Population Genetics

• Variation


  Characteristics that vary from one generation to the next and individual to individual
• Why variation is important


  To maintain diversity, greater chance of survival
• Charles Darwin


  • Studied how variations can lead to a natural advantage (natural selection) but he did not explain how variations take place
  • He didn't know about mutations, DNA, genes, alleles or how traits were inherited
• Natural selection


  1. Individuals of the same species are in a constant struggle for survival

  2. Individuals with more favorable variations are more likely to survive

  3. These individuals pass their variations on

  4. Survival is NOT random, best adapted and fit survive
• Evolution


  • Individuals with more favorable variations contribute proportionately more offspring to the next generations
  • These favorable variations become more common over time
• Microevolution


  Small changes in DNA that lead to an organism being better adapted to their environment
• Macroevolution


  This is speciation, formation of a new species
• Mutations


  DNA changes to produce an individual with better characteristics for survival
• From Light to Dark Moths


  • Moth collectors in England noted that most peppered moths collected in the early 1800's were light gray peppered with bits of black, but many years later most of the moths collected were almost completely black
• Changes in the environment (coal dust coating trees and turning them dark)


  • Lead the peppered moths to be eaten more, their numbers dropped
  • Black moths were more camouflaged and bred more, their numbers increased
• Microevolution (Peppered moths)


  Peppered moths were less fit, less camouflaged and less likely to survive
• The light peppered moth was common, while the dark form was very rare before the Industrial Revolution
• After the pollution from the Industrial Revolution started affecting trees, most of the collected peppered moths were of the dark form
• As the trees darkened with soot, the light-colored moths were easier to see and eaten by birds more, while the rare dark colored moths blended in better on the darker trees
• Over time, the dark colored moths became the more common of the two color forms. Natural selection favored the dark individuals, so they were more successful after the trees changed
• Since the 1950s, with the introduction of the Clean Air Act and cleaner burning fuels, the light peppered moth population has recently been more common in the population because it is better camouflaged
• Direct Evidence for Evolution


  • Different species lived on Earth at various times in the past (most now on Earth wasn't 1 million years ago)
  • The complexity of living organisms has increased from the past to present
  • Living species and their most closely matching fossils are usually found in the same geographic region
• Indirect Evidence for Evolution: Embryology


  • The study of organisms and the early stages of development
  • All embryos look similar because they have a common ancestors
• Indirect Evidence for Evolution: Analogous features

  Structures of different organisms are similar in appearance and function but not in evolutionary origin
• Homologous Feature


  Structures in an organism that have similar origins but different uses in different organisms
• Some organisms share similar DNA molecules and certain proteins
• Possible Sources of variation


  • Sexual reproduction
  • Crossing over
  • Different alleles
  • Random mutations
• Gene pool


  All the genes/alleles that occur in a specific population
• Genotype Frequency


  Frequency of genotypes in a population (AA or Aa or aa)
• Allele Frequency


  Frequency of alleles in a population
• Genetic Equilibrium


  No Change in allele frequencies
• Evolution


  Change in allele frequencies
• Conditions of the Hardy-Weinberg Principle


  • Large population
  • No migration
  • No mutation
  • No natural selection
  • Mating is random
• Genetic Drift


  Changes to allele frequency as a result of chance
• Founder effect


  If only a few individuals leave to establish a new population, it creates genetic drift and the allele frequency of the new population is probably not the same as the original

  
• Bottleneck effect


  A severe environmental event results in a drastic reduction in population size, the frequency of the survivors is very different from that in the original population
• Gene flow


  If individuals move from one population to another they remove their alleles from one population and are adding to another, which causes changes
• Mutations cause the gene pool to alter or the allele frequency to alter with the introduction of a new allele
• When natural selection occurs, individuals with certain alleles will have greater reproductive success than others do, increasing the relative frequency of their alleles in the next generation
• Sexual Selection


  • Traits favored in sexual selection include sexual dimorphism (when there is a striking difference between the male and female)
  • Behavioral differences also favour sexual selection
  • Sexual selection may compromise different selective pressures (bright feathers may be good for mating but detrimental to avoid predators)
• It is really hard to meet the conditions for Hardy-Weinberg Equilibrium to be met
• Even if the conditions aren't easy to meet, the Hardy-Weinberg Principle is still useful
• Heterozygotes Aa


  Have a selective advantage
• It's really hard to meet the conditions for Hardy-Weinberg Equilibrium
• Hardy-Weinberg requires that each allele has an equal chance of being passed on to the next generation