Theory of Sexual Selection Darwins The Descent of

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 Attractiveness. Symmetry

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

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