5.3 Sexual selection and mate choice

Sexual selection is a powerful force shaping evolution. It drives the development of exaggerated traits that enhance mating success, like peacock tails or lion manes. This process can sometimes conflict with natural selection, favoring traits that may reduce survival but boost reproductive success.

There are two main types of sexual selection: intrasexual and intersexual. Intrasexual selection involves same-sex competition, while intersexual selection is about mate choice. These processes lead to sexual dimorphism and the evolution of elaborate courtship displays and ornaments.

Sexual Selection Fundamentals

Sexual vs natural selection

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• Sexual selection drives evolution through competition for mates focusing on traits enhancing mating success often leading to exaggerated or costly features (peacock's tail)
• Natural selection promotes differential survival and reproduction favoring traits enhancing overall fitness typically promoting efficiency and adaptiveness (camouflage)
• Sexual selection primarily impacts reproductive success while natural selection affects survival and overall fitness
• Sexual selection can sometimes favor traits reducing survival (bright coloration attracting predators)

Types of sexual selection

• Intrasexual selection involves competition between same-sex individuals often male-male contests for access to mates (elephant seal fights)
• Intersexual selection occurs when one sex (typically females) chooses mates based on attractive traits or resources (bird courtship displays)

Mate choice and sexual dimorphism

• Mate choice process selects mating partners based on specific criteria often exercised by females due to higher reproductive investment (peahens choosing peacocks)
• Sexual dimorphism results from divergent selection pressures on each sex leading to differences in appearance (lion manes)
• Mate choice drives development of exaggerated traits in the chosen sex influencing direction and strength of selection (bird of paradise plumage)

Examples of sexually selected traits

• Peacock's tail attracts females through vibrant colors and patterns indicating male health and genetic quality
• Red deer antlers used in male-male combat for access to females signaling strength and dominance
• Bowerbird's bower demonstrates male skill and resource acquisition ability through elaborate structure building
• Firefly flashes allow species-specific light patterns for mate attraction and recognition
• Fiddler crab's large claw used in male combat and female attraction signaling male quality and fighting ability

Costs and benefits of sexual traits

• Costs include energy expenditure in developing and maintaining traits increased predation risk due to conspicuousness and potentially reduced survival probability
• Benefits involve increased mating success and reproductive output potential for good genes passed to offspring and honest signaling of individual quality
• Fitness impact creates trade-off between survival and reproductive success potentially increasing overall fitness despite survival costs
• Handicap principle suggests costly traits serve as honest indicators of quality only affordable by high-quality individuals
• Fisherian runaway selection creates positive feedback loop between trait and preference potentially leading to exaggerated traits beyond adaptive optimum (peacock's tail)