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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