Population Genetics III
Four Evolutionary Forces
- Random Drift
- Natural Selection
- Gene Flow
- Interaction of these forces
- Natural selection can also affect allele frequencies over time.
- Natural selection happens when there are differences among genotypes in their ability
to survive and reproduce.
- This differential ability is usually referred to as Fitness (normally represented as w).
Natural Selection Causes Evolution
- w11 w 12w a22 the fitness coefficients
- Genotypes are the units of selection
- Fitness is usually expressed as relative fitness. The genotype with the highest fitness is
assigned a fitness of 1.
- Relative fitness ranges from 1 (highest fitness) to 0 (genotypes don’t survive or
Another Way to See Natural Selection
- Another way to describe the role of selection is using the selection coefficient (s).
- The selection coefficient can be described as the proportional reduction in fitness of a
genotype with respect to the genotype with the maximum fitness (w=1).
- Thus, by definition w=1-s.
o S=portion being eliminated
- s ranges from 0 (genotype with the maximum fitness) to 1 (genotypes don’t survive or
The Effect of Selection on Allele Frequencies
- The effect of selection on allele frequencies will depend, among other things, on
- Differences in fitness between genotypes. When the differences are large, the effect of
selection is strong, and allele frequencies will change fast in the population from one
generation to the next
- The pattern of dominance, co-dominance or overdominance of the trait.
- You can simulate different scenarios in the hyperlink below!!!
Co-Dominant Selection: Mild
- For a dominant trait, you have 3 genotypes, but 2 phenotypes
- Selection will see the 3 genotypes differently
- wAA = 1 1
- wAB = 1 - s 0.99
- wBB = 1 - 2s 0.98
- p (1) + 2pq(1-s) + q (1-2s) = 1 Co-Dominant Selection: Strong
- w AA = 1 1
- w = 1 - s 0.75
- w BB= 1 - 2s 0.5
- Assuming different frequencies, the frequency of allele A, will increase. But in this case,
this increase will happen much faster (in comparison to the mild case)
- In just about 30 generations, allele B will be eliminated
- p (1) + 2pq(1-s) + q (1-2s) = 1
Selection Against Dominant Homozygotes and Heterozygotes
- w aa1 1
- w = 1 – s 0.75
- w2AA = 1 – s 2 0.75
- p (1) + 2pq(1-s) + q (1-s) = 1
Selection Against Recessive Homozygotes
- w AA = 1 1
- w Aa1 1
- w aa1 – s 0.75
- p (1) + 2pq(1) + q (1-s) = 1
Overdominant Selection (Selection for the Heterozygote)
- w AA = 1 – s 0.75
- w AB = 1 1
- w = 1 – 2s 0. 5
- p (1-s) + 2pq(1) + q (1-2s) = 1
- This is the case for sickle cell, where there is no elimination of an allele (there is an
- The homozygote for the B is eliminated more than A
- The heterozygotes has a higher rate of fitness relative to the two homozygotes
Examples of Selection Acting in Populations, and Its Effect on HW Proportions and Allele
A Population Evolving: No Selection