7.3 Evolution may lead to speciation

• Explain why individuals within a population of a species may show a wide range of variation in phenotype

  Genetic factors

  • Mutations = primary source of genetic variation
  • Crossing over between homologous chromosomes during meiosis
  • Independent segregation of homologous chromosomes during meiosis
  • Random fertilisation of gametes during sexual reproduction

  Environmental factors (depends on context - eg. food availability, light intensity)
• What is evolution?

  • Change in allele frequency over time / many generations in a population
  • Occurring through the process of natural selection
• Describe factors that may drive natural selection

  • Predation, disease and competition for the means of survival
  • These result in differential survival and reproduction, ie. natural selection
• Explain the principles of natural selection in the evolution of populations

  1. Mutations = Random gene mutations can result in [named] new alleles of a gene
  2. Advantage = Due to [named] selection pressure, the new allele might benefit its possessor [explain why] → organism has a selective advantage
  3. Reproduction = Possessors are more likely to survive and have increased reproductive success
  4. Inheritance = Advantageous allele is inherited by members of the next generation (offspring)
  5. Allele frequency = Over many generations, [named] allele increases in frequency in the gene pool
• Explain the effects of stabilising selection

  • Organisms with alleles coding for average / modal variations of a trait have a selective advantage (eg. babies with an average weight)
  • So frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease
  • So range / standard deviation is reduced

  
• Explain the effects of directional selection

  • Organisms with alleles coding for one extreme variation of a trait have a selective advantage (eg. bacteria with high resistance to an antibiotic)
  • So frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decrease

  
• Explain the effects of disruptive selection

  • Organisms with alleles coding for either extreme variation of a trait have a selective advantage
  • So frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease
  • This can lead to speciation

  
• Describe speciation (how new species arise from existing species)


  1. Reproductive separation of two populations (of the same species)
  2. This can result in accumulation of differences in their gene pools
  3. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring
• Describe allopatric speciation

  1. Population is split due to geographical isolation (eg. new river formed)
  2. This leads to reproductive isolation, separating gene pools by preventing interbreeding / gene flow between populations
  3. Random mutations cause genetic variation within each population
  4. Different selection pressures / environments act on each population
  5. So different advantageous alleles are selected for / passed on in each population
  6. So allele frequencies within each gene pool change over many generations
  7. Eventually different populations cannot interbreed to produce fertile offspring

  
• Describe sympatric speciation (part 1 steps 1+2)

  1. Population is not geographically isolated
  2. Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding / gene flow within one population, eg
  3. Gamete incompatibility
  4. Different breeding seasons (eg. different flowering times)
  5. Different courtship behaviour preventing mating
  6. Body shape / size changes preventing mating
• Describe sympatric speciation (part 2 steps 3-6)

  1. Different selection pressures act on each population
  2. So different advantageous alleles are selected for / passed on in each population
  3. So allele frequencies within each gene pool change over many generations
  4. Eventually different populations cannot interbreed to produce fertile offspring
• Explain genetic drift and its importance in small populations (part 1)

  • Genetic drift = a mechanism of evolution in which allele frequencies in a population change over generations due to chance (NOT natural selection)
  • Some alleles are passed onto offspring more / less often by chance
  • Regardless of selection pressures and whether alleles give a selective advantage
• Explain genetic drift and its importance in small populations (part 2)

  • So strongest effects in small populations with no interbreeding with other populations (no gene flow), as gene pool is small and chance has a greater influence
  • Eg. when a population is sharply reduced in size (bottleneck effect)
  • Eg. when a small, new colony forms from a main population (founder effect)
  • This can reduce genetic diversity - some alleles have much higher frequencies, others are lost