When does mitosis happen?

Mitosis occurs whenever more cells are needed. It happens throughout the entire lifespan of a living organism (human, animal or plant) but most rapidly during periods of growth. This means, in humans, the fastest rate of mitosis happens in the zygote, embryo and infant stage.

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).