The generally accepted view is that evolution requires "descent with modification". That is to say, the next generation is a copy of the previous generation, but not an exact copy, and it is a selection among these differences that drives changes in the population.
These days, nearly all of the variation in the next generation is the result of sexual shuffling of the DNA. Life on this planet is a set of variations on a theme. There is still mutations in the genome, but that kind of change is more chaotic and therefore is just as likely to be harmful to the next generation.
Still, mutations do become part of the genome since not all the mutant changes are drastic, or are protected by redundant systems, or are found in creatures that survive by luck or exist in a generally expanding habitat where the pressure to compete is less. Once a mutation is established in the population it will be subject to selection same as any other variation.
Billions of years ago, when there were much fewer genes and sex had not been "invented", mutations were the primary source of change. It's still the source of truly novel changes, but evolution is more successfull when changes are not abrupt leaps of novelty, but instead more variations on the same old theme.
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