Random mutations are the key to the creation of new alleles. All other methods merely rearrange the existing alleles in the gene pool.

Sexual reproduction also facilitates variation in populations of organisms. Crossing-over, independent assortment of homologues, and the random joining of gametes are examples processes that promote variation.

Diploidy, the presence of two copies of each chromosome in a cell, also promotes variation.

Outbreeding, or mating with unrelated partners, increases the possibility of different combinations of alleles.

Balanced polymorphism encourages the existence of more than one allele for a certain gene because of the survival benefits of both alleles. In heterozygote advantage, the heterozygote with both the dominant and the recessive allele has a better chance of survival than both the homozygous dominant and homozygous recessive variaties. Hybrid vigor describes the superior quality of offspring between two inbred strains of plants.

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