G&E: Variation and Selection

Question 1

What is the main source of genetic variation?

How else can it be caused?

Answer 1

Mutation

• Meiosis (independent segregation, crossing over)*
• Random fertilisation*
• Environment*

Question 3

Evolution

Answer 3

The frequency of an allele in a population changing over time.

Question 4

Describe the process of natural selection:

Answer 4

1. Individuals of the same species vary due to different alleles.
2. Predation, disease and competition (selection pressures) create struggle for survival.
3. Some individuals are better adapted to selection pressures than others.
4. Creates differential levels of survival and reproductive success in a population.
5. Means a greater proportion of the next generation inherit the beneficial alleles.
6. So, frequency of beneficial alleles in gene pool increases from generation to generation.

Question 5

Stabilising selection

Answer 5

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

Question 6

When does stabilising selection occur?

How does it affect the phenotypes?

Answer 6

When the environment isn’t changing.

Reduces the range of phenotypes.

Question 7

Directional selection

Answer 7

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

Could be in response to an environmental change.

Question 8

Disruptive selection

Answer 8

Where individuals with alleles for extreme phenotypes at either end of the range are more likely to survive and reproduce.

Question 9

What is the opposite of stabilising selection?

Answer 9

Disruptive selection - characteristics towards the middle of the range are lost.

Question 10

What are the three types of selection?

Draw them on a graph:

Answer 10

Disruptive

Stabilising

Directional selection