Genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring)
other answer
There are three sources of genetic variation: mutation, gene flow, and sexual reproduction. A mutation is simply a change in the DNA. Mutations themselves are not very common and are usually harmful to a population. Because of this, mutations are usually selected against through evolutionary processes
