BIOL 3445 Lecture Notes - Lecture 29: Myxoma Virus, Retinal Detachment, Balancing Selection
BIOL 3445.001 | Lecture #29 | 5/1/2018
Evolutionary Medicine
OBJECTIVES
― Explain why maladaptation might lead to disease
― How do pathogens evolve in response to their hosts?
― How is virulence a result of balancing selection?
― How can we use phylogenetics to determine the origin of new diseases?
― Explain how human population structure can influence the frequency of disease-
causing alleles
― Explain how cancer cell lines compete with each other
― How are we “maladapted” to modern life?
MALADAPTAION AND DISEASE
― We suffer from disease because natural selection is never perfect
― Consider the human eye:
o We have a blind spot, so we need to compensate by scanning
o The tissues of the eye are easily damaged (retinal detachment)
o Warping the lens causes near and farsightedness
― Why do maladaptations make us vulnerable to disease?
o Pathogens evolve faster than hosts
o Natural selection always lags behind environmental change
o Trade-offs make some problems “unsolvable”
o Evolution is constrained by history
o Some traits increase reproduction at the cost of disease resistance
o Some “diseases” are actually old adaptations
EVOLVING PATHOGENS
― Viruses, bacteria, pathogens – anything that uses you as a host
― Pathogens evolve to:
o Evade the host’s immune system
o Use host resources
o Spread from host to host
― Plasmodium causes clumping of infected red blood cells to escape the risk of
destruction by the spleen
o The adaptation of plasmodium to avoid destruction leads to increased
virulence (damage to the host)
▪ The level of virulence is the result of a balance between selection
for competition and transmission
▪ Pathogens that use up host resources can outcompete other
strains, but they may suffer if the host dies before their strain can be
passed on to a new host
o Clumped RBC avoid going through spleen, but they block up the veins
and arteries which may lead to blood vessels bursting
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BIOL 3445.001 | Lecture #29 | 5/1/2018
― Kanta Subbaro showed that increased virulence can evolve quickly when you
remove selection for transmission
o Studying the SARS virus in mice
o Problem: SARS did not initially produce disease in mice
▪ Unable to study the effects of the disease if it won’t spread/infect
o Solution: Allowing the virus to evolve inside hosts and then “manually”
infecting new mice led to the evolution of a deadly SARS strain in just
fifteen (15) steps
▪ Passages: spreading a disease by manually infecting individuals
▪ Artificially selecting for virulence while selecting against transmission
o Competition among strains inside the mice selected for virulence
― Selection for virulence depends on how readily transmission to new hosts occurs
o Rabbit myxoma virus was initially very virulent (killed quickly) when
introduced to Australia
o As rabbit numbers dropped, transmission of the virus became difficult
o Selection favored less virulent strains that left hosts able to function longer
(and increase transmission)
o As the population increases, the disease will select for virulence and
mortality rates will increase until the population decreases, etc.
― So, selection can lead to increased virulence when conditions allow easy or fast
transmission between hosts
o In crowded or unsanitary conditions, viruses evolve to become virulent
― Pathogens evolve resistance to our medicines
o Antibiotic resistance is now a huge problem
o Some strains of bacteria are resistant to all known antibiotics
o Bacteria can evolve resistance rapidly and can pass resistance genes to
neighbors through horizontal gene transfer
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Explain why maladaptation might lead to disease. Explain how human population structure can influence the frequency of disease- causing alleles. Explain how cancer cell lines compete with each other. We suffer from disease because natural selection is never perfect. Consider the human eye: we have a blind spot, so we need to compensate by scanning, the tissues of the eye are easily damaged (retinal detachment, warping the lens causes near and farsightedness. Viruses, bacteria, pathogens anything that uses you as a host. Pathogens evolve to: evade the host"s immune system, use host resources, spread from host to host. Plasmodium causes clumping of infected red blood cells to escape the risk of destruction by the spleen: the adaptation of plasmodium to avoid destruction leads to increased virulence (damage to the host) So, selection can lead to increased virulence when conditions allow easy or fast transmission between hosts. In crowded or unsanitary conditions, viruses evolve to become virulent.
