Population GeneticsEvolution

Population Genetics

• Know:

- Population

- Species

- Gene pool

Hardy-Weinberg Theorem

• Allele frequencies of a gene pool won’t change (no evolution) IF these conditions are met:

- Very large population

- No gene flow (no immigration/emigration)

- No net mutations

- Random mating

- No natural selection

Hardy-Weinberg Theorem

• Idea was to show that evolution does occur

• If any one of the five conditions aren’t met, the gene pool changes

• Evolution= change in a gene pool over time (generations)

Hardy-Weinberg Equilibrium

• Equation predicts the allele frequency from generation to generation if all H-W conditions are met.

• p= frequency of dominant allele (percent decimal)

• q= frequency of recessive allele (percent decimal)

• p + q = 1

Hardy-Weinberg Equation

• p2 + 2pq + q2 = 1

• Allows you to see genotypes instead

• p2 = frequency of homozygous dominant

• q2 = frequency of homozygous recessive

• 2pq = frequency of heterozygous

*Always solve for q first! Why?

Origin of the Equation

p + q = 1

Example

A = pink flowers a = white flowers

Out of 500 plants… 320 AA + 160 Aa + 20aa

- 500 plants = 1000 alleles (two for each trait)

- Frequency (percent) of dominant allele?

ExampleA population of rabbits may be brown (the dominant phenotype) or white (the recessive phenotype). Brown rabbits have the genotype BB or Bb. White rabbits have the genotype bb. The frequency of the BB genotype is .35.

• What is the frequency of the B allele?

• What is the frequency of the b allele?

ExampleA population of rabbits may be brown (the dominant phenotype) or white (the recessive phenotype). Brown rabbits have the genotype BB or Bb. White rabbits have the genotype bb. The frequency of the BB genotype is .35.

• What is the frequency of heterozygous rabbits?

Hardy-Weinberg says…

• p/q should remain the same if evolution is not occurring!

• You must compare p/q values from many generations to determine this.

Microevolution

• Change in allele frequencies of a population over time

• Smaller scale

• Occurs when HW conditions not met

Genetic Drift

• chance events cause unpredictable changes in allele frequencies from one generation to the next

• most obvious in small populations

Types of Genetic Drift

• Bottleneck Effect = size of population greatly reduced due to natural disaster or human action

• Founder Effect = few individuals become isolated from larger population & form new population

Bottleneck Effect

• Ex. Northern Elephant Seals

• population reduced to approx. 20 in late 1800s due to hunting

• population now 30,000+

• very little genetic variation

Bottleneck Effect

Founder Effect

• Ex. Polydactyly in the Amish Community

• much of the Old Order Amish was founded by Mr. & Mrs. King and their descendants

• they carried the trait and passed it on

Founder Effect

Effects of Genetic Drift

• Significant in small populations

• Can cause random changes in allele frequencies

• Can lead to a loss of genetic diversity within populations

• Can cause harmful alleles to become fixed

Gene Flow

• Transfer of alleles into or out of a population due to movement of fertile individuals

• Can help organisms better survive in local conditions

• Ex. Spread of insecticide resistance in mosquitoes

Gene Flow

Modes of Selection

• Natural selection can alter frequency of inherited traits in three ways:

• Directional

• Disruptive

• Stabilizing

Directional Selection

• Conditions favor individuals exhibiting one trait

• Frequency and inheritance shifts in one direction

Disruptive Selection

• Conditions favor traits at phenotypic extremes

• Individuals with intermediate phenotypes not favored

Stabilizing Selection

• Conditions favor intermediate phenotype; does not select for phenotypic extremes

Types of Selection