Sexual Selection

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Sexual Selection
differences among indivs in mating opportunity

• NS who SURVIVES
• SS who REPRODUCES
• SS can explain some deleterious/costly
  characteristics that cant be explained by NS
• Ornaments/Courtship Displays/Fights
• Sexual Dimorphism

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Types of Sexual Selection

• Competition (usually M)
• Sperm is cheap
• ? RS with of matings

• Choice (usually F)
• Eggs are expensive
• No ? in RS with of matings

? e.g. Newts
offspring
mates
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Sexual Selection depends on Investment

• Higher investment limited resource
• ? Usually Indiscriminate M Choosy F
• But, Sex role reversal/ biparental care/nuptial
  gifts

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Male-male competition

• 1) COMBAT
• Competition for access to F or territory
• Correlation b/w dominance rank of matings
• Size often important in competition
• (Sexual Size Dimorphism Degree of Polygyny)

e.g. 1 Elephant Seals
e.g. 2 Male marine iguanas larger than optimal
for survival
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2) Sperm competition

• Fertilization more important
• than mating
• 1) Large ejaculate/lots of sperm
• 2) Mate guarding
• 3) Copulatory plugs
• 4) Sperm removal
• Comparative method relative testis size often
  correlates to mating system
• e.g. Primates

45 kg 110g
70 kg40g
200 kg 30 g
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3) Infanticide

• Males increase repro opportunities
• Not good for female
• Often female has no choice

• e.g. Lions

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Female Choice Benefits?

• On Basis of Resources vs. In Absence of Resources
• e.g. Scorpionflies
• making males pay
• Nuptial gift
• arthropod
• salivary
• forced copulation

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• Polygyny Threshold
• e.g. Lark Bunting
• choose on the
• basis of M territory
• Fitness benefits

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Direct Benefit

• F may choose on the basis of
• gift, paternal care, territory
• If no M investment, how do F choose?

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Non-resource based Female Choice

• Mutant F with no preference for showy M
• non-showy M offspring survive better
• M offspring not preferred as mates
• survival benefit must gt repro benefit

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Runaway Sexual Selection

• Innate female preference for showy M

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The Runaway Process

• SS trait associated with higher survival
• Mutant F (preference) ? RS b/c high quality
  sons
• 2? advantage M with trait preferred by high
  proportion of popn as mutation spreads
• Reinforcement
• Offspring carry genes for trait preference
  (linkage disequilibrium)
• Advantage
• Survival ? Survival Mating ? Mating

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Honest Advertisement

• F preference for traits which
• demonstrate good quality
• Zahavi Handicap Principle
• Characters that reduce survival
• I must be good to pull this off (good genes)
• Indicator must be costly so M cant cheat
• Often cost of trait is less for high quality
  males

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Runaway vs. Good Genes

• Open-ended
• Heritable variation in male character
• SS b/c sons have mating advantage

• Open-ended
• Heritable variation in male fitness
• SS b/c sons have survival advantage

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Indicators of Health

• e.g. guppies orange diet quality
• Special case of good genes
• Hamilton Zuk parasite load plumage
  brightness
• Species (not individuals) with brighter plumage
  genlly higher parasite load
• ? able to use colour as indicator of
• risk of parasitism