1. Sex and Sex Ratio What is sex? Why sex?
Mechanisms of sex (gender) determination
Sex (gender) 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 twice as many females
Cost of meiosis - recombination breaks up favorable gene complexes
Finding a mate costs time and energy

4. Why Sex? The Benefits. Prevents Muller’s Ratchet
deleterious mutations accumulate in asexual lineages, not in sexuals
Provides long term benefit
Novel gene combinations are created
Red Queen Hypothesis: need to create new gene arrangements to combat pathogen evolution
Provides short term benefit

5. Host sexuality and parasitism
Freq. of males=sexuals
Trematode infections
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 infection by trematode parasites (Lively 1992)

6. Sex determination Types
Genetic (either chromosomal or genic)
Environmental
Social
Influences the degree to which a female can alter the sex ratio of her offspring

7. Chromosomal sex determination
Male heterogamety
Female
Male ZW ZZ
Female heterogamety
Male
Female
Why should mode of inheritance matter?
X chromosomes spend less time in males than females, while Z chromosomes spend more time in males than females. If selection is sex limited, then evolutionary rates will differ if traits are X, A or Z.
Note, also that Y genes never occur in females while W genes never occur in males XY XX

8. Distribution of sex chromosome heterogamety (numbers of families)
Group
XY males
ZW females
XY or ZW
Birds
all
Butterflies
Mammals
Flies 7 3
Fish 19 10 2
Lizards 4 1
Amphibians
All three authors emphasize birds vs mammals, but the best comparisons are fish, lizards and amphibians since these groups show recurrent evolution of male/female heterogamety.
Bull, 1983; Solari 1994

9. Haplodiploidy Mechanism Distribution
Haploid males develop from unfertilized eggs
Diploid females develop from fertilized eggs
Distribution
all hymenoptera, thrips, scale insects, some beetles
Creates asymmetries
in relatedness
Insert picture from pg. 339 Freeman & Herron

10. Environmental Sex Determination
Incubation
temperature
Terrapins
Turtles
Alligators
Insert Figure 9-4 from big folder

11. Social Sex Determination
Many fish can undergo sex reversal
Depends on mating system
Example: dominant female in Anthias sea bass harem changes into male when territorial male disappears
female
Insert picture from pg. 303 of Drickamer
male

12. Sex Ratio Allocation: Null Model
Produce more males
Produce more females

13. 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 a 1:1 stable sex ratio

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

15. 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 (e.g. through parental care)
and dominant individuals sire more offspring
then, they should produce more sons than daughters
and females in poor condition should produce more daughters

16. Condition-dependent sex allocation in red deer
Insert picture from pg. 306 in Drickamer

17. Dominance dependent sex allocation in yellow baboons
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Dominant females have more daughters than sons
(pass social rank to daughters)

18. Local Mate Competition
Mating b/w siblings takes place near hatching site
Brothers compete with each other for mating
Solution: Produce few sons.
Expect most offspring of first-laying female to be daughters
Expect second-laying female to adjust sex ratio according to the proportion of brood that are hers
Fig wasps, parasitoid wasps

19. Nasonia wasps adjust sex ratio
Line gives theoretical prediction
assuming females contribute
equal numbers of offspring and
adjust sex ratio to maximize
inclusive fitness.

20. Second laying females adjust sex ratio proportional to offspring produced
Sex ratio depends on proportion of
eggs that belong to the second female

21. 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
For example, galagos (bush babies) produce more males
Insert picture of Galago

22. 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 groups are male-biased
Males help feed young
Available nesting cavities are rare

23. Seychelles warbler sex ratios
As territories fill up, females on good territories produce fewer males (which disperse) but more females (which help)