Measuring Evolution within Populations

5 agents of evolutionary changeMutation Gene Flow

Genetic Drift Selection

Non-random mating

Migration Sexual Selection

Small population

ChemicalChanges to DNA

Natural SelectionDifferential 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 … p2 + 2pq + q2 = 1 p + q = 1

p = frequency of dominant allele (B) q = frequency of recessive allele (b) p2 = % of homozygous dominant individuals (BB) q2 = % of homozygous recessive individuals (bb) 2pq = % of heterozygous individuals (Bb)

Hardy-Weinberg Equations p2 + 2pq + q2 = 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? q2 = 1/2500 = 0.0004 q = √0.0004 = 0.02 (or 2%)

Hardy-Weinberg Equations p2 + 2pq + q2 = 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 p2 + 2pq + q2 = 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 = Hb

◦ Sickle allele = Hs

◦ Recessive trait = HsHs

◦ 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 = HbHs

Unusual for individuals with homozygous recessive genotype◦ 1 in 100 = HsHs

◦ Usually die before reproductive ageWhy is the Hs allele maintained at such high levels

in African populations?

Suggests some selective advantage of being heterozygous… HbHs

Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its

life cycle in red blood cells

Single-celled eukaryote parasite (Plasmodium) spends part of its

life cycle in red blood cells

1

2

3

liver

Heterozygote advantage In tropical Africa where malaria is common:

◦ Homozygous dominant (normal)◦ Reduced survival or reproduction due to malaria (HbHb)

◦ Homozygous recessive◦ Reduced survival or reproduction

due to sickle cell disease (HsHs)◦ Heterozygote carriers

◦ Survival & reproductive advantage (HbHs)