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| The study of microevolution looks at changes in the frequencies of alleles from one generation to the next. Such analyses allow detailed examination of the factors that can alter allele frequencies in the short term and also provide us with inferences about long-term patterns of evolution. Changes in allele frequencies stem from four evolutionary forces: mutation, natural selection, genetic drift, and gene flow. Mutation is the ultimate source of all genetic variation, but it occurs at low enough rates that additional factors are needed to explain polymorphic frequencies (whereby two or more alleles have frequencies greater than 0.01). The other three evolutionary forces are responsible for increasing or decreasing the frequency of a mutant allele. Natural selection changes allele frequencies through the process of differential survival and reproduction of individuals having certain genotypes. Genetic drift, the random change in allele frequencies from one generation to the next, has the greatest effect in small populations. Gene flow, the movement of alleles between populations, acts to reduce genetic differences between different groups. |