Lecture 6- Evolution of Sex Flashcards
4 processes useful to sexual reproduction
meiosis
recombination
segregation
syngamy
what is syngamy
fusing of 2 gametes
what is the twofold cost of sex
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
cost of meiosis
loss of half the genetic relatedness
cost of mating
needing to search for mates, engage in mating, potential competition/conflict/predation/infection risk
cost of recombination and segregation
potential breaking up of beneficial allele combinations or creation of deleterious ones
what does D<0 represent
negative linkage disequilibrium- the combination of 2 alleles occurs less frequently than would be ecpected
what is epistasis
situation where the phenotypic expression of a gene is modified by other genes- the effect of a gene is dependent on its genetic background
positive epistasis
combined effect of mutations is more beneficial than the sum effects of each individual mutation, vice versa for negative
how might sex have initially evolved?
asymmetric DNA transfer in unicellular organisms- but there may be more complex mechanisms underpinning its maintenance
how might sex lower fitness in the short term
splitting up beneficial allele combinations, but possibly useful long term as creates more diversity
when is variance a benefit in the short-term?
when extreme genotypes have a fitness advantage- otherwise, it is beneficial to stick with intermediate genotypes
when will sex and recombination be selected for?
either:
high linkage diseq, negative epistasis- breaking up negative allele combos
low linkage diseq, positive epistasis- doesn’t drive out advantageous alleles
red queen hypothesis
organisms must continually evolve to maintain high enough relative fitness compared to interacting organisms
how can parasites remove the advantage of reproducing asexually
adapting to infect common clonal genotype- negative frequency-dependent selection emerges
example of this in snails
-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
how can migration impact linkage equilibrium
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
Williams’ lottery model
‘bed hedging’- producing variable offspring is more likely to create higher mean fitness- more likely that one will do well
issues with lottery model
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
tangled bank hypothesis
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
evidence for/against tangled bank
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
ruby in the rubbish idea
asexual populations have a higher risk of losing beneficial mutations
hill-robertson interference
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
muller’s ratchet
when there is no recombination, deleterious alleles build up due to amplified genetic drift- this results in damage to a population
muller’s hatchet
sex is useful because it helps purge deleterious mutations- ‘truncation selection’
fisher-muller hypothesis
sex leads to the fusion of beneficial alleles, helping facilitate adaptation
why all of these theories may not be true
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
pluralist approaches- advantages
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
