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Sex and Sex Ratio
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What is sex?
Why sex?
Mechanisms of sex determination
Sex ratio allocation
What is sex?
Genetic recombination
What determines gender?
Why Sex? The Costs.
• 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
Why Sex? The Benefits.
• Muller’s Ratchet
– 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
Evolutionary Arms Race
• Some spp of snails are both sexual and asexual
• Sexual proportion (freq. of males) increases with
parasite infection
Sex determination
• Influences the degree to which a female can
alter the sex ratio of her offspring
• Types
– Chromosomal
– Environmental
– Social
Chromosomal Sex Determination
• Heterogamety
– Male: (XY) mammals, flies
– Female: (ZW) Birds, Butterflies
Chromosomal Sex Determination
• Haplodiploidy: all hymenoptera, thrips,
scale insects, some beetles
– Haploid males develop from unfertilized eggs
– Diploid females develop from fertilized eggs
Creates asymmetries
in relatedness
Environmental Sex Determination
• Incubation
temperature
– Terrapins
– Turtles
– Alligators
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
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
Adaptive Sex Ratio Bias
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Maternal condition influences offspring
investment (Trivers-Willard Effect)
2. Local mate competition
3. Local resource competition
4. Local resource enhancement
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
Condition-dependent sex allocation
in red deer
Dominance dependent sex
allocation in yellow baboons
• Dominant females have more daughters than sons
(pass social rank to daughters)
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 ‘superparasites’ on fly pupae
Depends on proportion of
eggs that belong to the
second female
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)
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?
Seychelles Warblers
As territories fill up, produce
fewer males (which disperse) but
more females (which help)