Lecture 8: Variation, selection and adaption Flashcards
Classic example of natural selection (moth) INFO
Peppered moth (Biston betularia)
Two morphs:
1) Typical (typical) form
2) Melanic (carbonara) form
GENETICS: -controlled by a single gene with 2 alleles.
-Melanic allele (M) is dominant to the typical allele (+)
Classic example of natural selection (moth) HISTORY
The melanic form was first seen in Manchester in 1848
-its frequency increased 1-10% in industrial areas by 1898
-had a freq of >99% in Manchester in 1948.
Typical form remained common in rural areas
Typical form:
grey/ lighter form
Melanic form:
Black form
The relative fitness of different genotypes is calculated by..
taking the most favoured genotype as the standard with a fitness of 1.0 from % recaptured
The selection coefficient (s) against a disfavoured genotype is calculated by
difference between the fitness of the standard (taken as 1.0) & the relative fitness of the genotype in question.
The increase in frequency of the melanic form in polluted areas was driven by
POSTIVE SELECTION
After the clean air act-
1959 onwards.
As pollution declines the melanic form is no longer cryptic to birds and so it loses its fitness advantage.
Adapted melanism in rock-pocket mice (Chaetodipus intermedius)
-coat colour determined by the melanocortin 1 receptor gene (MC1R).
D (dark) is dominant to d (light)
Rock-pocket mice two different environments..
- light-coloured sandy substrate in most of its range. Mice generally have lighter hair.
- In area with a lot of BASALT (black rock from volcano), the dark form i common.
Rock-pocket mice predators
owls and mammalian carnivores
HIV..
human immunodeficiency virus
HIV and AIDS
HIV is the pathogen causing aids (acquired immunodeficiency syndrome), with almost 100% fatality
When was AIDS first recognised & how many
in 1981; 42 million people infected worldwide, 5 million new infections per year
AID is a single/ double strand of DNA/RNA virus??
single stranded RNA virus
HIV makes an enzyme called..
reverse transcriptase (RT). RT converts RNA into double-stranded DNA; key step in the life cycle of HIV; prerequisite for replication
Whats 3TC and what does it do?
HIV drug.
It interferes with the normal RT, blocking the reproduction of HIV. HIV strains with mutant RTs that differ from the normal RT at the 184th a.a. are resistant to 3TC. The mutants have functional defects (rare in the absence of 3TC)
In 4 Weeks host becomes resistant, drug no longer effective.
Whats helping HIV to evade the drug 3TC?
- reverse transcriptase is error prone (HIV has a very high mutation rate [~3.4 X10^-5/site/generation])
- Generation time of 2.5 days
- An extremely large population size
HIV lifecycle and recombination
Recombination is an integral part of the HIV lifecycle, occurring when reverse transcriptase switches between alternative genomic templates during replication
Examples of positive selection
- peppered moth, rock-pocket mice, HIV
- evidence from genotypes and phenotypes
- positive selection can drive advantageous phenotypes and genotypes to high frequencies
Positive selection is just one example of
natural selection
Sickle-cell anaemia
- a recessive disorder caused by a mutation in the beta-haemoglobin gene; 80% die before reproduction
- S (disease form) is recessive to A (normal form)
- AA: no anaemia
- AS: usually healthy some with mild anaemia
- SS: severe anaemia
link between anaemia and malaria
Anaemia kills the malaria.
AA: susceptible to severe malaria
AS: quite resistant
SS: mostly killed by anaemia
AS red blood cells and sickling
Do not normally sickle. They do when they become infected with Plasmodium falciparum (causes malaria). The cell is destroyed and the parasite killed in the process.
The S allele (for anaemia) is maintained at..
high frequencies by natural selection
Balancing selection..
another form of natural selection; it can happen when heterozygotes are fitter than homozygotes. (such as S allele in africa malaria infested parts)
Kuru and the PRNP gene
cannibalism banned in the 1950s.
Heterozygotes have highest fitness. PRNP was under balancing selection when kuru was present.
Kuru:
a prion disease similar to mad cow disease and Variant CJD (vCJD); no cure, fatal. Spread by consumption of prion infected meat. When men die women and children consume flesh in cannibalistic rituals. Genetic variation at the prion protein gene (PRNP) is associated with susceptibility to prion disease in various species.
Two forms of natural selection
1) positive selection e.g drug resistance
2) Balancing selection e.g. Kuru
