Crossing over. At the chiasmata during prophase I of meiosis, breakage may occur and exchange of genetic information may take place. New gene combinations which result in variations.
Independent assortment. The arrangement of homologous chromosomes at the equator of the spindle during metaphase of the first meiotic division is random. These homologous chromosomes segregate into different daughter cells which contain different genetic combinations which brings about variation
Fertilisation permits parental genes to be brought together in different combinations. This is the cause of variations in members of the same family.
Mutation is the spontaneous change in the genetic makeup of an organism. These changes may be inherited by the offspring therefore beginning variation.
Hardy-Weinberg equilibrium requires no immigration or emigration, a large population, random mating, and no spontaneous mutations (all of which are virtually unavoidable in nature). Natural selection would violate these conditions.
