes away.

[lasereye]

I understand the theory of natural selection. What I thought you were saying was that the mutations themselves are not random in nature but that somehow the beneficial ones are more likely to occur.