Types of selection

What is one method by which evolution occurs?

• The frequency of an allele in a population changes over time - this is evolution (look at Chapter 4)

  • Natural selection is one method by which evolution occurs

Explain natural selection

• Individuals of the same species vary because they have different alleles  (maybe due to mutation)

• Predation, disease, climate change and competition (selection pressures) create a struggle for survival

• Because individuals vary, some are better adapted to the selection pressures than others

• This means that individuals with a phenotype that increases their chance of survival are more likely to survive, reproduce and pass on their genes which includes the passing on of the advantages alleles

  • As a result, the allele frequencies will be different in the next generation, with favourable alleles increasing in frequency

How can natural selection be categorised?

Natural selection can be categorised based on its effects on phenotypes in populations:

What are the three types of natural selection?

• directional, stabilising, and disruptive selection

Explain directional selection

• This kind of selection usually occurs when there is a change in the environment

• Its effect on populations: it selects for one extreme phenotype over other phenotypes

Explain stabilising selection

• This kind of selection is more likely to occur when the environment is not changing

• Its effect on populations: it selects the average phenotype and selects against extreme phenotypes

Explain disruptive selection

• This kind of selection is where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce. It's the opposite of stabilising selection because characteristics towards the middle of the range are lost

• It occurs when the environment favours more than one phenotype

Provide an example of disruptive selection

• Birds with large beaks are specialised to eat large seeds and birds with small beaks are specialised to eat small seeds

• In an environment where the majority of seeds are large or small and very few are medium-sized birds with a medium-sized beaks may have a reduced chance of survival as they are unable to eat either large or small seeds effectively

• Birds with large or small beaks are most likely to survive, reproduce and pass on their alleles. So the alleles for a large and small beak increase in frequency, but the alleles for a medium-sized beak decrease in frequency.

• Over time the proportion of the population that have either large or small beaks increases

Draw the graphs for directional, disruptive and stabilising selection

check on notes