P10- Lecture 10- Evolution of Disease- Feb. 24

P10- Lecture 10- Evolution of Disease- Feb. 24 - Darwinian...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Darwinian Medicine Why Nurses are like Mosquitoes A Darwinian view of disease s 1) History matters - Disease and infirmity in an evolutionary context s 2) Evolving pathogens and their environment s 3) The evolution of virulence 1. History matters Yesterday's genes in today's body s Myopia! x Myopia has negative effects x Myopia is very common x Myopia has a heritable component History matters Yesterday's genes in today's body s Myopia x Why hasn't natural selection eliminated the genes contributing to myopia? x Genes and environment interact! 3 Barrow Alaska Age 6-35 : 42% myopic; Age 36-88 : 5% myopic Few adults over 35 had attended formal school 3 Myopia is a recent "disease" arising from old genes being expressed in a new environment s Breast cancer Yesterday's genes in today's body x Deadly, and often strikes early x 12% of women in N.A. Why is it so common? x Risk of breast cancer related to exposure to reproductive hormones 3 Trade-off between reproductive success and survival; AND ... 3 "Modern" women live in "unnatural" conditions Monthly menstrual cycling is not the historical norm! Historically, women had many fewer (<1/3) menstrual cycles than "modern" women - spent much more time pregnant or lactating History matters Yesterday's genes in today's body s Breast cancer History Matters - Trade-offs s Sickle-cell anemia x A recessive allele with very serious effects when homozygous. q = 0.2 in some African pop. x Why does it persist? History Matters - Trade-offs s Sickle-cell anemia x Heterozygote advantage- stabilizing selection 3 Heterozygotes are more resistant to malaria 3 Frequency of heterozygotes (e.g. 2pq = 0. 32) is much greater than the frequency of recessive homozygotes (e.g. q2 = 0.04). 3 Balanced polymorphism History Matters - Trade-offs s Cystic Fibrosis x A recessive allele with very serious effects when homozygous - lung infections, early death x High % among Europeans x Why does it persist? x Heterozygote advantage 3 Heterozygotes are more resistant to typhoid fever 3 Frequency of heterozygotes (e.g. 2pq = 0.04) is much greater than the frequency of recessive homozygotes (e.g. q2 = 0.0004). History Matters - Trade-offs s Cystic Fibrosis x Heterozygotes may have greater resistance to typhoid fever 2. Evolving Pathogens s Antibiotic resistance in bacteria x Bacteria evolve quickly generation time, lots of genetic exchange x Widespread use of antibiotics selects for resistant strains x Bacterial evolution by natural selection can take the "wonder" out of overused "Wonder Drugs" 3 3 Short E.g. Penicillin s E.g. AZT resistance in HIV 3. The Evolution of Virulence s What is Virulence? s Common Wisdom x Virulent diseases will eventually evolve towards reduced virulence and benign coexistence x Why? What's bad for the host is bad for the disease organism... s This is not necessarily true!! The Evolution of Virulence s Pathogens must do two things x 1) Reproduce 3 Often causes sickness in the host 3 Higher rates of reproduction do more damage to the host x 2) Disperse to new hosts 3 This may or may not depend on the condition of the host The Evolution of Virulence s Rhinovirus - the common cold x Relies on host mobility for transmission x Infects only those areas that promote transmission 3Coughing and sneezing x High virulence would be related to more reproduction, but lower transmission if the host was immobilized x Colds are relatively mild The Evolution of Virulence s Rhinovirus - the common cold x High virulence would be related to more reproduction, but lower transmission if the host was immobilized x Colds are relatively mild The Evolution of Virulence s Small Pox - A deadly disease x Plague of Antonius, Rome. A.D. 165-180 3 Killed millions x Arrived in Mexico in 1520 3 By 1618 the Native Population had dropped from 20 million to 1.6 million x Devastating throughout New World after arrival of Europeans 3 One Mandan Village went from 2000 to 40 in a few weeks s Small Pox - The Evolution of Virulence How can it be so virulent? x Employs a "sit and wait" strategy x A durable pathogen - can remain viable for 10 years outside host x Even if host is killed, it can infect new hosts 3 E.g. through bedding, etc. s Vectors promote the evolution of virulence s How? x Pathogens that don't rely on their host for The Evolution of Virulence transmission favor reproduction more than host mobility virulence The Evolution of Virulence s Animal Vectors - E.g. Mosquitoes/Malaria x Carry pathogen from host to host x Mobility of host is not necessary (in fact, immobile host may be less able to fend off mosquitoes!!) x Transmission is pretty effective, so even the long term survival of the host is not very important x Pathogens reproduce in many tissues to increase chance of being picked up by vector x Bad news The Evolution of Virulence s Animal Vectors E.g. Nurses/E. coli x Nurses handle babies, especially sick babies The Evolution of Virulence s Animal Vectors - E.g. Nurses/E. coli x Ease of transmission via nurses favors virulent strains of E. coli 3More virulence means more bacteria and more diarrhea 3Presence of nurses as vectors increases the chance of transmission even if the host (i.e. the baby) dies sooner! The Evolution of Virulence s Cultural Vectors - E.g. water/Cholera x Causes severe diarrhea and death x Virulence of cholera strains varies x Contamination of drinking water favors virulence. Why? The Evolution of Virulence s Cultural Vectors - E.g. water/Cholera The Evolution of Virulence The Evolution of Virulence New World meets Old World What happened? New World meets Old World What happened? s Relatively little effect on Old World health Syphilis (maybe) Why such a difference?? s Apparently two major causes. Relative to the New World, the Old World had: s 1) A long history of animal domestication x Domesticated animals are the source or a reservoir for many human diseases (e.g. Swine flu) New World meets Old World Why such a difference? Why such a difference? x Large and/or dense populations allow s 2) A long history of large, dense populations pathogens to persist in the population x Maintains a reservoir of diseases x Maintains selection for disease resistance Summary s Human Diseases may reflect changes in our environment (e.g. myopia) or selective trade-offs (e.g. sickle-cell anemia or cystic fibrosis) s Pathogens evolve in response to selection pressures (e.g. antibiotic resistance) s Pathogens will not necessarily evolve towards Summary reduced virulence x Virulence reflects a trade-off between reproduction and transmission x Anything that facilitates transmission may also favor increased virulence (animal vectors, poor hygiene, promiscuity) Summary s Differences in the virulence of New World and Old World diseases reflect x Different histories of animal domestication x Different histories with respect to population size/density ...
View Full Document

This note was uploaded on 05/11/2010 for the course BIOLOGY 100 taught by Professor Richard during the Spring '10 term at George Mason.

Ask a homework question - tutors are online