Some important theoretical background: we want to develop a picture of what happens to a new mutant in a population, lets say a single nucleotide change a one position in the DNA. This is the starting point for molecular evolution. If the new mutant is governed by genetic drift, its fate should be quite different than another nucleotide mutation that is governed by selection (see below). To describe molecular evolution Kimuraformulated the Neutral theory of molecular evolutionwhich is remarkably simple. If:u = mutation rate / gene / generation, N = population size, then the number of new mutations occurring per generation in a population = 2Nu (2 because we are considering diploid organisms).Now, when a new mutation occurs in a population its initial frequency = 1/2Nbecause it is the one new variant out of a total of 2N genes in the population. This is also its probability of fixationbecause the probability of you reaching into a barrel of 2N marbles and getting the one new marble is 1/2N. Thus taking these two
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