Answer:
A lot of factors cause genetic variation in a population. The root of all genetic variation is mutations, which occur randomly.
A lot of genetic variation in fact results during meiosis, the process by which gametes (sex/reproductive cells) are formed. The 3 main ways are crossing over, independent assortment of chromosomes, and random fertilization. Crossing over occurs during Prophase I of meiosis, in which genetic material is exchanged between nonsister chromatids of homologous chromosomes. Independent assortment refers to the following: When cells divide during meiosis, homologous chromosomes are randomly distributed during anaphase I, separating and segregating independently of each other. Random fertilization is where any sperm can fertilize any egg cell. The many combinations of eggs and sperms mean variation.
Another source of genetic variation include gene flow. Gene flow is where populations have their members enter other populations (migration), influencing the gene pool of populations. This source is more controversial, however.
Random mating can also increase genetic variation.
