20. Natural selection and speciation

• What is evolution

  Change in the frequency of alleles within a population over many generations
• Why do allele frequencies change

  Differential reproductive successes
• What causes genetic variation

  Mutation

  meiosis

  random fusion of gamete's
• What is the process of natural selection

  There is a random mutation giving rise to new alleles of a gene. Organisms with advantages alleles will have a phenotype with a selective advantage in the environment. They are more likely to reproduce and can then pass on the advantageous allele. Causes an increase in frequency of this allele.
• What is an anatomical change 


  Change to the body (anatomy)
• What are the three types of natural selection

  Directional

  stabilising

  disruptive
• What is directional selection

  Occurs when the environment changes and it favours one extreme phenotype. (The mode changes)
• What is stabilising selection

  Occurs in an unchanging environment and acts against both extreme phenotypes (decreased range and standard deviation)
• What is disruptive selection 


  Occurs in a fluctuating environment where it favours both extremes and acts against those in the middle. (Mean splits )
• What is speciation

  Process where one gene pool gives rise to more than one gene pool
• What is allopatric speciation

  When two populations become geographically separated by a physical barrier. Prevented interbreeding.
• What do different environments in allopatric speciation lead to

  Different selection pressures
• What is sympatric speciation

  The populations still inhabit the same area but evolve into different species. (Reproductive isolation)
• What are the causes of sympatric speciation

  Seasonal isolation - reproduce at different times of year

  mechanical isolation - physically not possible to breed
• How does evolution happen via genetic drift

  When chance causes alleles to be passed on.