3.7.3.1 Variation and selection

Question 1

Define selection pressure

Answer 1

Environmental factor which affects an organism’s chance of survival (e.g. disease, predation, competition)

Question 2

** Define differential reproductive success

Answer 2

Individuals with a selective advantage are more likely to survive and reproduce

Question 3

Summarise the stages of natural selection

Answer 3

1. Phenotypes vary bc of DIFFERENT ALLELES which have arisen due to MUTATION
2. SELECTION PRESSURES create struggle for survival
3. Some individuals are BETTER ADAPTED than others so SURVIVE, REPRODUCE & PASS ON BENEFICIAL ALLELE (differential reproductive success)
4. GREATER PROPORTION of the next generation inherit BENEFICIAL ALLELES
5. The FREQUENCY OF BENEFICIAL ALLELES INCREASES over many generations in a gene pool

Question 4

Sources of variation

Answer 4

• Random mutation
• Random fertilisation
• ( in meiosis) crossing over & independent segregation

Question 5

Explain how different subspecies may have evolved from a common ancestor

Answer 5

• mutation linked to different trait / phenotype
• selection for different trait
• leading to differential reproductive success
• increase in allele frequency

Question 6

What is disruptive selection?

Answer 6

• selection against mean / selection for allele with extreme trait at both ends
• occurs when environment favours more than one phenotype

Question 7

What is directional selection? and example

Answer 7

• selection against an extreme
• occurs in response to an environmental change
• e.g. antibiotic resistance, giraffe neck

Question 8

What is stabilising selection? and example

Answer 8

• selection against both extremes

- occurs in a stable environment, plant height, human birth weight