22 Natural Selection

Question 1

How is the relative fitness of different genotypes calculated?

Answer 1

Take the most favoured genotype as the standard with a fitness of 1.0
- Divide % recaptured of melanic morph by % recaptured of typical morph
- So chance of survival for melanic moth only 34% of that for a typical moth in an unpolluted wood

Question 2

How is the selection coefficient against a disfavoured genotype calculated?

Answer 2

The difference between the fitness of the standard (1.0) and the relative fitness of the genotype in question
s = 1.0 - 0.34 = 0.66

Question 3

What is positive selection?

Answer 3

A form of natural selection
For a gene with 2 alleles A1 and A2, allele A1 is under positive selection if it offers a selective advantage over A2 (relative fitness A1A1>A1A2>A2A2)
- So A1 increasing in frequency and replacing A2

Question 4

How is there positive selection in rock-pocket mice?

Answer 4

Selection maintains the dark allele at high frequencies on lava flows
- If selection was weak, allele frequencies would be more even due to gene flow

Question 5

How is HIV able to rapidly develop resistance to drugs?

Answer 5

• Reverse transcription is error prone - HIV has high mutation rate
• Short generation time
• Extremely large population size

Beneficial alleles arise by mutation quickly, then driven to high frequencies by positive selection

Question 6

Why is recombination important to HIV?

Answer 6

It occurs when reverse transcriptase switches between alternative genomic templates during replication
- Recombinant strains are resistant to drugs used in combination therapy

Question 7

What is Sickle-cell anaemia?

Answer 7

• Recessive disorder caused by mutation in Beta-haemoglobin gene
• AA: no anaemia
• AS: usually healthy, sometimes mild anaemia
  -SS: severe anaemia

Question 8

Why does the allele for Sickle-cell anaemia persist at high frequencies?

Answer 8

There is extensive overlap of the geographical distribution of SCA and malaria

S allele maintained at high frequencies by natural selection
- May be beneficial to be a heterozygote in areas with high susceptibility to malaria
- As are quite resistant to malaria but also don’t have severe anaemia
- Form of balancing selection

Question 9

When does balancing selection occur?

Answer 9

When heterozygotes are fitter than homozygotes

Question 10

What can departures from HWE indicate?

Answer 10

Natural selection

Question 11

What level can natural selection be observed at?

Answer 11

Both phenotype and genotype level