Stabilising selection

• Stabilising selection - Selection favours average individuals, because the extremes are at a selective disadvantage

  • preserves the characteristics of the population
• Stabilising selection:

  • Individuals with characteristics towards the middle of the range are more likely to survive and reproduce
  • Occurs when the environment isn't changing
• Stabilising selection:

  • If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured
  • These individuals are more likely to pass their alleles on to the next generation
  • Those individuals with phenotypes at the extremes are less likely to pass on their alleles - and so tends to eliminate the phenotypes at the extremes
• Example of stabilising selection: HUMAN BIRTH WEIGHTS

  • Very low and very high birth weights are selected against, leading to the maintenance of the intermediate birth weights
• Example of stabilising selection: HUMAN BIRTH WEIGHTS

  • At very low birth weights, babies are unable to maintain their own body temperature due to their large SA:V so are unlikely to survive
  • At high birth weights, babies are more likely to die during childbirth as they are too large to fit through the pelvis
• Example of stabilising selection: HUMAN BIRTH WEIGHTS

  • The infants mortality rate is greater at the two extremes
  • The infants with the highest and lowest birth masses are more likely to die (being selected against) while those around the mean are less likely to die (being selected for/favoured)
  • The population's characteristics are being preserved rather than changed
• Stabilising selection results in phenotypes around the mean being selected for and those at both extremes being selected against