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Long Term Advantages of Sex

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1 Long Term Advantages of Sex
Is sex better than Asex? (Assume: female’s reproductive mode doe not affect number of offspring Or survival probabilities of her offspring) TEST Dunbrack et al 1995 Used Tribolium castaneum; red morph assigned to be sexual and black morph Assigned to be asexual Selective agent used was malathion  added to flour Fitness of blacks and reds measured each generation

2 Which is more fit – asexual or sexual phenotype????
CONCLUSIONS: sexual forms have long term advantages REASONS FOR ADVANTAGES? Muller’s Ratchet Due to genetic drift, populations will continually accumulate more mutations per genome that cannot be removed by selection TEST: Anderson and Hughes using Salmonella 2. Changing selection in changing environments

3 Short Term Advantages of Sexual Reproduction
BENEFITS OF SEX Changing environments impose hostile and novel selection in an unpredictable manner on organisms RED QUEEN hypothesis: B/c of constantly changing ‘selection environments’ between generations, individuals who produce more variable offspring have higher fitness than those that reproduce less variable offspring • Curtis Lively et al through 2007; studied snails in New Zealand (Potamopyrgus antipodarum)

4 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 obligately asexual 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. asexual reproduction? (ie. Support for the Red Queen?)

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9 Purging Bad Mutations Hypothesis
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


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