2. Variation and Selection

Natural Selection

Where traits better suited to their environment are more likely to survive, reproduce, and pass on their advantageous alleles to their offspring.

Selection pressures

Pressures that affect the chances of organisms surviving

e.g. predation, disease, competition

3 types of natural selection:

• stabilising selection

• directional selection

• disruptive

• stabilising selection

• where individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.

• This occurs when environments are changing.

• This reduces the range of possible phenotypes

directional selection

• Where individuals with alleles for a single extreme phenotype are more likely to survive and reproduce.

• This could be in response to an environmental change.

• The mode changes.

Disruptive selection

• Where individuals with alleles for extreme phenotypes are more likely to survive and reproduce.

• This is the opposite for stabilising selection because characteristics towards the middle of the range are lost.

• This occurs when the environment favours more than one phenotype.