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BIO152H5 Chapter Notes -Progne, Common Kestrel, Carotenoid

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Maria Arts

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Tutorial 11 notes
Models of Sexual Selection
What is Sexual Selection?
Reproductive systems in which individuals differentiate among potential mates and
choose one based upon some characteristic
Intrasexual Selection: members of one sex (usually males) compete with one another for
the opportunity to mate
Intersexual Selection: members of one sex (females) choose among members of the
opposite sex (males) à female choice
What is Sexual Selection?
In order for sexual selection to take place, there must be variation in the male secondary
sexual characteristic
à But why do females prefer certain secondary sexual characteristics over others? And does
sexual selection conflict with natural selection?
Case Study: Widowbirds
African bird (Euplectes progne) characterized by pronounced sexual dimorphism
During the non-breeding season, very little difference between male and female
Case Study: Widowbirds
During the breeding season, males molt and produce black feathers, bright epaulets and
elaborately long tail feathers
Males secure and defend a territory from other males à intrasexual selection
They build one or more nest frames where females lay eggs
Case Study: Widowbirds
Males perform a flight display to attract females à intersexual selection
• http://www.arkive.org/jacksons-widowbird/euplectes-jacksoni/video-00.html
Beyond building the nest frame, males do not participate in rearing the offspring
Question: How do we explain the sexual selection observed in widowbirds?
Models of Sexual Selection

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We will discuss six models that explain sexual selection:
1. Direct Benefits Hypothesis
2. Good Genes Hypothesis (Hamilton-Zuk)
3. Sexy Sons Hypothesis
4. Sensory Bias Hypothesis
5. Genetic Compatibility Hypothesis
6. Runaway Selection Hypothesis (Fisher)
Direct Benefits
Females receive direct benefits from their mate
This can include: increased fecundity, territory, nuptial gifts, etc.
Direct Benefits
European Kestrel males have bright orange plumage
Hypothesis: females prefer brightly-coloured males AND brightness of the males’
plumage is positively correlated with direct benefits to females
Direct Benefits
Good Genes/Hamilton-Zuk
The full expression of secondary sexual characteristics can be hindered by parasites
Males fully expressing the characteristic have “good genes” that confer parasitic
Females therefore choose ornamented males
Good Genes/Hamilton-Zuk
The house finch is sexually dimorphic with females preferring males with bright plumage
House finches are affected by several parasites including feather mites
Hypothesis: males with brighter plumage will have fewer parasites
Good Genes/Hamilton-Zuk
What is missing from this study?
Are the offspring of the bright-coloured males also bright-coloured?
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