Selection

• what is directional selection?

  where individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.
• what could directional selection be in response to?

  an environmental change
• give an exampel of directional selection?

  bacteria evolving antibiotic resistance
• how does directional selection work?

  • some individuals in a popoulation have allels that give them resistance to an antibiotic
  • the population is exposed to the antibiotic, killing bacteria without the resistant allele
  • resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring
  • after some time, MOST organisms in the population will carry the ANTIBIOTIC RESISTANCE ALLELE
• what is stabilising selection?

  where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce
• when does stabilising selection occur?

  when the environment IS NOT CHANGING, and it REDUCES THE RANGE OF POSSIBLE CHARACTERISTICS
• what is an example of stabilising selection?

  human birth weight
• how does stabilising selection work?

  • humans have a range of birth weights
  • very small babies are less likely to survive - partly because they find it hard to maintain their body temp
  • giving birth to large babies can be difficult , so large babies are also less likely to survive
  • conditions are most favourable for medium-sized babies- so weight of human babies tends to shift towards the middle of the range
• what are two types of natural selection?

  directional

  stabilising
• what does natural selection do?

  Alters allele frequency in a population