Sexual Selection Natural Selection: Individual Lifetime Reproductive Success Survival, Reproduction Tradeoff Greater Current Reproduction vs Survival (Future.

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Presentation transcript:

Sexual Selection Natural Selection: Individual Lifetime Reproductive Success Survival, Reproduction Tradeoff Greater Current Reproduction vs Survival (Future Reproduction)

Sexual Selection Does Not Oppose Natural Selection Natural Selection for Traits Enhancing Mating Success May Reduce Survival, But Not Fitness

Sexual Selection Two Processes, Defined by PI Difference Intrasexual (Direct Male Competition) Intersexual (Indirect Male Competition)

Sexual Selection Intrasexual: Direct Competition among Members of Sex with Lower PI for Mating with Sex with Greater PI “Male-Male Competition”

Sexual Selection Intersexual: Indirect Competition among Members of Sex with Lower PI Sex with Greater PI Increases Mating Variance in Sex with Lower PI through Mate Choice “Female Choice”

Sexual Selection Potential for Sexual Selection Ratio Variance in Reproductive Success: Lesser PI Sex / Greater PI Sex Small: Monogamy Large: Polygyny (Polyandry)

Sexual Selection: Female Defense

Sexual Selection: Polygyny V[Male Rep Success] > V[Female Rep Success] Sexual Selection More Intense in Males Body Size, Aggression, etc stop

Secondary Sexual Characters Bright Colors, Ornaments, Elaborate Displays of Males Sexual Selection: Advance Mating Success & Enhance Fitness, Despite Cost Evolve?

Secondary Sexual Characters Intrasexual Competition: Some Display Function Darwin: Sexual Selection through Intersexual Competition, Female Choice Why Females Attracted by Bright Colors? Number of Hypotheses

Secondary Sexual Characters “Sexy Son” Hypothesis: Given Female Preference, Female Chooses Male Genes Likely to Render Her Sons Attractive Doesn’t Answer Question ?

Secondary Sexual Characters & Female Choice Hamilton, W.D. & M. Zuk Science 218:384. Females Assess Potential Mate’s Ability to Resist Parasites Female “Obtains” Parasite-Resistance Genes for Offspring Resistance versus Tolerance

Secondary Sexual Characters Assumptions Expression of Secondary Sexual Traits Increases with Health/Vigor of Male Resistance to Parasites Heritable Parasites Affect Host Viability

Secondary Sexual Characters Predict that Species with Greater Parasite Burdens Would Show Brightest Colors Assumptions Not Met When Prevalence of Parasites Low High Parasite Prevalence: Both Sexes May Benefit from Signal of Health

Secondary Sexual Characters & Female Choice North American Passerine Birds Male Brightness (6 point scale) Correlates Positively with Correlates Positively with Parasite Prevalence (Proportion Males Infected by Class of Blood Parasites) Supports Hypothesis (Tolerance)

Secondary Sexual Characters & Female Choice Within Population: Parasite Load vs Mating Success? Tolerance versus Resistance Are Brighter Males Withstanding Parasite Load Physiologically? Are Brighter Males Resistant, and Parasitized Less? Open Questions

Secondary Sexual Characters Barn Swallow (Hirundo rustica) Males: Long Tailfeathers Female Choice & Males with Larger Tail Ornaments Survive Better

Secondary Sexual Characters Moller, A.P Proc. Natl. Acad. Science USA 91:6929. Tail Length & Heritability Denmark

Secondary Sexual Characters

Barn swallow Females Choose Ornamental Trait Secondary-Sexual Trait Positively Correlated with Survival in Male Mate Trait Heritable: Son’s Survival Positively Correlated with Father’s Survival

Parasitism & Male Traits Alternative Hypothesis: “Trade-off” Males Pay Cost of Secondary Traits More Testosterone Implies Increase in Male Secondary Sexual Traits Aggressiveness: Intra-sexual Competition Female Choice: Intersexual Component

Parasitism & Male Traits Testosterone: Immunosuppressive Increased Male Traits Can Decrease Immunocompetence More Testosterone May Imply Greater Parasite Load, Decreased Survival ? Maintained by Sexual Selection ?