Chapter 6: Mendelian Genetics in Population
Mendelian genetics: Describes transmission of alleles from individual parents to individual offspring.
Population genetics: describes the transmission of alleles in a population from one generation to the next, follows allele and genotype frequency.
Ex: Finches of 1 species confined to a particular island are a population
Population genetics begins with a null model (idealized fantasy) used to compare to the real one.
Gene pool: (allele pool) the collection of alleles present within a population
Allele Frequency: the proportion of alleles present with in a population
Genotype Frequency: the proportion of a genotype in a population
This equilibrium will hold true no matter the frequencies of the alleles in the parent generation. HWE is the foundation of population genetics.
Conclusion 1: At Equilibrium, allele frequencies do not change from generation to generation (p’=p)
Conclusion 2: Allele frequencies can be used to predict genotype frequencies (p^2 + 2pq + q^2 = 1)
If the population is displaced from equilibrium it will be corrected in 1 generation.
No selection on any genotype
No mutation
No migration
No genetic drift (no random events affect the population)
Mating within the population is random
An infinitely large random mating population that is free from outside evolutionary forces.
All 5 assumptions must be met to meet HWE, which means there is NO evolution.
Using Hardy-Windberg, you can determine a codominant locus (blood-type) or the frequency of rare recessive alleles in a population (PKU).
Adding selection changes the allele frequency. The fitness of a genotype depends on the relationship between genotype and phenotype. Selection can cause allele frequency to change across generations when individuals with some genotypes survive at higher rates. Alleles that confer survival or reproductive success are selected for and become fixed in a population (HWE violated p’ does not equal p).
Cumulative changes in allele frequency are driven by selection
In the ADH gene of Drosophila → frequency change in the alcohol dehydrogenase genes (ADH) in lab populations of Drosophila exposed to alcohol. The flies with the ADH F (fast) form process alcohol faster and do better when ethanol is present. ADH S (slow) form decrease. No change in controls (HWE conclusion 1 holds).
Selection violates both conclusions of HWE.
Selection will have major effects when →over long periods of time even when the frequency of the favored allele is low and selection is weak and over short periods of time only when the frequency of the favored allele is high and selection is strong.
Patterns of Selection: The allele favored by dominant vs. recessive or by common vs. rare
When a recessive allele is common and a dominant allele is rare, then evolution by natural selection is RAPID (Steep Slope) (Big q^2).
When a recessive allele is rare, then evolution by natural selection in SLOW (small q^2)
Most recessive copies will be hidden in heterozygotes (asymptomatic carriers), so they are effectively hidden from Selection (Remember Natural Selection works on Phenotypes, not Genotypes). Therefore natural selection will be unable to change frequency and fix alleles even if the selection is very strong.
Favoring Homozygotes and Heterozygotes
A gene has 2 alleles (V, L) VV and VL are viable, and LL is lethal. The expectation is that the pattern will be the same as the rare vs. common experiment. In reality, the alleles reach an equilibrium. This is explained by heterozygote superiority.
Heterozygote Superiority: AKA heterosis or overdominance, the heterozygotes have a higher fitness than both homozygotes.
In Africa, the allele for sickle cell anemia (HBS) is maintained at 10%, this is because heterozygotes display resistance to malaria.
Heterozygote superiority maintains genetic diversity indefinitely.
Heterozygote Inferiority: AKA underdominance is less common than superiority, where heterozygotes have lower fitness than both homozygotes. (Compound Chromosomes (s) in fruit flies).
Heterozygote inferiority reduces genetic diversity, one allele tends to be fixed and the other is lost.
Elderflowers produce 2 colors (purple and yellow) and do not produce a reward for their pollinators, either through nectar or pollen. This is an example of the rare color advantage, selection can maintain different alleles in a population if each is advantageous when rare. The rare allele has higher fitness. There is also fluctuation around equilibrium value but this depends on allele frequency.
Mutation by itself is NOT a potent or rapid evolutionary force. There is virtually no change in allele frequency so there is virtually no effect.
Mutation in combination with selection is a potent evolutionary force because mutation provides the variation for selection to act upon. A balance between selection and mutation might explain the persistence of deleterious alleles in a population.
Mutation-Selection Balance: As selection removes deleterious alleles, mutation resupplies them.
μ → the mutation rare at which A is changed to a
S → the selection coefficient (0 < s < 1) basically the strength of selection against an allele
S=1 is lethal and S=0 is 100% survival.
Mendelian genetics: Describes transmission of alleles from individual parents to individual offspring.
Population genetics: describes the transmission of alleles in a population from one generation to the next, follows allele and genotype frequency.
Ex: Finches of 1 species confined to a particular island are a population
Population genetics begins with a null model (idealized fantasy) used to compare to the real one.
Gene pool: (allele pool) the collection of alleles present within a population
Allele Frequency: the proportion of alleles present with in a population
Genotype Frequency: the proportion of a genotype in a population
This equilibrium will hold true no matter the frequencies of the alleles in the parent generation. HWE is the foundation of population genetics.
Conclusion 1: At Equilibrium, allele frequencies do not change from generation to generation (p’=p)
Conclusion 2: Allele frequencies can be used to predict genotype frequencies (p^2 + 2pq + q^2 = 1)
If the population is displaced from equilibrium it will be corrected in 1 generation.
No selection on any genotype
No mutation
No migration
No genetic drift (no random events affect the population)
Mating within the population is random
An infinitely large random mating population that is free from outside evolutionary forces.
All 5 assumptions must be met to meet HWE, which means there is NO evolution.
Using Hardy-Windberg, you can determine a codominant locus (blood-type) or the frequency of rare recessive alleles in a population (PKU).
Adding selection changes the allele frequency. The fitness of a genotype depends on the relationship between genotype and phenotype. Selection can cause allele frequency to change across generations when individuals with some genotypes survive at higher rates. Alleles that confer survival or reproductive success are selected for and become fixed in a population (HWE violated p’ does not equal p).
Cumulative changes in allele frequency are driven by selection
In the ADH gene of Drosophila → frequency change in the alcohol dehydrogenase genes (ADH) in lab populations of Drosophila exposed to alcohol. The flies with the ADH F (fast) form process alcohol faster and do better when ethanol is present. ADH S (slow) form decrease. No change in controls (HWE conclusion 1 holds).
Selection violates both conclusions of HWE.
Selection will have major effects when →over long periods of time even when the frequency of the favored allele is low and selection is weak and over short periods of time only when the frequency of the favored allele is high and selection is strong.
Patterns of Selection: The allele favored by dominant vs. recessive or by common vs. rare
When a recessive allele is common and a dominant allele is rare, then evolution by natural selection is RAPID (Steep Slope) (Big q^2).
When a recessive allele is rare, then evolution by natural selection in SLOW (small q^2)
Most recessive copies will be hidden in heterozygotes (asymptomatic carriers), so they are effectively hidden from Selection (Remember Natural Selection works on Phenotypes, not Genotypes). Therefore natural selection will be unable to change frequency and fix alleles even if the selection is very strong.
Favoring Homozygotes and Heterozygotes
A gene has 2 alleles (V, L) VV and VL are viable, and LL is lethal. The expectation is that the pattern will be the same as the rare vs. common experiment. In reality, the alleles reach an equilibrium. This is explained by heterozygote superiority.
Heterozygote Superiority: AKA heterosis or overdominance, the heterozygotes have a higher fitness than both homozygotes.
In Africa, the allele for sickle cell anemia (HBS) is maintained at 10%, this is because heterozygotes display resistance to malaria.
Heterozygote superiority maintains genetic diversity indefinitely.
Heterozygote Inferiority: AKA underdominance is less common than superiority, where heterozygotes have lower fitness than both homozygotes. (Compound Chromosomes (s) in fruit flies).
Heterozygote inferiority reduces genetic diversity, one allele tends to be fixed and the other is lost.
Elderflowers produce 2 colors (purple and yellow) and do not produce a reward for their pollinators, either through nectar or pollen. This is an example of the rare color advantage, selection can maintain different alleles in a population if each is advantageous when rare. The rare allele has higher fitness. There is also fluctuation around equilibrium value but this depends on allele frequency.
Mutation by itself is NOT a potent or rapid evolutionary force. There is virtually no change in allele frequency so there is virtually no effect.
Mutation in combination with selection is a potent evolutionary force because mutation provides the variation for selection to act upon. A balance between selection and mutation might explain the persistence of deleterious alleles in a population.
Mutation-Selection Balance: As selection removes deleterious alleles, mutation resupplies them.
μ → the mutation rare at which A is changed to a
S → the selection coefficient (0 < s < 1) basically the strength of selection against an allele
S=1 is lethal and S=0 is 100% survival.
