1. Sex and Sex Ratio What is sex? Why sex?
Mechanisms of sex determination
Sex ratio allocation

2. What is sex?
Genetic recombination
What determines gender?

3. Why Sex? The Costs.
Add pictures of asexual aphid and Maynard Smith logic game from Freeman & Herron, pg
Cost of males - could produce 2 times as many females
Cost of meiosis - favorable gene complexes recombine
Energetic and time costs of finding a mate

4. Why Sex? The Benefits. Muller’s Ratchet Genetic Recombination
deleterious mutations accumulate in asexual lineages
Genetic Recombination
novel gene combinations are created
Red Queen Hypothesis: need to create new gene arrangements to combat pathogen evolution

5. Evolutionary Arms Race
Insert snail picture from pg. 223 of Freeman-Herron
Some spp of snails are both sexual and asexual
Sexual proportion (freq. of males) increases with parasite infection

6. Sex determination
Influences the degree to which a female can alter the sex ratio of her offspring
Types
Chromosomal
Environmental
Social

7. Chromosomal Sex Determination
Heterogamety
Male: (XY) mammals, flies
Female: (ZW) Birds, Butterflies
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8. Chromosomal Sex Determination
Haplodiploidy: all hymenoptera, thrips, scale insects, some beetles
Haploid males develop from unfertilized eggs
Diploid females develop from fertilized eggs
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Creates asymmetries
in relatedness

9. Environmental Sex Determination
Incubation temperature
Terrapins
Turtles
Alligators
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10. Social Sex Determination
Some fish undergo sex reversal
Depends on mating system
Ex: dominant female in sea bass harem changes into male when territorial male disappears
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11. Sex Ratio Allocation: Null Model
R. A. Fisher: Parental strategies should evolve towards equal investment in offspring of the two sexes
If sex ratio falls below 50%, increased production of rare sex is favored
Assuming random mating, rare sex will experience greater reproductive success
Frequency-dependent selection leads to an ESS of 1:1 sex ratio

12. Adaptive Sex Ratio Bias
Maternal condition influences offspring investment (Trivers-Willard Effect)
Local mate competition
Local resource competition
Local resource enhancement

13. Trivers-Willard Effect
Population sex ratio is 1:1, but individual sex allocation depends on condition
If moms in good condition transfer competitive ability to sons more than daughters
Then, they should produce more sons than daughters assuming that
Dominant individuals sire more offspring

14. Condition-dependent sex allocation in red deer
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15. Dominance dependent sex allocation in yellow baboons
Dominant females have more daughters than sons (pass social rank to daughters)
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16. Local Mate Competition
Mating b/w siblings takes place near hatching site
Males compete with each other for matings.
Solution: Produce few sons. Expect most offspring of first-laying female to be daughters
What if a second female arrives to lay eggs?
Ex: Nasonia wasps; haplodiploid ‘super-parasites’ on fly pupae

17. Depends on proportion of eggs that belong to the second female
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18. Local Resource Competition
Offspring that stay near their birth site may compete with their parents for resources
In many species, one sex disperses farther or at a greater rate than the other sex
Solution: Produce more of the dispersing sex (Ex: galagos produce more males)
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19. Local Resource Enhancement
In some spp., offspring of one sex delay dispersal and remain at the natal site to help parents raise their siblings
Benefits of helpers must be greater than cost of increased competition
Ex: Red-cockaded woodpecker male-biased
Prediction: Sex-ratio biased towards helping sex on high-quality territories?

20. Seychelles Warblers As territories fill up, produce
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As territories fill up, produce
fewer males (which disperse) but
more females (which help)