Types of Natural Selection (DP IB Biology): Revision Note

Types of Natural Selection

• There are three main types of selection:

  • Directional

  • Stabilising

  • Disruptive

Directional selection

• The population changes towards one extreme of a range of variation

  • As that extreme becomes better adapted

• This tends to happen when environmental conditions change

• For example, a fall in average temperatures can affect plants that are not resistant to frosts (spells where the air temperature falls below 0°C)

• If there is no allele that can give the species a degree of frost resistance, then the species will become extinct in that habitat if cold temperatures endure for more than one generation

• If an allele exists that gives a degree of frost protection, then the species will be able to

  • Survive the frost

  • Go on to reproduce successfully

• Such an allele may code for a new protein that can lower the freezing point of water/cell contents by a few degrees and prevent the formation of damaging ice crystals

• The species has developed a selective advantage over other species

• A disadvantageous allele does not have to be fatal to an individual organism

• However, it must prevent the individual from reproducing successfully

  • Which is effectively the same thing from an evolutionary point of view

  • Because the allele will disappear from the gene pool as the reproductively unsuccessful individuals die

• The species can change its genome abruptly by directional selection

Directional selection diagram

Directional selection acting on fish body size. Increases in ocean temperatures are selecting for smaller body sizes in fish. Warmer seas cause fish metabolism to speed up and so increase 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.

Stabilising selection

• Selects in favour of the average individual in a population

• Occurs when environmental conditions are stable / do not change

• Selection tends to favour individuals with a range of alleles whose characteristic is the most advantageous

• Stabilising selection is the most common form of natural selection

• An example is the coat colour of mice

  • The colour stabilises as the one which gives the most camouflage against the surroundings e.g. brown fur versus a forest floor

• Birth mass is also an example

  • Where a normal distribution clusters around a mean birth mass

  • Too low and too high can lead to problems of survival for an infant

• Stabilising selection

  • Discards extreme phenotypes

  • And instead favours the majority of the population that is well adapted to their local environment

  • Decreases diversity within a population

  • Works mostly on traits that are polygenic

  • Is often characterised by a normal distribution (a bell-shaped curve)

Stabilising selection diagram

Stabilising selection on human birth weight

Disruptive selection

• Selects against the average individual in a population

• Is the rarest of the three forms of natural selection

• Like directional selection, disruptive occurs when habitats or resources undergo a change

• Disruptive selection can lead to the formation of an entirely new species (speciation)

  • For this reason, is sometimes referred to as 'diversifying selection'

• Darwin's finches in the Galápagos Islands are one of the best-known examples

• Fifteen different species evolved from a common ancestor

• Multiple types of beaks have adapted to different food sources over time

• On one island, Santa Cruz

  • Ground finches eat more seeds and some arthropods

  • Tree finches eat more fruits and arthropods

  • Vegetarian finches feed on leaves and fruit

  • Warblers typically eat more arthropods

• When food is abundant, their diets can overlap

• When food is scarce, these specialisations give each species the ability to compete for a certain type of food better than other species

• This helps each species to occupy its own niche

Disruptive selection diagram

Disruptive selection acting on beak size in a bird population

Types of natural selection overview diagram

Summary of the three main forms of natural selection and their effects on the average phenotype of a population