8.3

• Stabilising selection

  • Reduces variation in the population - conserves phenotypes already present - selects against extreme phenotypes
• Disruptive selection

  • Increases the diversity of a population - common when conditions are diverse and small subpopulations evolve different phenotypes suited to their niche - selects for extreme phenotypes
• Directional selection

  • The 'classic' natural selection - shows a change from one phenotype to another, which is more advantageous to the environment
• Population bottleneck

  The effect of a catastrophic event or series of events that dramatically reduces the size of a population (by at least 50%) and causes a severe decrease in the gene pool of the population, resulting in large changes in allele frequencies and a reduction in genetic diversity
• Founder effect

  The loss of genetic variation that occurs when a small number of individuals become isolated, forming a new population with allele frequencies not representative of the original population (also referred to as a voluntary bottleneck)
• Genetic drift

  Random changes in the gene pool of a population that occur by chance, not because they confer any advantage or disadvantage to the offspring. Has a major effect on small populations, e.g. after a population bottleneck
• Hardy-Weinberg equation

  Can be used to estimate the frequency of alleles in a population
• Hardy-Weinberg equation variables

  • p = the frequency of the dominant allele (represented by A)
  • q = the frequency of the recessive allele (represented by a)
  • p^2 = frequency of AA (homozygous dominant)
  • 2pq = frequency of Aa (heterozygous)
  • q^2 = frequency of aa (homozygous recessive)
• For a population in genetic equilibrium: p + q = 1.0
• (p + q)^2 = 1 hence p^2 + 2pq + q^2 = 1
• Conditions for the Hardy-Weinberg Equation

  • No mutations
  • Random mating
  • Large population
  • No migration into or out of the population (i.e. the population is isolated)
  • No selection pressure
• What is a Population?

  All the organisms of a particular species that live in the same place
• Selection pressures

  • Predation
  • Disease
  • Competition (for food, habitats, mates)
  • Environmental conditions e.g. temperature
• How selection pressures change allele frequencies within a population

  Organisms with advantageous characteristics are more likely to survive and produce offspring. Therefore their favourable alleles get passed on, while unfavourable alleles die out.
• Stabilising selection


  Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against. Results in low diversity.
• Disruptive selection


  The opposite of stablising selection, in that both extremes of the normal distribution are favoured over the mean. Over time, the population becomes phenotypically divided and new species may develop.
• Genetic drift


  A change in a population's allele frequencies that occurs due to chance rather than selective pressures. In other words, it is caused by 'sampling error' during reproduction.
• Population bottleneck


  Where a catastrophic event dramatically reduces the size of a population, thereby decreasing the variety of alleles in the gene pool and causing large changes in allele frequencies.
• Founder effect


  When a small number of individuals become isolated, forming a new population with a limited gene pool, with allele frequencies not reflective of the original population.
• Hardy-Weinberg principle


  Allows us to estimate the frequency of alleles in a population, as well as if allele frequency is changing over time.
• Assumptions of the Hardy-Weinberg principle

  • No mutations occur to create new alleles
  • No migration in or out of the population
  • No selection, so alleles are all equally passed on to the next generation
  • Random mating
  • Large population
• Hardy-Weinberg equation for calculating allele frequency


  The frequencies of each allele for a characteristic must add up to 1.0. The equation is therefore; p + q = 1 Where p= frequency of the dominant allele, and q= frequency of the recessive allele.
• Hardy-Weinberg equation for calculating genotype frequency

  The frequencies of each genotype for a characteristics must add up to 1.0. The equation is therefore; p2 + 2pq + q2 = 1 Where p2= frequency of homozygous dominant, 2pq= frequency of heterozygous, and q2= frequency of homozygous recessive.