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Evolutionary Genetics: One-Locus

Selection at one locus can be described mathematically given initial allelic frequencies (A & a) and selection coefficients (s). The specific nature of the seleciton is determined by the type of dominance at that locus. The frequency of an allele ( f(a)=q ) in the next generation can be predicted with the following equations:

No Dominance: WAA=1, WAa=1-1/2s, Waa=1-s

Complete Dominance: WAA=1, WAa=1, Waa=1-s

where W is the fitness of a particular genotype.

Evolutionary Genetics: One-Locus

Adjust the initial allelic frequencies (p,q) and genotype fitnesses (W) to the right:

For best results, begin by setting the fitness values to 1.0, and initial allelic frequencies to 0.5. Then, decrease one or more of the fitness values by a small amount.

Selection against the A allele occurs when WAA<WAa and/or WAA<Waa. The strength of this selection is a function of the differences among the three fitness values. Selection of this type (directional) generally leads to fixation of one of the two alleles.

Selection against the a allele occurs when Waa<WAa and/or Waa<WAA.

Heterozygote advantage occurs when WAA<WAa>Waa. In this case both alleles are preserved in the population. Note that allelic frequencies do not change when WAA=Waa.

Heterozygotes can also be selected against, when WAA>WAa<Waa. Note that allelic frequencies do not change when WAA=Waa.

The population represented to the right posseses 5000 randomly breeding individuals. This large size is chosen to minimize the effects of drift so that the effect of selection would be clear. There is no background mortality in this simulation.