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Evolution and Human Health - Coggle Diagram
Evolution and Human Health
Cholera epidemic in 1854
lethal diarrhea and consequently dehydration
John Snow in 1855 prepared a map of affected neighborhood, revealing victims and water pumps
Germ Theory
Early discoveries
Inspired by Pasteur, other scientists found bacteria responsible for anthrax, gonorhea, tb
Robert Koch in 1883: vibro cholerae
Louis Pasteur in 1858 proposed that diseases were caused by microorganisms
Led to identification of pathogens
Infection-fighting developments
antiseptic surgery
discovery of antibiotics
improvements in sanitation
Evolving Pathogens
pathogens have evolved in response to selection pressures imposed by medicine
antibiotics: chemicals that kill bacteria
penicillin
first antibiotic
saved thousands of lives
Today is ineffective against bacteria that previously were highly vulnerable
Select for resistance in bacteria
Frequency of antibiotic resistance tracks changes in the number of prescriptions
Cost of resistance in bacteria
a. resistance rates fell in Iceland when Penicillin use dropped, suggesting that resistance imposes a cost on bacteria
b. if resistance comes at a cost, then in the absence of antibiotics, sensitive bacteria will have higher fitness
c. However, the cost does not always persist
d. resistant bacteria can get equal in fitness compared to sensitive ones, even in the absence of antibiotics
Study: Scrag et. al. 1997
a. Studied streptomycin-sensitive
E. coli
and screened for resistant mutants
b. Streptomycin interferes with protein synthesis by binding to a ribosomal protein (rpsL)
c. But point mutation in rpsL provided resistance to streptomycin
d. Competed resistant and sensitive strains against each other
initially: resistant strains at disadvantage
Later: resistant strains evolved, and new mutations compensated for the cost
e. With time, the resistant strains outcompeted sensitive strains
Avoiding evolution of resistance:
reducing antibiotic usage might not restore vulnerable bacterial population
thus, steps to avoid bacteria from developing resistance need to be taken:
reduce infection rate
limit use of antibacterial soaps and cleaners
Design and use drugs that target few bacteria
antibiotic use in animal feed should be eliminated
Change in frequencies of genotypes within a population
populations of pathogens and host are in conflict
Pathogens: want to consume host to make more pathogens
Host: wants to kill and and protect itself from pathogens
Pathogens naturally evolve responses:
large population size
short generation times
high rates of mutation
evolve quickly
Is selection imposed by the human immune system responsible for detectable evolution in populations of influenza A viruses?
Influenza A:
responsible for annual flu epidemics
ordinary season in the USA kills ~20k people
1918 epidemic:
affected 20% of world pop
within a few months, killed ~50-100 million people
genome composed of 8 RNA strands that encode 13 proteins
has two major surface proteins:
Hemagglutinin:
Main viral coat protein
primary protein recognized and attacked by host immune system
Neuraminidase
Must survive by one of two of these:
constantly finding new host that doesn't recognize hemagglutinin
altering its hemagglutinin
Flu Virus Evolution
virus has antigenic sites (sites recognized by immune system)
hemagglutinin has 5 antigenic sites
viruses with novel antigenic sites should have a selective advantage
Study: Fitch et al (1991): Mutations on antigenic sites
examined frozen flu virus strains from 1968-1987
estimated rate of evolution and phylogeny from samples
found two patterns:
Most flu samples examined represented side branches of one main evolutionary tree
Surviving flu strains evolved at a steady rate
Expected and found: Main strain would have had more mutations in the antigenic sites than the extinct strains
Increased variability in antigenic sites gave surviving strain an advantage
Human immune system does exert strong selection on flu virus hemagglutinin genes and virus populations evolve in response
flu virus evolves ~1 million times faster than humans
Origin of pandemic flu strains
Radical alteration of hemagglutinin genes
Nucleoprotein gene most important for determining host specifity
strain evolutionary history determined by building phylogeny
each hemagglutinin/neuraminidase group constitutes a clade
Flu strains can have some closely related and some distantly related genes because of horizontal gene transfer
If a host is infected with two different strains, these strains can swap RNA strands
potential for new strains
very novel hemagglutinin genes
leads to pandemic!