Selection

Question 1

Directional selection

Answer 1

When there is an environmental change.
Selective pressure occurs for species to change in response to it.
It favours individuals that are different.
E.g. Antibiotic resistance

Question 2

Distribution curves - directional selection

Answer 2

Individuals at one extreme of range are selected for.
Individuals are selected against at the other side of the range.
There is a shift in the mean.

Question 3

Stabilising selection

Answer 3

When there is no change in the environment. It remains stable.
Individuals with phenotypes closest to the mean are favoured for.
Individuals with extreme phenotypes selected against, reducing them.
Distribution curves become narrower.
E.g. Human birth weights.

Question 4

Natural selection (Darwin)

Answer 4

Individuals vary due to mutations taking place.
Some are better adapted for survival, meaning they Are more likely to reproduce and pass on alleles.
Over time, the frequency of advantageous alleles increases in the population.