Download Sexual Selection

Theory of Sexual Selection
Darwin’s The Descent of Man, and
Selection in Relation to Sex
Sexual Selection
Darwin’s attempt to explain traits that defied
natural selection
Intersexual Selection
• “female-choice”
• Bateman’s Principle: the sex which
invests the most in producing offspring
becomes a limiting resource over which
the other sex will compete
Intersexual
Selection
Intrasexual Selection
• “Male-male competition”
• Strategies used to compete with
members of the same sex for access to
members of the opposite sex
• Female ornamentation produced at
expense of their offspring which makes
them lose fitness
Intrasexual
Selection
Sexual Dimorphism
• Differences between individuals of
different sex of the same species
• Male is larger in most mammals
• In insects, spiders, birds, reptiles,
amphibians the female is usually larger
Sexual Dimorphism
Sexual Selection and
Physical Attractiveness
Coloration in Animals
Female Mandrill: facial coloration
proportional with age (signals
reproductive ability)
Three Spined Stickleback:
Females prefer males with
intense red coloration
(signals parasite resistance,
because parasites reduce
intensity)
Physical Attractiveness- Skin
Coloration
Physical Attractiveness- Skin
• Skin color distribution influences perception of
female face
• Attractive signals correspond to high
Estrogen/Testosterone ratios
• Skin problems may signal a disturbance in E/T
ratio and reduced reproductive ability
Physical AttractivenessSymmetry
Physical Attractiveness- Symmetry
• Phenotypic indicator of biological fitness
• Fluctuating Assymetries (FAs) result from
genetic and environmental stresses during
development
• Indicates developmental homeostasis
• Cue of high parasite resistance (parasite theory)
– Defense against parasites: genetic diversity
(dependent on individual heterozygosity)
– Heterozygosity related to symmetry of bilateral traits
Physical Attractiveness- Scent
• Androstenone- secondary sex characteristic
(men give off three times more than
women)
• Secondary sex characteristics correlated
with offspring survival
• During ovulation, female is most fertile
• Increase in sensitivity to androstenone
Relativity in Physical Attractiveness
• Female choice is modified by female’s viability
• Female three-spined sticklebacks in worse
condition show preference for less brightly colored
males
• In humans, relative height
• Humans adjust preferences to increase pool of
partners
• Women who considered themselves physically
attractive showed greater preference for symmetry
in male faces
Summary
• Intrasexual and Intersexual Selection
• Sexual Dimorphism
• Physical Attractiveness
–
–
–
–
Coloration
Symmetry
Femininity
Scent
• Relativity
References
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