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Alleles become more or less common either by chance in a process called [[genetic drift]] or by [[natural selection]].<ref>[http://evolution.berkeley.edu/evosite/evo101/IIIMechanisms.shtml Mechanisms: The Processes of Evolution] {{webarchive|url=https://web.archive.org/web/20080527163721/http://evolution.berkeley.edu/evosite/evo101/IIIMechanisms.shtml |date=27 May 2008 }} Understanding Evolution, Accessed 20 May 2008</ref> In natural selection, if an allele makes it more likely for an organism to survive and reproduce, then over time this allele becomes more common. But if an allele is harmful, natural selection makes it less common. In the above example, if the island were getting colder each year and snow became present for much of the time, then the allele for white fur would favor survival since predators would be less likely to see them against the snow, and more likely to see the gray mice. Over time white mice would become more and more frequent, while gray mice less and less.
Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties.<ref>[http://evolution.berkeley.edu/evosite/evo101/IIICGeneticvariation.shtml Genetic Variation] {{webarchive|url=https://web.archive.org/web/20080527163711/http://evolution.berkeley.edu/evosite/evo101/IIICGeneticvariation.shtml |date=27 May 2008 }} Understanding Evolution, Accessed 20 May 2008</ref> So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those that are useful, causes
==Inherited diseases==
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