Population Population geneticsgenetics

• In the early 1900’s In the early 1900’s the science of the science of population genetics population genetics was born.was born.

• This science This science explained the explained the relationship relationship between genetics between genetics and evolutionand evolution.

Wilhelm Weinberg Wilhelm Weinberg 1862 - 19371862 - 1937

Population Population geneticsgenetics

• Population genetics Population genetics was an important was an important discovery because discovery because populations are the populations are the smallest unit that smallest unit that can evolve. can evolve.

• Traits within Traits within populations show populations show variation.variation.

Genetic Genetic EquilibriumEquilibrium

Peppered moths

Think: What conditions would favor black moths over white?

• Some variations are Some variations are discontinuous.discontinuous.– Ex: male Vs femaleEx: male Vs female– Ex: Tall pea plants Vs Ex: Tall pea plants Vs

short pea plants.short pea plants.

• Some variations are Some variations are continuous.continuous.– Polygenic traits usually Polygenic traits usually

cause continuous traitscause continuous traits– Ex: human heightEx: human height– Ex: human skin colorEx: human skin color– Ex: size of fish in a pond.Ex: size of fish in a pond.

Variation Within A PopulationVariation Within A Population

• Continuous traits Continuous traits demonstrate a bell demonstrate a bell shaped curveshaped curve– Most individuals are Most individuals are

in “average”.in “average”.– Individuals with Individuals with

“extreme” variation “extreme” variation are few in number.are few in number.

think: Explain the change in the graph after selection occurs.

Distribution of traitsDistribution of traits

• Traits are Traits are caused by:caused by:– EnvironmentEnvironment– HeredityHeredity

• Usually both Usually both factors play a factors play a role.role.

Causes of VariationCauses of Variation

Are these plants different heights because of genetics or nutrition?

3 causes of variation in a population.3 causes of variation in a population.

1.1. Mutations to DNA.Mutations to DNA.2.2. Independent assortment & crossing Independent assortment & crossing

over.over.3.3. Random fertilization.Random fertilization.

What Increases Variation?What Increases Variation?

• Gene Pool;Gene Pool;– All genes found All genes found

in an in an interbreeding interbreeding population.population.

• Allele frequency;Allele frequency;– % of a particular % of a particular

allele in a allele in a population.population.

Allele Frequencies In A Gene Allele Frequencies In A Gene PoolPool

Hardy-Weinberg Genetic Hardy-Weinberg Genetic EquilibriumEquilibrium

Flamingo Flamingo populationpopulation

• Allele Allele frequencies frequencies remain the remain the same generation same generation after generation.after generation.

• This will be true This will be true if 5 conditions if 5 conditions are met.are met.

Hardy-Weinberg Genetic Hardy-Weinberg Genetic EquilibriumEquilibrium

Tule Elk population at Pt Tule Elk population at Pt ReyesReyes

1.1. No change due No change due to mutations.to mutations.

2.2. Individuals do Individuals do not move in or not move in or out of the out of the population.population.

3.3. The population The population is and remains is and remains large.large.

4.4. Random mating.Random mating.5.5. No selectionNo selection

Disruption of Genetic EquilibriumDisruption of Genetic Equilibrium

Tule Elk population at Pt. ReyesTule Elk population at Pt. Reyes

• If any of the 5 If any of the 5 factors do not factors do not occur, they can occur, they can cause the gene cause the gene frequency to frequency to change in a change in a population.population.

• If the gene If the gene frequencies frequencies change evolution change evolution has occurred.has occurred.

Summary review1. What is the shape of a

graph of normal distribution?

2. What is a gene pool?

3. What are 3 causes of genetic variation?

4. Given the requirements of genetic equilibrium do populations remain unchanged? Why?