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There are a number of forces that influence genetic diversity. Some can be influenced by us, others occur by coincidence. At population level there are forces that increase genetic diversity, and forces that decrease it. Mutations are events that create new alleles. When it happens in germ cells it has an increasing effect on genetic diversity. Also migration may have an increasing effect, but only immigration (new animals moving into the population). Emigration (animals moving out of the population) usually has a decreasing effect on genetic diversity, especially when the population size is small. Also selection has a decreasing effect: only animals with a specific genetic make-up are allowed to breed, at the expense of others. This will have an effect on allele frequencies, away from the equal frequencies. A final force with a decreasing effect on genetic diversity is the force of coincidence (genetic drift), and related to that is inbreeding. Genetic drift cannot directly be influenced by our selection decisions. More explanation on genetic drift is in the next paragraph.