EB13 Flashcards

Question

Describe the mutational deterministic hypothesis

Answer 1

if the combo of two deleterious mutations decreases fitness more drastically than would be expected from independent effects of each one (synergistic epistasis) AND genomic deleterious mutation rate is greater than 1. THEN sex is favoured because it combines multiple slightly deleterious mutations into highly unfit genotypes that are selected out of the popualtions leaving healthier genotypes over represented.

Answer 2

1. many estimates suggest mutation rate is not generally high enough >1 to male this model work 2. deleterious mutations dont show a general tendency towards synergistic epistasis. *can be seen in both E.coli and yeast experiments, most double mutatnts show little or no epistasis. - combined harm less than expected. no general synergistic pattern 3. current consensus are that the conditions required are not widespread in nature.

Answer 3

very easily testable model

Answer 4

from weismanns hypothesis to muller fisher hypothesis to hill-robertson effects * all facets of same idea that sex breaks apart or combines mutations art different site and therefore improves population response to selection, especially in finite popualtions, where drift is a problem - focuses on how recombination helps bring good mutations together but could also split up bad.

Answer 5

sexual and aseuxal yeast populations: genetically engineered to have the same background but sex is KO. fitness differences observed under lab conditions. evolve strains over time to see how quickly they adapt.

Answer 6

1. adaptation was faster in sexual than asexual 2. yeast is isogamous so there was no two fold cost. 3. sex can fix good mutations indepndently of bad. 4. if selection stops - e.g. environment not constantly changing than asexuals regain the advantage.

Answer 7

1. parasites and pathogens are ubiquitous and are rapdily and relentlessly selected to infect the most common host genotype - they have short generation times, large population and high mutation rates - strong selection pressure to infect. 2. any successful combo of alelles soon becomes disproportionally susceptible, favouring new and different combos 3. sex breaks down common genotypes and replaces them with novel ones for the parasite. 4. therefore sexuals have adv. over asexuals as asexuals inherit the same vulnerabilities as their parents.

Answer 8

A time lagged negative frequency dependent selection

Answer 9

now here you see it takes all the running you can do just to keep in the same place

Answer 10

1. taxa with both sexuals and asexuals, asexuals tend to be found in physically harsh environments with low biotic diversity. (less in biotically rich habitats, more harsh). 2. this is consistent with advantage of sex linked to co-evo interactions, rather than changing physical conditions.

Answer 11

sexual snails co-exist with apomictic females (two fold cost of sex) 1. proportion of sexual snails within lakes correlated with frequency of sterilizing parasitic worms. 2. across multiple years asexual snails were more heavily infected than sexual. 3. proportion of asexuals over time is inversely linked with overall prevelance of infection and not generally increasing despite two fold advantage. 4. frequency of asexuals decreased over a decade as they became disproportionally susceptible to parasites. 5. meanwhile frequency of sexuals was roughly stable so parasites prevented any given clone from becoming successful enough to replace sexuals.

Answer 12

1. c. elegans hermaphrodites can self-fertilise or outcross with males. 2. adaptation to uncahinging serratia strain increased the rate of outcrossing but highe rates were only sustained by co-evolution with the parasite. 3. obligately selfing. c. elegans all went extinct in the coevolution treatment, but not in others. as parasite adapted to infect the selfing strain more strongly 4. obligately outcorssing strains became more resistant to the parasite during coevolution experiment. * ** clear evidence of selective benefits of sex with parasites

Answer 13

1. effects of sex on coevolution difficult to pindown in a model. - unsure how general adv. may be. 2. unsure effect if many parasites involved, if parasites have sex? 3. major concern: parasites select for genetic diversity not sex per se. a diverse mix of genetically different clones could potentially cycle up and down in frequency and simualte benefit of sex. 4. dont know whether beneift of sex in RQ models comes from fluctating rare advantage or from a generally improved response to selection. Hill roberston effect?

Answer 14

mix of both hill robertson and RQH. neither has definitive theoretical or empiral support * RQ has empiral support in some cases but hard to make theoretical models are dynamics are so complicated. whereas HR are simple and general to model but hard to find in nature. ** mutational hypothesis could still be important in helping to overcome the costs of sex.

EB13 Flashcards

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