4: Natural selection Flashcards
Describe positive selection + example
For a gene with 2 alleles A1 and A2, allele A1 is under positive selection if it offers a selective advantage A2
e.g Peppered moth
Increase in frequency of melanic form in polluted areas → driven by positive selection
Clean air acts 1959 onwards → melanic form is no longer a fitness advantage
e.g Adaptive melanism in rock-pocket mice
Describe positive selection in the drug resistance of HIV
- How it works
- How it evolved so quickly
- Single stranded RNA virus causing AIDS = used to be 100% fatal
- Reverse transcriptase converts RNA to DNA - key life stage
- 3TC drug interfers with RT, block repro
- But, mutant HIV strains are resistant to 3TC
Why did it evolve so quickly?
- RT is error prone → has a high mutation rate (~3.4 x - 10-5/ site/gen)
- Gen time of 2.5 days
- An extremely large pop. size
What type of selection is sickle-cell anaemia/malaria?
Balancing selection
Describe how the link between sickle-cell anaemia and malaria is an example of balancing selection.
- Recessive disorder caused by mutation in the β-haemoglobin gene - 80% die before reproduction
- S (disease form) is recessive to A (normal form)
- AA = no anemia
- AS = usually healthy, sometimes mild anemia
- SS = severe anemia (most killed by it)
S allele persists in high frequencies in parts of Africa
- AA = susceptible to severe malaria
- AS = quite resistant to malaria
- SS = mostly killed by anemia but resistant to malaria
= heterozygotes highest fitness
What type of selection is Kuru disease associated with?
Balancing
Describe Kuru disease and how it is an example of balancing selection
→ prion disease, no cure, fatal
- Spread by consumption of infected meat (brain)
- In the Fore tribe of Papua New Guinea cannibalistic funerary, women & children consume human brains
- Genetic variation in the prion protein gene (PRNP) associated with susceptibility
= The PRNP gene was under balancing selection when kuru prevalent
= Heterozygotes have the highest fitness
