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Natural Selection
The AIDS Story
Basketball Player “Magic” Johnson
• 1991 at age 32 retires
• He has AIDS and possibly just a few
years to live (8 - 10 years)
• Fast forward to 2007 - 16 years later
and he is still alive
• How and why
• Medical advances and knowledge of
evolution
HIV and AIDS
• AIDS (acquired immune deficiency
syndrome)
• Killed 40 Million already and is on the
increase! - worry baby!
• Disease of the immune system
• AIDS is caused by HIV virus
• Virus infects and kill white blood cells
• T cells
• T cd4 cells
• Host loses immunity and dies from other
infections
Course of HIV infection
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Virus invades
Infects T cells and releases ‘babies’
Body fights
Virus mutates
Body fights
Virus mutates
Virus evolves and escapes detection
The Theory of Natural Selection
• The Origin of Species by Charles Darwin
– 1) Common descent
– 2) Natural Selection
• Common descent - we all appear to have
descended from a common ancestor accepted easily
• Natural Selection - How variety has come
about since - hard to comprehend by many
early scholars - it the greatest scientific theory
of all.
Natural Selection is based on
4 general observation
1. Individuals in a population vary
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Variation exist
2. Some variation is inherited
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Genetic material involved
3. More offspring are produced then will
survive
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Multiple births
4. Survival and reproduction are not random
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Those who are not suited have fewer offspring
or not at all……
Testing Natural Selection
• Artificial selection
– Humans and Dogs
– Dogs originated from the Grey Wolf
– Breeding for physical traits
– Many breeds of dogs - all one species
– Human induced and not natural selection
Natural Selection in the Lab
– High concentration of alcohol causes cell death
– Fruit Flies tested by feeding them food with alcohol
– As with any population there is variation in the
natural population
– Most flies find alcohol poisonous, but there are
some (about 10% occurring naturally) who have a
gene that can reduce to a less toxic form
– Conducted experiment for 57 generations
– Compared results
Wild populations
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No human interference
Galapagos finches
1977 drought filled many
Natural selection selected those that
had the largest bills
• Better at breaking tough seeds
• In 1978 this finch population had a 5%
greater bill size
Subtleties of Natural Selection
• DODO
• “The Dodo could not adapt to human
hunters , so it went extinct”
• Wrooooooooonnnnnnnnggg!!!!
• Change and evolution do not work at
the individual level - just at the
population and gene frequency levels
Subtleties of Natural Selection
• Natural selection does not result in perfection
• Better adapted to the current circumstances
• Bacteria example
– Growth of bacteria under starvation conditions
– Those that produce a toxin, which kills neighbours lives
– Change to food-rich environment and these same bacteria
lose out to the others
– The normal bacteria do not waste time making the toxin
Natural Selection and HIV
•
4 conditions are present for evolution of HIV
virus
1. Virus in the bloodstream vary
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Reproduction equals mutation
2. Variation is passed to offspring
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Mutated RNA molecules infect new cells
3. More virus produced than can survive
–
Most HIV is eliminated by the antibodies of the host
4. Virus survival is not random
–
Antibodies do not recognize these and they will exist
for longer
Evolutionary Arms Race
• Virus is changing and so is the host
– As each variant becomes more
pronounced the host matches and makes
new antibodies
– HIV begins to be cleared from the blood
– Virus mutates and other strains survive…
Medical Solutions
• Drugs which interfere with the life cycle
of the virus
AZT failure
• No matter who took the AZT alone they
sooner or later died of AIDS
• The virus was mutating to become
resistant to the AZT
• Single drug therapy could give you
some extra time but you would die..
Combination drug therapy
• Combining 3 or 4 drugs which act at different
phases of the life cycle of the virus has worked
extremely well
• A single virus cannot (at least not easily) mutate
simultaneously at all these parts of its life cycle
• Resistance to multiple drugs is uncommon
• Decreasing HIV replication decreases rate of
evolution
• Multiple drug resistance may be less deadly
– 15% of patients develop drug-resistant HIV but it does
not kill them as quickly - takes 3-5 years longer
Problems with Combination
Drug THerapy
• Drug resistance
• HIV seems less serious a disease
• Therapy is expensive
Preventing AIDS
• Can humans evolve resistance to HIV
– In some Europeans there is a 1% chance you will
never get AIDS
– They carry a protein - CCR5 - makes their T cells
resistant to infection
– Issues to many to accept
• ABC
– Abstinence
– Be Faithful - monogamous
– Condoms
Living with AIDS
• ‘Magic’ Johnson is still alive and free of AIDS
- why?
• He is meticulous
– Takes his medications on time!
• and has access to the best medical care in
the World
– He is rich!
• What will happen to him - will he die from
being hit by a bus?