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IntroductionEvolution occurs when alleles in a population change over time. The four forces that influenceallele variation are: mutation, gene flow, genetic drift and natural selection. Mutation is the onlyforce that allows new alleles to arise through DNA changes causing new variations. Genetic flowincreases diversity because alleles are exchange amongst different populations of the samespecies. Genetic drift, on the other hand, decreases diversity because alleles are randomly lost.Natural selection, unlike the other three forces of evolution, is not random. It always favorsalleles that improve fitness. In addition natural selection requires the trait to be variable andheritable. In this experiment an excel simulation document provided by the instructor was used toinvestigate natural selection. The excel program calculated the allele frequency for beak size ofDarwin’s finches in relationship to the amount of rainfall. The stimulation was run three times,with a population of 200 and 10 generations for each trial. The first trial was set up to the defaultrainfall,1000 mm/year, which is the average condition on the island. The second trial was set upto 500mm of rainfall per year, and the third one to 1500mm/year of rainfall. Rainfall variation isexpected to influence the size of the finches beak because it impacts the type of food available.