CHAPTER 4

• Population genetics


  The central issue is genetic variation - its extent within populations, why it exists, and how it changes over the course of many generations
• Gene pool


  All of the alleles of every gene in a population
• Population


  A group of individuals of the same species that occupy the same region and can interbreed with each other
• Local population (deme)


  Smaller groups within a larger population, members are more likely to breed with each other than with the general population
• Polymorphism


  The observation that many traits display variation within a population
• Polymorphism in the Hawaiian happy-face spider

  • Individuals differ in alleles that affect color and pattern
• Polymorphic gene


  A gene that commonly exists as 2 or more alleles in a population
• Monomorphic gene


  A gene that exists predominantly as a single allele
• Genetic variations in the human β-globin gene

  • Single nucleotide polymorphism causing sickle cell disease
  • Deletion eliminating gene function

  
• Allele frequency


  Number of copies of an allele in a population / Total number of all alleles for that gene in the population
• Genotype frequency


  Number of individuals with a particular genotype / Total number of individuals in the population
• Hardy-Weinberg equilibrium


  A mathematical expression that relates allele and genotype frequencies in a population, under certain conditions the frequencies do not change over generations

  
• Conditions for Hardy-Weinberg equilibrium

  1. No new mutations

  2. No genetic drift

  3. No migration

  4. No natural selection

  5. Random mating
• Allele frequency (p) and (q)

  p + q = 1
• Calculating genotype frequencies using Hardy-Weinberg

  • Frequency of GG = p^2

  Frequency of Gg = 2pq

  Frequency of gg = q^2
• Hardy-Weinberg equation can be used to predict frequency of carriers for recessive genetic diseases

  
• Chi-square test can be used to determine if a population is in Hardy-Weinberg equilibrium

  
• Expected number of NN individuals

  0.007 x 200 = 1.4 (or 1 rounded to the nearest individual)
• Expected frequency of genotype MN

  2pq = 2(0.915)(0.085) = 0.156

  
• Expected number of MN individuals

  0.156 x 200 = 31.2 (or 31 rounded to the nearest individual)
• The hypothesis that the alleles for this gene are in HW equilibrium is accepted
• Mutation


  A change in the nucleotide sequence of an organism's DNA

  
• Mutations


  • Primary sources of genetic variation
  • Only mutations in the cell lines that produce gametes can be passed to offspring
  • Mutations that occur in somatic cells are lost when the individual dies
  • Point mutations
• Genetic drift


  A change in the frequency of an allele within a population over time
• Founder effect


  • When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population
• Bottleneck effect


  • A drastic reduction in the size of a population can lead to a loss of genetic diversity
• Migration


  The movement of individuals or gametes into or out of a population
• Example of migration


  • Allele frequency of A is 0.7 and 0.3 in the donor and recipient populations, respectively

  20 individuals migrate and join the recipient population which has 80 members

  m = 20/(20+80) = 0.2

  pC = 0.3 + 0.08 = 0.38
• Non-random mating


  A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
• Types of non-random mating

  • Intrasexual selection
  • Intersexual selection
• Natural selection


  Causing adaptive evolution; evolution that results in a better match between organisms and their environment
• Modes of natural selection

  • Directional selection
  • Disruptive selection
  • Stabilizing selection
• Mechanisms preserving natural selection

  • Diploidy
  • Balancing selection
  • Neutral variation
• Diploidy


  Most eukaryotes are diploid: large amount of genetic variation is hidden from selection in the form of recessive alleles
• Balancing selection


  Natural selection maintains two or more forms in a population
• Neutral variation


  DNA variation that has little or no impact on reproductive success