Lecture 6- Evolution of Sex

Question 1

4 processes useful to sexual reproduction

Answer 1

meiosis
recombination
segregation
syngamy

Question 2

what is syngamy

Answer 2

fusing of 2 gametes

Question 3

what is the twofold cost of sex

Answer 3

idea that all else being equal, an asexual population can reproduce twice as fast as a sexual one- so asexual mutants should be able to spread and outcompete sexual individuals

Question 4

cost of meiosis

Answer 4

loss of half the genetic relatedness

Question 5

cost of mating

Answer 5

needing to search for mates, engage in mating, potential competition/conflict/predation/infection risk

Question 6

cost of recombination and segregation

Answer 6

potential breaking up of beneficial allele combinations or creation of deleterious ones

Question 7

what does D<0 represent

Answer 7

negative linkage disequilibrium- the combination of 2 alleles occurs less frequently than would be ecpected

Question 8

what is epistasis

Answer 8

situation where the phenotypic expression of a gene is modified by other genes- the effect of a gene is dependent on its genetic background

Question 9

positive epistasis

Answer 9

combined effect of mutations is more beneficial than the sum effects of each individual mutation, vice versa for negative

Question 10

how might sex have initially evolved?

Answer 10

asymmetric DNA transfer in unicellular organisms- but there may be more complex mechanisms underpinning its maintenance

Question 11

how might sex lower fitness in the short term

Answer 11

splitting up beneficial allele combinations, but possibly useful long term as creates more diversity

Question 12

when is variance a benefit in the short-term?

Answer 12

when extreme genotypes have a fitness advantage- otherwise, it is beneficial to stick with intermediate genotypes

Question 13

when will sex and recombination be selected for?

Answer 13

either:
high linkage diseq, negative epistasis- breaking up negative allele combos
low linkage diseq, positive epistasis- doesn’t drive out advantageous alleles

Question 14

red queen hypothesis

Answer 14

organisms must continually evolve to maintain high enough relative fitness compared to interacting organisms

Question 15

how can parasites remove the advantage of reproducing asexually

Answer 15

adapting to infect common clonal genotype- negative frequency-dependent selection emerges

Question 16

example of this in snails

Answer 16

-shallow water has a high level of trematode infections, and there tends to be more sexual reproduction here
-in deper water, where parasites are rarer, sex is rarer- there is less advantage to having the rarer genotype, removing the advantage of continuous adaptation

Question 17

how can migration impact linkage equilibrium

Answer 17

migration causes an influx of extreme genotypes, which can then increase linkage disequilibrium- sex and recombination may then be very useful short-term, due to differences in local selection

Question 18

Williams’ lottery model

Answer 18

‘bed hedging’- producing variable offspring is more likely to create higher mean fitness- more likely that one will do well

Question 19

issues with lottery model

Answer 19

little evidence, would predict that sexual reproduction happens in less stable environments but that doesn’t seem true, asexual reproduction is associated with more unpredictable habitats

Question 20

tangled bank hypothesis

Answer 20

diversity means siblings can all move into different microsites, reducing competition and allowing each individual to colonise an area- spatial heterogeneity would favour genetic polymorphism

Question 21

evidence for/against tangled bank

Answer 21

sex seems to be somewhat associated with constant environments such as the tropics
however it also seems to be associated with few, larger offspring rather than a lot which can all go off and colonise

Question 22

ruby in the rubbish idea

Answer 22

asexual populations have a higher risk of losing beneficial mutations

Question 23

hill-robertson interference

Answer 23

in clonal populations, advantageous alleles can become linked with deleterious ones (clonal interference), which reduces the efficacy of natural selection- recombination helps stop this happening

Question 24

muller’s ratchet

Answer 24

when there is no recombination, deleterious alleles build up due to amplified genetic drift- this results in damage to a population

Question 25

muller’s hatchet

Answer 25

sex is useful because it helps purge deleterious mutations- ‘truncation selection’

Question 26

fisher-muller hypothesis

Answer 26

sex leads to the fusion of beneficial alleles, helping facilitate adaptation

Question 27

why all of these theories may not be true

Answer 27

sex doesn’t have to increase variation
genetic variation may not be beneficial
evolution doesn’t always promote genetic exchange, even if it might be favourable in theory

Question 28

pluralist approaches- advantages

Answer 28

most individual hypotheses are a bit reductionist, so combining them can give a better idea of interactions between mechanisms etc, can help overcome constraints of individual hypptheses