Microevolution is a change in allele frequencies in a population from one generation to the next. Allele frequencies are the relative proportions of different alleles (versions of a gene) for a given gene within a population (an interbreeding group of individuals of the same species). Genetic variation arises through mutation, chromosome errors, and sexual reproduction. Evolution is based on the accumulation of many variances and mutations. Variation allows natural selection to alter the frequency of alleles that already exist in the population, and mutation allows novel traits or characteristics to become a part of a population. These, therefore, affect survival of some individuals in the environment and are essential for evolution to occur. Evolution depends on the survival and reproduction of individuals, but individuals themselves cannot evolve. Microevolution, which happens at the population level, can occur through mutation, natural selection, migration, or non-random mating, and becomes exaggerated as population size decreases. When microevolution is not happening, a population is in Hardy-Weinberg equilibrium.