7.4-7.5: Population Genetics

Population Genetics

• population: a group of individuals of the same species living in the same area at the same time

• population genetics involves measuring a population’s generations by their:

  • genotype frequencies

  • allele frequencies

  • phenotype frequencies

• evolution within a population can be measured by the change in frequencies over time

• gene pools and allele frequencies

  • gene pool: a population’s genetic makeup

    • original generation

      • 9 individuals = 18 alleles

      • W → 8 alleles/18 total = 0.44 = 44%

      • w → 10 alleles/18 total = 0.55 = 55%

• microevolution: the gradual change in the frequency of a gene in a population over time

• when allele frequencies stay the same → not evolving → “Population is in the Hardy-Weinberg equilibrium”

• what causes/ maintains H-W equilibrium?

  • 1. A large population size → more “resistant” to random fluctuations in allele freq.

  • 2. Random mating (no sexual selection) → random chance determines alleles passed to next generation

  • 3. No gene flow (no migration) → gene flow would disrupt the allele freq. of both populations

  • 4. No mutations → would introduce new alleles + change freq.

  • 5. No natural selection → would increase/decrease freq. of certain phenotypes (and their alleles)

• if one of these conditions is disrupted, a population has the potential to experience evolution

• Mechanisms of Evolution

  • 1. Natural Selection

  • 2. Genetic Drift: change in a population’s genetic makeup due to random events (chance events, random)

    • founder effect: when a few individuals emigrate and establish new population in a new location

    • bottleneck effect: when a sudden event causes a major reduction in population size (overhunting, natural disasters)

    • has greater effects on smaller populations

    • creates populations w/ less genetic diversity than initially

    • can “fix” harmful alleles in the population (make stay, keep them around n greater proportions than you would expect in other populations)

  • 3. Gene Flow

    • movement of alleles from one population to another

  • 4. Random Mutations

Hardy Weinberg Equations

• p+q=1

  • for allele frequencies

• p² + 2pq + q² = 1

  • for genotype frequencies

• p = frequency of dominant allele

• q = frequency of recessive allele

• p² = freq. of homozygous dominant

• 2pq = freq. of heterozygous

• q² = freq. of homozygous recessive