Mutation includes a range of effects, from changes to individual nucleotide bases along the DNA to wholesale rearrangements of chromosomes. It is the ultimate source of new genetic variation for a species. Sexual reproduction can lead to recombination of variation created by mutation to produce novel genotypes.
Natural selection is essentially a bookkeeping process.
If there is heritable variation in phenotypic traits that affect fitness, then by definition the genetic composition of a population will change over time, over generations. Natural selection can be directional, stabilizing or disruptive. For qualitative traits, selection will lead to defined changes in allele and genotype frequencies. For qualitative traits, the response to selection will be determined by both the strength of the selection and the heritability of the trait. Selection can occur at any stage of an organisms life cycle.