21 Evolution

• Biological evolution


  Descent with modification from a common ancestor
• Modification


  Evolution only occurs when there are changes in allele frequency within a population over time

  
• Descent


  Evolution involves passing on of heritable genetic changes to the next generation
• Common ancestor


  Present day species are related through a common prehistoric ancestor that have changed and adapted over time
• Galapagos finches


  • Example of evolution
• Evolution of the horse (Equus ferus)


  • Example of evolution

  
• Microevolution


  Change in allele frequency within a population or species over time
• Macroevolution


  Gradual changes in allele frequencies over many thousands of years may eventually give rise to a new species
• Microevolution and macroevolution are fundamentally identical processes but on a different time scale
• Microevolution may eventually lead to macroevolution, given enough time
• Microevolution examples


  • Antibiotic resistance in bacteria, pigmentation of peppered moth
• Macroevolution examples


  • Darwin's Galapagos finches, mammals and birds diverged from reptiles, reptiles diverged from amphibians, all tetrapods diverged from fish
• Five agents of evolutionary change

  Natural selection, disruption of gene flow, genetic drift, non-random mating, mutation

  
• Natural selection


  Results in the survival of individuals with the most favourable adaptations to a particular environment, leading to differential reproductive success in a population
• Principles of natural selection

  1. Overproduction of offspring

  2. Constancy of numbers

  3. Struggle for survival

  4. Variation within a population

  5. Survival of the fittest

  6. Like produces like

  7. Formation of new species over time
• Variation is the raw material for natural selection to act on
• Meiosis and sexual reproduction are significant sources of variation
• Mutations introduce new alleles and enrich the gene pool, increasing variation

  
• Types of natural selection

  • Directional selection
  • Disruptive selection
  • Stabilising selection
• Directional selection favours phenotypes at one extreme
• Disruptive selection favours individuals on both extremes of a phenotypic range
• Stabilising selection favours the more common intermediate variants in a population
• Directional selection


  • Melanic form of the peppered moth has selective advantage in polluted areas
  • Intermediate phenotypes are selected against
  • Favours individuals on both extremes of a phenotypic range
  • Possible to result in polymorphism, where two or more forms are found in one species
• Disruptive selection


  • Extreme phenotypes are selected against
  • Favours the more common intermediate variants in a population
  • Does not promote evolutionary change but maintain phenotypic stability with a population over time

  
• Stabilising selection


  • Babies who are heavier than and lighter than optimum birth weight are at a selective disadvantage
  • Slightly increased mortality works against extremes of birth weight in humans
• It is incorrect to imply that industrialization caused the melanic form of peppered moth to appear
• Example 1: The peppered moth

  1. Before 1848, there was a greater proportion of the lighter form

  2. With the industrial revolution the environment changed

  3. The lighter form of moth became more visible and easy prey to birds

  4. The darker forms of moth were well camouflaged and thus proliferated
• The change in the numbers of the two types of moths


  Is due to the selection pressure exerted by the predatory birds which can now more easily spot the lighter form of moths
• Directional Natural Selection


  Shown by the example of the peppered moth
• Example 2: Antibiotic resistant bacteria

  1. Antibiotic resistant and non-resistant bacterial strains exist naturally

  2. Resistant strains usually arise by spontaneous mutations

  3. Antibiotics kill most non-resistant bacteria

  4. Antibiotic resistant bacteria have a selective advantage in the presence of antibiotics

  5. The frequency of the antibiotic resistance allele increases in the population
• Directional Natural Selection


  Shown by the example of antibiotic resistant bacteria
• Completing the course of antibiotics ensures that the dosage is sufficient to clear most of the susceptible bacteria leaving only the antibiotic-resistant bacteria for the immune system to deal with
• Gene flow


  The transfer of alleles from one population to another through the movement of fertile individuals or their gametes
• Gene flow tends to reduce differences in allele frequencies between neighbouring populations
• Disruption to gene flow can result in differences in allele frequencies over time
• Disruption of gene flow is required for speciation to occur
• Genetic drift


  A change in allele frequency due to chance events

  
• Genetic drift tends to reduce genetic variation in populations through losses of alleles
• Founder effect


  • A few, random individuals from a larger population became pioneers of a newly isolated population and are not likely to carry all the alleles present in the original population
  • The new population is usually small and reproductively isolated
  • Rare alleles may become more common
• Bottleneck effect


  • A catastrophic event leading to a drastic reduction in population results in the reduction of allele frequencies and even elimination of alleles in a random nature
  • The few, random surviving individuals constitute a random genetic sample of the original pre-catastrophe population