Types of selection

• What is stabilising selection?

  When the median trait has a selection advantage and therefore continues to be the most frequent in the population. The range decreases as the extreme traits are lost over time.
• Name a common example of stabilising selection.

  Human birth weight

  Middling birth weight is a selective advantage as extremely low birth weights may be disadvantageous to survival due to the underdevelopment of the baby. Likewise, an extremely large birth weight may lead to complications during the birth. Therefore, babies born with a middling weight are more likely to survive and pass on the advantageous allele to their offspring later in life.
• Which trait has a selective advantage?

  The modal trait
• When does the modal trait occur?

  when there is no change in the environment
• Does the modal trait change?

  No, it remains the same
• What happens to standard deviation as the number of individuals with the extreme trait decrease?

  Standard deviation decreases.
• What is directional selection?

  Directional selection is when one of the extreme traits has a selective advantage.
• Directional selection.

  • One of the extreme traits has a selective advantage.
  • Directional selection occurs when there is a change in the environment.
  • The modal trait changes.
• Name an example of directional selection.

  Antibiotic resistance in bacteria.
• How does antibiotic resistance occur in bacteria?

  A random mutation creates an allele that provides resistance to antibiotics in bacteria. If this population of bacteria are then exposed to this antibiotic, only those with the resistance allele survive, and all others die. This allele is then passed on over many generations and results in most of the bacteria carrying this allele.
• What 5 things should you include when talking about natural selection?

  • Random mutation
  • Advantageous allele
  • Survival and reproduction
  • Inheritance of the allele
  • Increase in allele frequency