5 agents of evolutionary change
Mutation Gene Flow Non-random mating
Chemical
Changes to DNA
Genetic Drift
Migration
Selection
Sexual Selection
Small population
Natural Selection
Differential Survival

Hardy-Weinberg Equilibrium
Hypothetical situation
◦ Serves as a null hypothesis
A non-evolving population
◦ Remove all agents of evolutionary change
◦ 1. No genetic drift (a very large population)
◦ 2. No gene flow (no migration in or out)
◦ 3. No mutation (no chemical change to DNA)
◦ 4. Random mating (no sexual selection)
◦ 5. No natural selection (equal survival)

Hardy-Weinberg Equations
If the previous conditions are met, then … p 2 + 2pq + q 2 = 1 p + q = 1 p = frequency of dominant allele (B) q = frequency of recessive allele (b) p 2 = % of homozygous dominant individuals (BB) q 2 = % of homozygous recessive individuals (bb)
2pq = % of heterozygous individuals (Bb)

Hardy-Weinberg Equations p 2 + 2pq + q 2 = 1 p + q = 1
Cystic fibrosis is a recessive condition (cc) that affects 1 in
2,500 babies in the Caucasion population in the U.S.
What is the frequency of the recessive allele?
q 2 = 1/2500 = 0.0004
q = √0.0004 = 0.02 (or 2%)

Hardy-Weinberg Equations p 2 + 2pq + q 2 = 1 p + q = 1
Cystic fibrosis is a recessive condition (cc) that affects 1 in
2,500 babies in the Caucasion population in the U.S.
What is the frequency of the dominant allele?
p = 1 - q p = 1 - 0.02 = .98 (or 98%)

Hardy-Weinberg Equations p 2 + 2pq + q 2 = 1 p + q = 1
Cystic fibrosis is a recessive condition (cc) that affects 1 in
2,500 babies in the Caucasion population in the U.S.
What is the percentage of heterozygotes in the population?
2pq = 2(.98)(.02) = 0.04 (or 4% … or 1 in 25)

Example of strong selective pressure
Tay Sachs
◦ Primarily in Ashkenazi Jews and Cajuns
◦ Recessive disease = aa
◦ Lysosomal storage disease
◦ Lack of 1 functional digestive enzyme in lysosome
◦ Build up undigested fat in brain cells
◦ Children die before the age of 5
So where do new cases of
Tay-Sachs come from?

Example of heterozygote advanage
Sickle Cell Anemia
◦ Inherit a mutation in gene coding for one of the subunits in hemoglobin
◦ Oxygen-carrying blood protein
◦ Normal allele = H b
◦ Sickle allele = H s
◦ Recessive trait = H s H s
◦ Low oxygen levels cause RBC to sickle
◦ Clogs small blood vessels
◦ Damages organs
◦ Often lethal

Sickle Cell Frequency
High frequency of heterozygotes
1 in 5 in Central Africans = H b H s
Unusual for individuals with homozygous recessive genotype
◦ 1 in 100 = H s H s
◦ Usually die before reproductive age
Why is the H s allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous… H b H s

Malaria
1 liver
2
3
Single-celled eukaryote parasite
(Plasmodium) spends part of its life cycle in red blood cells

Heterozygote advantage
In tropical Africa where malaria is common:
◦ Homozygous dominant (normal)
◦ Reduced survival or reproduction due to malaria (H b H b )
◦ Homozygous recessive
◦ Reduced survival or reproduction due to sickle cell disease (H s H s )
◦ Heterozygote carriers
◦ Survival & reproductive advantage (H b H s )