There are four forces in evolution: natural selection, genetic drift, gene flow, and mutation. These forces exist in every natural population and their relative magnitudes determine the genetic state of the population. For example, an overwhelmingly high rate of mutation will elevate the genetic variation, and strong selection against variants will reduce it. Strong drift, such as that active in a population of small numbers, will also reduce genetic variation as rare variants are loss by chance over time. The effect of gene flow on genetic variation depends on the state of the source population.