KEY CONCEPT Unifying themes connect concepts from many fields of biology.
Evolution is the unifying concept of biology. Two Central Themes of Biology Adaptation - How and in...
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Transcript of Evolution is the unifying concept of biology. Two Central Themes of Biology Adaptation - How and in...
Evolution is the unifying concept of biology
Two Central Themes of Biology
Adaptation - How and in what ways doorganisms function and become bettersuited to their environments?
Diversity- What are the evolutionaryrelationships among species andand how do they arise?
Evolution : key points
• Evolution means change• Modern organisms are
descendants of pre-existing life forms
• The evidence for evolution is overwhelming
• Natural selection is a primary mechanism of evolution
A flowchart of evolutionary reasoning
Evolution via natural selection is a logical outcome given:
• Individuals within species are variable
• Some variations are passed on to offspring
• In every generation, more offspring areproduced than survive
• Survival and reproduction are not random: individuals that survive and reproduce are ones with the most favorable variations
Evolution is important to many fields of biology
• HIV and AIDS research•AZT worked initially, but then
failed….why?
•Why is HIV fatal?
•Why are some people resistant to HIV?
•Where did the HIV virus come from?
2007 DataUNAIDS
Change in HIV Prevalence: 1990 vs 2007
Credit: UNAIDS
2007 DataUNAIDS
Prevalence AmongPregnant WomenIn Botswana
HIVHuman
ImmunodeficiencyVirus
Credit: NIAID
Credit: CDC
HIV-I (green)emerging from acultured lymphocyte
Credit: NIAID
Normal T-cells HIV Infected T-cells
AZT blocks reversetranscription
gp120 protein binds to CD4 protein and co-receptor
Kuritzkes D R PNAS 2001;98:13485-13487
©2001 by The National Academy of Sciences
AZT worked initially, but then failed…..Why?
• Mutations of reverse transcriptase occur frequently during viral reproduction
• These mutations yield virions with differenttypes of reverse transcriptase,that differ with respect to AZT inhibition
• Some mutant virions were better able tosurvive and reproduce in an environmentwith AZT than others
• These mutations were passed on to theoffspring of AZT-resistant virions
Why is HIV Fatal?
Evolutionary Biologists Think About Problems From The Perspective of the Organism
Why is it advantageous for me-the-HIV-virus toreplicate so quickly that I kill my host?
So…….
Viruses are Deadly but Short-Sighted
1) They are so good at evolving new epitopes, they rapidly burn through the host’s supply of T-cells.
2) After infection, HIV populations evolve towardmore aggressive replication.
3) Eventually, during infection HIV populations evolve the ability to use CXCR4 to gain access to naïve helperT-cells.
4) Virulence may be associated with transmission.
VIRUS DNA VARIATION
Evolution of HIV Within a Single Host
Days Since HIV Infection
0 6 12
X4 virionsOriginate
DuringThe Infection
Cycle
Why are some people resistant to infection by HIV?
Some people have a mutant form of CCR5 (co-receptor) on their CD4 cells.
Natural selection on human populations mayresult in an increase in HIV resistance alleles.
Prediction:
CCR5-delta32
Where did HIV originate?
Phylogeny: a tree that shows the genealogy of a groupof strains, species or populations.
More closely related species should have more similarcharacteristics.
Basic concept underlying a phylogenetic tree:
Phylogeny of SIV and HIVindicates that HIV originatedindependently at least twotimes from non-human primates.
Phylogeny based on reversetranscriptase gene.
Phylogeny based on gp120 gene.
Phylogeny of chimp SIV and HIV indicates that HIV originated independently at least three times from chimp SIV.
What does Evolutionary Biology have to say about AIDS?
1) Resistance alleles should increase in human and viral populations.
2) The search for a vaccine may be futile.a) Combinational therapies may slow HIV evolution.
3) The best defense is education.