The main driving force of evolution is natural selection, which occurs when the environment picks for the organism which variations are useful by killing off those who haven't got them. That is how...
Mutations serve up the variations in organisms to natural selection. The reproductively successful variations pass on these successful traits to progeny and allele frequencies change because of this...
Serve up variations in genomes so that natural selection can choose among them in the immediate environment. The change in alleles over time in a population of of organisms is evolution.