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?)

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