Sex Flashcards
define sex
recombination of genes from separate sources into a single organism.
in EK, what complications does sex involve?
- alternation between diploid and haploid (often just gametes) life stages.
- reduction of genetic info by meiosis
- crossing over during meiosis
- fusion of haploid gametes with those of another individual of the opposite sex (syngamy).
what 4 problems does sexual reproduction present which don’t occur in asexual?
- the 2 fold cost - asexual females can produce twice as many offspring as sexual females, and pass on 100% DNA. therefore, sex is 2x as costly.
- meiosis is costly as it is prone to errors in replication.
- finding a mate/competition is costly.
- Linked co-adapted genes are separated, eg heterozygous for having sickled RBC is advantageous if in Malaria prone zones. heterozygosity can be removed by meiosis.
how did diploidy evolve?
Perhaps to provide a replacement for defective genes. Firstly, there may have been 2 identical sets of chromosomes, one to act as a back up copy to cover for mutations, and DNA could be repaired using the counterpart molecule.
also, could expose deleterious mutations so can be removed from the lineages.
Why is diploidy expensive?
more DNA to copy and repair. therefore, as a coping strategy in bad conditions, can switch to haploidy w/o crossing over. (Reduction division).
why may crossing over have evolved?
May be a by product mechanism of double stranded repair. promoted variation.
why may syngamy have evolved?
to restore heterozygosity after double strand repair and crossing over.
Would crossing over or syngamy have evolved first
- If x-ing over first, then the 2 chromosomes would have been identical, so doesn’t create much variation (but still good for DNA repair)
- If syngamy came first, x-ing over (between homologous, but not identical chromosomes) increases variation
what is a classic theory as to why sex is maintained?
what is a problem of this theory?
Fisher 1930 - speeds up evolution by bringing good genes together. if only asexual, unlikely that two good mutations will spread to fixation, whereas with sex, beneficial mutations will be brought together immediately.
However - it is only a benefit in a large pop, with mutations of small benefit, as in small populations when mutations give large benefits they spread v rapidly. therefore sex doesn’t actually speed things up.
describe muller’s ratchet theory
at a given time,there are individuals with 0 (most fit), 1 (2nd fittest), 2 etc mildly deleterious mutations. in a small pop, by chance those with 0 mutations may die, fail to breed or mutate. makes the best individuals those with 1 mutation. the ‘ratchet’ has clicked round and pop has lower fitness. this can lead to extinctions, which may be why asexual species are so rare.
how does sex counteract Muller’s ratchet effect?
Purges deleterious recessives so sex can restore a perfect genome. deleterious recessives are eliminated from the pop in offspring which die.
why must there be short term advantages to sex
what are they?
some organisms can do both sexual and asexual rep.
- variable offspring allows exploitation of spatially unpredictable environments.
- can evade parasites better - red queen hypothesis, Hamilton 1980.
- purges synergistic genes.
describe some evidence for hamilton’s red queen hypothesis
Curt Lively found that, in a population of freshwater snail that could self-fertilise facultatively, it was more likely to self-fertilise in ponds where parasites were rare, but outbreed in pond where parasites were more common.
what is a mating type?
different to sex
eg angiosperms have incompatibility systems and 2 sexes.
eg Stylonychia has 48 mating type, each one can mate with an of 47 others but not its own.
mechanism to avoid inbreeding.
what is a classic theory for the evo of different sexes
Parker et al 1972 - GAME THEORY
determines ESS - evolutionary stable strategy
