Genetic variation in a population increases the chance that some individuals will survive. 

• Genetic variation gives us phenotypic variation
• Phenotypic variation is required for natural selection
• Genetic variation is stored in a population's gene pool

Allele frequencies measure genetic variation, they are a measure of how common any particular allele is in a population.

Sources of genetic variation:

• Mutation - a random change in the DNA of a gene
• Recombination - forms new combinations of alleles
• Hybridization - the crossing of two different species