Why Sex Flashcards
sex
- genetic recombination that involved meiosis with crossing over and mating
- creates variance, even within siblings
the paradox of sex
- the costs of sex (4)
- cost of recombination
- cost of mating
- cost of meiosis
- cost of producing males
cost of recombination
sex produces some bad allele combinations and may break coadapted gene complexes
cost of recombination
- sickle cell anemia
healthy heterozygotes with high survival rate can produce unhealthy offspring
- 1/4 offspring are susceptible to malaria
- 1/4 offspring have sickle cell disease
cost of recombination
- swallowtails (2)
to avoid predation, swallow tails replicate features of distasteful species
- fit combinations: tail/bright or no tail/dull
mating between these phenotypes produce a swallowtail that does not successfully mimic distasteful species
- produce unfit recombinant: no tail/bright
cost of mating
- it takes more time and energy to find a mating partner than if an organism were to asexually reproduce
cost of meiosis
only half of genome is passed on and is diluted through generations
- sexual: 50% genetic material transmitted
- asexual: 100% genetic material transmitted and not diluted over generations
cost of producing males
1/2 population is not reproducing when 50% of offspring are males; two-fold cost of sex
- sexual: female population size remains the same
- asexual: female populations doubles in size after each generation
what is evidence that sex can be eliminated (2)
- asexual species have arisen from sexual ones multiple times, even in vertebrates
- some species have sexual and asexual phases that alternate
hypotheses for the maintenance of sex: long term advantage (2)
- Muller’s Ratchet
- Fisher’s rate of evolution
hypotheses for the maintenance of sex: short term advantage (2)
- mutational: negative epistasis
- ecological: Red Queen hypothesis
Muller’s Ratchet (3)
- in asexual species of finite size, deleterious mutations should accumulate and eventually lead to extinction
- sex would weed out deleterious mutations by recreating mutation-free variants
- assumes that mutations are independent of one another
Fisher’s Rate of Evolution
sex brings together advantageous mutations, thus increasing evolutionary rate
- asexual: mutations have to be sequential and in same lineage
- sexual: recombination can quickly create lineage with good mutations
Fisher’s Rate of Evolution
- problems (2)
- in small populations, beneficial mutations would not be frequent enough for sex to combine them any faster than those combinations arising in asexual populations
- sexual reproduction would not be better than asexual reproduction in small populations
Muller’s Ratchet
- problems
- acting alone, it operates too slowly to provide a significant short-term advantage of sex
Fisher and Muller hypotheses
- implication
- strength
- both imply long term effects and species-level selection
- not powerful enough to explain sex alone seeing that asexual species do appear more prone to extinction
short term advantage: mutational hypothesis (3)
- negative epistasis: non-additive effect of deleterious mutations/additional mutations cause disproportionally greater decline in fitness
- sexual reproduction concentrates “bad” alleles in some offspring, thus requiring fewer selective deaths for sexual reproduction to win
- sexual reproduction combines bad mutations at same loci together
short term advantage: ecological hypothesis; sibling (2)
- sex is advantageous in the face of an uncertain future; sex can create variety in phenotype so offspring have some fitness advantage in changing environments
- if environment changes slowly, process works only for very fecund organisms with high competition/low survival
short term advantage: ecological hypothesis; Red Queen (3)
- pathogens and parasites are fast changing components of the environment
- sex and recombination create diverse offspring to which pathogens and parasites are not well matched
- implication: sex has evolved to confound our germs
parasitic pressure and sexual mode (2)
- high parasitic pressure leads to greater number of sexual organisms
- lower parasitic pressure leads to greater number of asexual organisms
short term advantage mutations hypothesis:
- requirements (3)
- high mutation rates (>1 per genome per generation)
- large population size
- negative epistasis: each deleterious mutations leads to greater decline in fitness than previous one
short term advantage ecological hypothesis:
- requirements (2)
- sever effects of parasites on their hosts
OR - ‘hard’ rather than ‘soft’ selection: only healthiest hosts survive and reproduce
advantages of combining hypotheses: (3)
- mutational and ecological explanations can complement each other
- we don’t discriminate among them, but rather estimate parameters
- different mechanisms may be relevant to different species or act simultaneously within the same population
rate of environmental change
- examples (2)
- low rate: climate change
- high rate: pathogens or parasites
evolutionary traction
- in asexual populations, deleterious alleles may fix by hitchhiking along favourable mutations
when do asexuals prevail (4)
- graph: zone I
- when there is high rate of evolutionary change and low rate of genomic change
- when there is high rate of genome mutation rate and low environmental change
- when both factors are low
when do sexual prevail (2)
- graph: zone II
- when both rate of environmental change and genome mutation rate are high
when is Muller’s Ratchet sufficient to explain sexual reproduction (2)
- graph: zone III
- when genome mutation rate is high (rate of environmental change does not necessarily matter as long as mutation rate is high)
