17.2.2 Natural Selection: Types of Selection

• Environmental factors that affect the chance of survival of an organism are selection pressures
  • For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects’ for faster, more powerful lions that are better hunters

• These selection pressures can have different effects on the allele frequencies of a population through natural selection
• There are three types of selection:
  • Stabilising
  • Disruptive
  • Directional

Stabilising selection

• Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
  • This means that allele frequencies stay as they are unless there is a change in the environment
• A classic example of stabilising selection can be seen in human birth weights
  • Very low and very high birth weights are selected against leading to the maintenance of intermediate birth weights
    • It is disadvantageous to have a very low birth weight because it increases the risk of health complications for the baby
    • It is disadvantageous to have a very high birth weight as this increases the risk of birth complications

Stabilising selection on human birth weight

Directional selection

• Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
• This usually happens when there is a change in the environment or a new selection pressures which leads to a certain allele becoming advantageous
• For example, a recent finding has shown that climate change is having an effect on fish size in certain habitats; the increase in temperature is selecting for a smaller body size and against a larger body size
  • Warmer seas cause fish metabolism to speed up and so increases their need for oxygen; oxygen levels are lower in warmer seas
  • Larger fish have greater metabolic needs than smaller fish, and so they feel the effect of increased temperatures more strongly
  • Organisms are sensitive to changes in temperature primarily because of the effect that temperature can have on enzyme activity
  • Fish with a smaller body size are therefore fitter and better adapted to living in seas experiencing increased temperatures
  • Fish body size is determined by both genetic and environmental factors
  • Fish of a smaller size are more likely to reproduce and pass on their alleles to offspring
  • Over generations, this leads to an increase in the frequency of alleles that code for a small body size and a decrease in the frequency of alleles that code for a larger body size

Directional selection acting on fish body size

Disruptive selection

• Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles
  • In other words, individuals with intermediate phenotypes or alleles are selected against
• Disruptive selection maintains polymorphism; the continued existence of two or more distinct phenotypes in species
• This can occur in an environment that shows variation
• For example, birds that live on the Galapagos Islands use their beaks to forage for different sized seeds
  • Different sizes of seed are more efficiently foraged by a shorter or longer beak than by a medium-sized beak
  • The size of the bird's beaks are either small or large with the intermediate, medium-sized beak selected against

Disruptive selection acting on beak size in a bird population