evolution of sex Flashcards
how does sex produce variation?
it reshuffles chromosome and allows for recombination through crossing over
explain why asexual reproduction is always the ESS
because sexual reproduction is wasteful- a female must produce a male and a female in order for offspring to occur in the third generation and only the female produces offspring. where as in asexual reproduction the female produces a female which can both individually produce offspring therefore the population increases exponentially. even if in sexual reproduction produces more offspring, asexual will always be faster- this is the two fold cost of sex
what is the two fold cost of sex?
members of a sexually reproducing population must produce both sexes, but effectively only half of them can “give birth” to offspring ot their own. In contrast, all offspring of an individual from an asexually reproducing population can potentially “give birth”. This is the two-fold cost of sexual reproduction
what is the downside of sex in terms of fitness of genetic information passed on?
- only half of the information from an individual is passed on to the next generation
why is sex bad in term of reshuffling?
if an organism is perfectly adapted under a certain genotype, then reshuffling of this genotype will result in the potential of less fit offspring- an example of this is sickle cell anaemia: heterozygous is optimal fitness because of opposing selection pressure against either homozygous- anaemia or malaria. when reshuffled the fitness decreases.
what are the 3 downsides of sex?
- reshuffling optimal fitness genotypes
- 50% of genes are passed on from one parent to offspring g
- two fold cost of sex= only half of the population can produce offspring- the other is wasted.
what percentage of animals produce sexually?
99.9%
in what kind of population is sexual reproduction best? (3)
Optimal genotypes not common:
- non-equilibrium situation
- selection (positive or negative)
Components of optimal genotype dispersed:
- alleles present in different individuals
- negative linkage-disequilibrium (sex allows recombination)
explain how optimum genotypes can be obtained in a sexual population
- takes a long time as the congregation of beneficial mutations on the same chromosome is very rare:
- for example you begin with a wild type
- a beneficial mutation A, B and C would allow the wild type to reach optimum.
- the chances of the B allele mutating in an genome which already has A is very rare- more likely to occur in wild type.
- this takes a long time but eventually happens
- then C mutation occurs not in individual with A and B. also even if it does, a deleterious mutation may occur which can’t be recombined off and causes the holder to die
explain how sexual reproduction is quicker in terms of the ABC example used for the asexual mechanism reaching optimum
-overall recombination can break up linkage disequilibrium
- if ABC is needed for optimum
- A can occur initially in one organism
- then B occurs in another and through recombination can be recombined onto the same chromosome as A
then AB can recombine onto one with C
- this occurs in a much shorter time frame
due to sex facilitating recombination, what can be hypothesised and proved through an experiment in terms of adaptation?
it would follow that sexual organisms can adapt t harsh environments quicker than asexual ones- this can be tested though using a budding yeast which produces asexually and sexually- when under pressure it reproduces sexually.
- an experiment used yeasts which had a gene that controlled sexual reprod knocked out
- then both populations were grown in hot harsh environment
- this showed that in a benign environment no chang in fitness occurred
- in the harsh environment the sexual population increased in fitness more than the asexual and a lot quicker
when is sex good in terms of negative selection?
- in order to reach optimum fitness, deleterious mutations will need to be removed. recombination can recombine bad mutations on one chromosome and good ones on another
- in asexual populations if a deleterious mutation becomes fixed via drift then it can’t be removed. gradually mor build up and get worse and the individual dies
what is muller’s ratchet?
- long term extinction of asexual populations in asexual populations if a deleterious mutation becomes fixed via drift then it can’t be removed. gradually mor build up and get worse and the individual dies
what study supports the hypothesis that sexual selection is good because of its ability to facilitate quickly adaptation?
diseases and predators are in competition with us- evolutionary arms race- try to exploit us and we try to out compete them- so we are constantly adapting- out evolve each other- this supports sexual reproduction- the red queen dynamic- constant evolution without either one dominating indefinitely
fluctuating selection: genetic interaction-
asexual- one parasite type is good at exploiting one host type and not another type. when host is infected with parasite 1 it has a lower fitness that infected by parasite 2 because 1 can exploit very well. the interaction between parasite and host go up cyclically- whenever host 1 is frequent then parasite 1 has an advanatge over 1, then 1 suffers from fitness and goes down so parasite 2 overtakes on host 2, then host 2 becomes less fit, goes down and host 2 goes down- continues as a cycle.
if you are sexual then you wont become the preferentially targetted because you change constantly and parasites cant specialise.
what supports the theory that sexual selection is selected for because of the constant pressure to out compete parasites and pathogens
two types of aquatic snails- one sexual, one not. there is a parasite which sterilises the host- fitness eradicated.
one prediction of red queen dynamic is that parasitsm will select for sex because can diversify. when parasite pressure is higher, more sexual- sexual reproduction is advanatgeous for paarsitsm which is something that almost all organisms are exposed to
another prediction: whenever an asexual clone becomes common it will be targetted by the aparsite and be driven to lower frequencies- so numbers should decrease after a while- in 1994 marked lineages, came back 7 years later and the lineages has reduced in frequency.
can see that high frequency asexualgenotypes become targetted as soon as become common.
so asexual lineages are fixed genotype and can be targetted and specialised against
new asexual cloes out-compete sexuals- arent specialised against but as soon as they become common, they are targetted. but sexual selection is more resistant
