P10- Lecture 11- Evolution of Disease cont. & Sexual Selection- Feb. 26

P10- Lecture 11- Evolution of Disease cont. & Sexual Selection- Feb. 26

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Unformatted text preview: 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) x 2) 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 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 Sex and Sexual Selection What is sex? s No, it's not about recreation (at least here)! s Sexual reproduction x Two parents giving rise to genetically unique offspring through the fusion of gametes produced by meiosis. Top three reasons NOT to have sex s Inefficient - better to produce all females x Requires twice as many adults to produce a given number of offspring in sexual species Figure 12.13 Top three reasons NOT to have sex s Inefficient - better to produce all females x Requires twice as many adults to produce a given number of offspring in sexual species s Costlyx Devote time and/or energy to producing and maintaining sexual organs, finding and courting mates, etc. s Riskyx STDs, fights, injury s Seems very sensible, but relatively few Asexual organisms species are obligate asexual reproducers x Asexual species tend not to persist. x Bdelloid rotifers an exception- parthenogenesis 3 An "evolutionary scandal" (Maynard Smith 1978) 3 > 40 million years without sex How can bdelloids evolve without sexual reproduction? I.e.: Do you have to have sex in order to have diversity in a species? http://www.sciencedaily.com/releases/2007/03/070320090458.htm s Numerous species are facultative asexual reproducers x E.g. Daphnia x Asexual when conditions are good x Sexual during times of stress s s Sex reversal & hermaphrodites Ex: protogyny (female 1st) protandry (male 1st) Ex: Caribbean bluehad wrasses Do we need males? The value of males and thus, SEX s "Sex is the Queen of Problems in Evolutionary Biology" (Bell, 1982) s 1. The Tangled Bank Hypothesis x Sex provides genetic variability in offspring x Variability comes from new gene combinations (independent assortment and recombination) x Variability promotes chances of success in a complicated world niche differentiation Why is sex so common? s 2. The Red Queen Hypothesis x Sex provides varied offspring x Allows more rapid evolution x Can keep up in the evolutionary "race" 3 e.g. host/parasite evolution 3 Is sex a strategy to combat parasites? ...
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