BSC2010-evolution-3-2024 (1)

Sexual Selection and The Evolution of Sex

Defined by Darwin as the "advantage which certain individuals have over others of the same sex and species, solely in respect to reproduction.”Involves natural selection focusing on mating success.Selection for phenotypic differences enhancing mating success among individuals of the same or opposite sex.Explored in Darwin’s work, The Descent of Man and Selection in Relation to Sex.

Asymmetrical Investment in Offspring

  • Female Investment: Invest directly in offspring (e.g., eggs).

  • Male Investment: Invest in opportunities to mate. Asymmetry leads to two types of selection: intersexual (mate choice) and intrasexual (competition).

Intersexual Selection

  • Example population: 10 male fish, 1 female.

  • Female's choice:

    1. May choose a mate.

    2. Mate with the strongest male. Females will be choosy to maximize the quality of offspring, as their investment per offspring is greater.

Effects of Female Choice

Leads to Sexual Dimorphism (distinct difference in size or appearance between sexes).

How Do Females Choose?

  • Non-adaptive choices: Sensory bias or species recognition.

  • Directly adaptive: Phenotypic traits indicate higher fitness.

  • Indirectly adaptive: Traits signal higher fitness indirectly.

Intrasexual Selection

  • Example population: Two male fish, 10 females.

  • Male reproductive success is limited by access to mates.

  • Prediction: Males will compete for mating opportunities.

Consequences of Male Competition

Leads to Sexual Dimorphism (distinct differences between male phenotypes).

Types of Male-Male Competition

  • Pre-copulatory: Fighting for mates.

  • Post-copulatory: Competing at the level of sperm.

Sperm Competition

Examines success rates of matings in relation to offspring sired.

Unique Cases in Sexual Selection

The Male Seahorse: An exception to typical competition dynamics.

Evidence of Sexual Selection in Humans

Investigates factors influencing mate selection and competitive behavior in human populations.

Natural Selection vs. Sexual Selection

  • Natural Selection: Adaptations in both sexes for survival.

  • Sexual Selection: Enhances reproductive success and focuses on one sex.Not always leading to better adaptation to the environment.

The Evolution of Sexual Reproduction

Sexual Reproduction: Production of haploid gametes through meiosis, leading to zygote formation.Females are organisms carrying the majority of provisioning for offspring through eggs.

Costs and Benefits of Sexual Reproduction

  • Costs:

    • Slower population growth.

    • More energetically costly.

  • Benefits:

    • Increases genetic variability via recombination.

    • Greater resistance to disease and changing environments.

The Two-Fold Cost of Sex

Populations grow faster via parthenogenesis compared to sexual reproduction.Example scenario:

  • Generation 0: 100 females (sexual) vs. 100 females (parthenogenetic).

  • Generation 1: 240 female progeny vs. 120 female progeny.

  • Generation 2: 576 female progeny vs. 144 female progeny.

Asexual Reproduction

Reproduction without meiosis (no fusion of gametes).Methods include fission, budding, and occur in species from aphids to some fish and reptiles.More common in plants, illustrating an alternative evolutionary strategy.

Conclusion

Despite the costs, sexual reproduction is evident across many species, suggesting evolutionary advantages.