How can mutation cause gene variation?

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).

Mutations lead to genetic variation by producing random changes in an organism's genetic code.

This causes a different protein to be produced, or none at all.

Usually has a negative effect, due to an important protein, e. g. an enzyme, not being produced, so the organism cannot do a certain thing (e. g. digest fats), occasionally makes an organism better able to survive (positive effect). This means that it is more likely to survive and pass the mutated (beneficial) gene on to it's offspring.