Population Genetics - Selection

How Natural Selection works on population genetics

Changes allele frequencies unidirectionally
Rate of frequency change depends on strength of selection, how important it is
If the deleterious allele is lost from the population, it has been purged

Absolute Fitness

The likelihood that a certain genotype survives to adulthood.
$w_{i j}$ is the absolute fitness of genotype $A_{i} A_{j}$

Mean fitness

Proportion of survivors in population after selection, according to H-W:

\bar{w} = p^{2} w_{11} + 2 p q w_{12} + q^{2} w_{22}

Frequencies of the new genotypes must be divided by $\bar{w}$ to sum to 1:
!EEB-020-PopulationGenetics-2025, p.18

Genotype to allele frequencies:

The frequency of $A_{1}$ is $p = p^{2} + p q$ or frequency of $A_{1} A_{1}$ heterozygotes + half the frequency of heterozygotes (since each heterozygote has one $A_{1}$ allele)

So, after selection with change in absolute fitness:
!EEB-020-PopulationGenetics-2025, p.19

Relative Fitness

Relative fitness is absolute fitness divided by mean fitness: $w_{i j} / \bar{w}$

This is important since we want to know how an individual preforms relative to others in the population. This tells us how advantageous a gene is

Measure of the selective advantage of a genotype or allele in a population
If $w_{i j} / \bar{w} > 1$ , then $A_{i} A_{j}$ will increase in frequency in the population
Often expressed relative to one genotype. For exaple, if $A_{2}$ results in selection change $s$ :
!EEB-020-PopulationGenetics-2025, p.20

Dominance and recessivity

There is a relationship between selection coefficient $s$ and dominance coefficcient $h$ .
$h > 0.5$ means the gene is dominant.
!EEB-020-PopulationGenetics-2025, p.22
For recessive mutations that are positive, it takes a longtime for the allele to become fixed (many generations)

Deleterious mutations (negative selection)

Very deleterious (deadly) are often highly recessive, otherwise they would be purged from the population quickly
They are not "seen" by selection, so they can be maintained within a population

Conclusions

Evolution is fastest when genetic variation in fitness is largest, i.e. when p = q = 0.5
Selection will cause loss of deleterious mutations and fixation of beneficial mutations.
We are still only considering infinite populations with random mating