Review of Review of Natural Natural

Selection TypesSelection Types

Effects of SelectionEffects of Selection

See Fig. 23.12

Coat color

Directional selection consistently favors phenotypes at one extreme

Effects of SelectionEffects of Selection

See Fig. 23.12

Coat color

Coat color

Stabilizing selection favorsintermediate phenotypes

Effects of SelectionEffects of Selection

See Fig. 23.12

Coat color

Coat color

Diversifying (disruptive) selection simultaneously favors both phenotypic extremes

Effects of SelectionEffects of Selection

See Fig. 23.12

Coat color

Coat color

Effects of SelectionEffects of Selection

Directional, diversifying (disruptive), and stabilizing selection

See Fig. 23.12

Coat color

Coat color Coat color Coat color

Population Population GeneticsGeneticsand the and the

Hardy-Weinberg Hardy-Weinberg EquationEquation

Population GeneticsPopulation Genetics

• In the early 1900s these two men discovered how the frequency of a trait’s alleles in a population could be described mathematically.

G H Hardy – British Mathematician Wilhelm Weinberg – German Doctor

Population GeneticsPopulation Genetics

• For every phenotype how many alleles do you have???– 2

• 1 from Mom and 1 from Dad

• These scientists figured out an equation that can be used to figure out the percentages of alleles and genotypes that are in a population.

Population GeneticsPopulation Genetics

• In order for their equation to work the population has to be in genetic EQUILIBRIUM– There is no change in the gene pool =

no evolution

Genetic EquilibriumGenetic Equilibrium• 1.) Population size is large• 2.) No gene flow in the population

• No new organisms introducing more alleles

• 3.) No mutations• 4.) No environmental factors causing

natural selection• No trait is favorable over another

• 5.) Random mating must occur

The Hardy-Weinberg The Hardy-Weinberg EquationEquation

• p2 + 2pq + q2 = 1• p2 = frequency of the homozygous

dominant genotype• 2pq = frequency of the heterozygous

genotype• q2 = frequency of the homozygous

recessive genotype

Hardy-WeinbergHardy-Weinberg• p – frequency of the dominant allele• q – frequency of the recessive allele• Because there are only 2 alleles, the

frequency of the dominant allele (p) and the frequency of the recessive allele (q) will add up to 1 or 100%

• p + q = 1

Hardy-Weinberg ExampleHardy-Weinberg Example• In a population of 100 people 28 of them

were found to have freckles and 72 were not. We learned in class during our genetics unit that having freckles is a recessive trait and not having them is because of a dominant trait. If this population is in genetic equilibrium then solve for the allelic frequencies and the variables in the hardy-weinberg equation:

Genetic DriftGenetic Drift

Genetic DriftGenetic DriftGenetic Drift occurs when the frequency of alleles change due to RANDOM PROCESSES! (NOT natural selection)

Bottleneck EffectBottleneck Effect

Bottleneck EffectBottleneck Effect

Founder EffectFounder Effect

Queens full of Jacks!Queens full of Jacks!• Let’s Mate!• red card=dominant allele=R• black card=recessive allele=r

P2 + 2pq + q2P2 + 2pq + q2RR Rr rr

Prediction 36% 48% 16%

1st gen.

2nd gen

3rd gen

Predicted vs ActualPredicted vs Actual• If this population is in equilibrium, we

should have the predicted % for our genotypes…

• We have…20 rr envelopes and 30RR envelopes

• Are we in equilibrium?

What should happen?What should happen?If we are evolving…If we are not…