Introduction

Mutation

Drift

Selection

Random drift of a new allele


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When a mutation occurs, there is a single copy of the new allele. Usually, it will be lost due to random drift, but occasionally it will get lucky and increase in frequency. The first graph shows the fate of a new allele in a population of 20 haploid individuals.

Random drift of a new allele

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1
50
Generation

The next graph lets you choose the population size. It automatically sets the initial allele frequency in this haploid population to 1/N (the dynamics would be exactly the same in a population of diploid individuals, in which case the initial frequency would be 1/2N). Set the number of generations to 5 times the number of haploid individuals, so there's time for the new allele to become fixed or lost.

Run a large number of replicates; you should see that the proportion of replicates fixed for the new red allele is about equal to the initial frequency. This is one of the fundamental results of the neutral theory: the probability of fixation of a new allele is 1/N (for a haploid population) or 1/2N (for a diploid population).

Random drift of a new allele

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Generation
Red allele
frequency
0.00
1.00
1
500
Number of haploid individuals:
Number of generations:
Number of replicates:


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This page was last revised March 6, 2009. Its address is http://udel.edu/~mcdonald/evoldriftnew.html.
©2009 by John H. McDonald. You can probably do what you want with this content; see the permissions page for details.