There are many occasions where mutations do not affect natural selections--some color variations, for example.  The mutations must affect longevity, virility, survivability, adaptability,etc.--factors that increase the probability of passing a trait to the next generation and the next and the next.  incidental mutations that do not affect the chances of being passed on are by definition outside the genetist's study,because they did not result in a new species, by definition. And of course mutations that hinder or prevent reproduction are not passed on.  What's amazing, however, is how slight a mutation can affect the chances of survival (and therefore reproduction). In the case of Darwin's finches of the Galopagos Islands, slight changes in beak shape allowed changes in diet.