The study of mutations, together with the analyses of population genetics, has been used to explain the mechanism of evolution. The elementary process of evolution is considered to be the changes in the frequency of occurrence of alleles in a population. Mutation, which causes the appearance of new alleles or changes the relative frequency of already existing alleles, is one important mechanism by which evolution occurs. Natural selection (see selection), by affecting reproductive success, influences the frequencies of alleles and other genetic variants in successive generations. For example, if the presence of a particular allele makes a homozygous individual unable to mate, the allele may be eliminated from the population.
Genetic drift —the random fluctuation in the frequency of an allele, resulting mainly from the vagaries of chance mating—is also an evolutionary mechanism. Although in large populations drift varies only a little above and below a statistical mean, in small breeding populations an entire generation might, by chance alone, be born with the same genotype with respect to a particular allelic pair of genes, thus leading to either the elimination or dominance of a particular gene. Because fluctuations in the proportions of alleles are more significant in the gene pools of small, isolated breeding populations, genetic drift is a mechanism of species diversity and evolution in such groups.
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