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Long Term Advantages of Sex
Is sex better than Asex?
(Assume: female’s reproductive mode doe not affect number of offspringOr survival probabilities of her offspring)
TEST Dunbrack et al 1995
Used Tribolium castaneum; red morph assigned to be sexual and black morphAssigned to be asexual
Selective agent used was malathion added to flour
Fitness of blacks and reds measured each generation
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Which is more fit – asexual or sexual phenotype????
CONCLUSIONS: sexual forms have long term advantages
REASONS FOR ADVANTAGES?
1.Muller’s RatchetDue to genetic drift, populations will continually accumulatemore mutations per genome that cannot be removed by selection
TEST: Anderson and Hughes . . .using Salmonella
2. Changing selection in changing environments
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Short Term Advantages of Sexual Reproduction
Changing environments impose hostile and novel selection in an unpredictable manner on organisms
BENEFITS OF SEX
RED QUEEN hypothesis: B/c of constantly changing ‘selection environments’ between generations, individuals who produce morevariable offspring have higher fitness than those that reproduceless variable offspring
• Curtis Lively et al. 1992 through 2007; studied snails in New Zealand (Potamopyrgus antipodarum)
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Snails infested by over 12 different trematode worm parasites
Parasite eats gonads – castrating host (0 fitness)
Two types of females in population: obligately sexual and obligatelyasexual
Proportions of sexuals/asexuals and trematode densities vary across population
Snails infested by over 12 different trematode worm parasites
Parasite eats gonads – castrating host (0 fitness)
Two types of females in population: obligately sexual and obligatelyasexual
Proportions of sexuals/asexuals and trematode densities vary across population
Why two reproductive phenotypes maintained in same species in N.Z.? -
Does varying selection influence the benefits to costs of sexual vs. asexualreproduction? (ie. Support for the Red Queen?)
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Alternate Hypotheses?
1. Trematode infection rates are higher in more dense populations of snails
2. More asexual females are found in less dense snail populations b/c the costs of finding a male (as required by sexual phenotypes) are higher when fewer individuals are present
COSTS
Purging Bad Mutations Hypothesis
Harmful mutations accumulate within an individual’s genetic line; sex exposes the recessive mutations to selection and thus allows them to be removed