Updated Chapter 11

Chapter 11: Sexual Selection

Sexual Dimorphism

• Definition: Males and females differ in various traits across many species.

  • Traits include size, coloration, shape, and body ornamentation.

• Commonality: Sexual dimorphism is widespread in many species.

Factors Influencing Sexual Dimorphism

• The reasons behind the prevalence of sexual dimorphism can be linked to adaptive traits through natural selection.

• Example: Purple-throated caribs (Eulampis jugularis)

  • Observation: Females possess longer and more curved beaks than males.

  • Adaptation Question: This leads to investigation if these traits are adaptations for feeding strategies.

Feeding Preferences and Adaptations

• Caribs show preference for nectar from Heliconia caribea.

  • Males: Larger and dominant, limit female access to feeding.

  • Females: Feed on Heliconia bihai, which has longer flowers suited for their beaks.

  • Natural Selection:

    • Favors males with shorter, straighter beaks for H. caribea and females with longer, curved beaks for H. bihai.

Sexual Selection Overview

• Darwin's Recognition: Natural selection is not the only force; sexual selection significantly influences reproductive success.

  • Sexual Selection: Enhances an individual's ability to attract mates, leading to sexual dimorphism.

  • Male and female selection pressures may differ based on reproductive strategies.

Heritable Variation and Mate Acquisition

• For sexual selection to occur, there must be heritable traits that affect mate acquisition.

  • Over time, variants that improve mating success will become more prevalent.

Asymmetries in Sexual Reproduction

• Sexual Selection Drives:

  • Females have a fixed maximum fecundity, while males can increase success by increasing the number of mates.

  • Males typically have greater variance in reproductive success compared to females.

• Parental Investment: Differences in the energy and time investment in offspring lead to distinct reproductive strategies.

  • Orangutans: Females care for the fetus and infants, while males provide no care in 90% of species.

Daily Energy Expenditure in Gametes

• Illustrates differences in energy expenditure per egg versus sperm; eggs require more energy to produce.

Fruit Fly Experiment (Bateman's Principle)

• Setup: Mating trials with virgin males and females to assess reproductive success.

  • Findings: Males' reproductive success positively correlates with the number of mates, while females do not see increased success past a single mating.

Sexual Selection Patterns

• Access to mates is often limiting for males, prompting stronger sexual selection pressure.

  • Intrasexual Selection: Males compete for access to females (direct control).

  • Intersexual Selection: Females choose mates based on traits or displays advertised by males.

Male-Male Competition

• In species with fighting males, traits like large body size or weaponry evolve.

  • Example: Galápagos marine iguanas exhibit larger males due to selection pressures.

• Natural Selection Impact: Found stabilizing selection on body size.

  • Medium-sized iguanas show higher fitness than larger or smaller counterparts.

Infanticide as a Competition Strategy

• Male lions often kill cubs of other males to expedite mating opportunities with the pride.

  • Average coalition time for males in the pride is typically 1-2 years.

Intersexual Selection Factors

• Females tend to be choosy for several reasons: to gain good genes, resources, or due to aesthetic preferences.

• Barn Swallows: Illustrates a model of intersexual selection where male tail variability influences female choice.

  • Males with elongated tail feathers attract mates faster and gain greater reproductive success.

Courtship Behaviors and Strategies

• Distinct calling traits in species like gray tree frogs influence female preferences.

• Hangingflies: The size of prey during copulation can enhance sperm transfer efficiency.

Runaway Selection Hypothesis

• Suggests that female preferences can reinforce the male trait selection over generations, even if traits are arbitrary.

Alternative Mating Strategies

• Small males in species like marine iguanas compensate through strategies such as sneaky copulation.

• Coho Salmon: Exhibit variations in mating strategies where smaller males are sneaker males that try to fertilize eggs discretely.

Sexual Selection Theories on Human Behavior

• Studies suggest that human dimorphism may not solely arise from sexual selection, due to cultural complexities.

  • Examples: Observations of aggressive behaviors affecting reproductive success in males.

Conclusion on Sexual Selection

• Sexual selection is a driving force in the evolution of various traits, often acting in opposition to natural selection but still enhancing reproductive fitness in individuals.