SELECTION

• WHAT IS NATURAL SELECTION?

  mechanism of evolution where more genetically fit organisms survive and pass on alleles (leading to a shift in allelic frequency)
• WHAT IS A SELECTION PRESSURE?

  environmental force altering the frequency of alleles in a population
• EXAMPLES OF SELECTION PRESSURES?

  predation, lack of food, competition for mates, antibiotics, disease
• WHAT IS AN ALLELE?

  variant of a gene caused by a different base sequence via mutation
• TYPES OF SELECTION?

  disruptive, stabilising, directional
• WHAT HAPPENS IN DISRUPTIVE SELECTION?

  • individuals containing allele coding for either extreme trait are more likely to survive
  • pass on alleles
  • allele frequency changes
  • more individuals with allele for extreme trait
• WHAT DOES DISRUPTIVE SELECTION LEAD TO?

  speciation
• SUMMARISE DISRUPTIVE SELECTION?

  natural selection causes shift in allele frequency to both extremes
• SUMMARISE STABILIZING SELECTION?

  natural selection causing shift in allele frequency towards mean value
• SUMMARISE DIRECTIONAL SELECTION?

  natural selection causing shift in allele frequency away from mean towards one extreme
• WHAT DO SELECTION PRESSURES NOT DO?

  cause mutations, only select for or against characteristics
• EXAMPLE OF STABILISING SELECTION?

  baby birth weight
• EXAMPLE OF DIRECTIONAL SELECTION?

  antibiotic resistance
• EXAMPLE OF DISRUPTIVE SELECTION?

  long/ short fur mammals (seasonal advantages) or peppered moths
• NATURAL SELECTION PROCESS?

  • random gene mutation
  • causes genetic variation in population
  • selection pressure
  • organism with advantageous allele survives and reproduces
  • pass on alleles to offspring
  • over many generations the frequency of the advantageous allele increases