Sexual Selection

Sexual Reproduction and Evolution

• Functions of Sexual Reproduction:

  • Introduces new genetic variations.

  • Increases overall variation within a population.

  • Enhances chances of producing more fit or adapted organisms.

Forms of Reproduction in Multicellular Eukaryotes

• Asexual Reproduction:

  • Apomyxis: Production of gametes through mitosis-like division resulting in clones, barring mutations.

  • Automixis: Gametes produced by meiosis; nuclei fusion can create genetic diversity.

Types of Reproductive Systems

• ZZ/ZW and XY Sex Determining Systems:

  • Differentiation in gamete size leads to different reproductive strategies across species.

Occurrence of Asexual Reproduction

• Asexual reproduction is less common than sexual reproduction.

• Some species alternate between sexual and asexual generations.

• Other species exhibit both reproductive forms simultaneously.

Sexual Reproduction Mechanics (Amphimixis)

• Involves two diploid individuals producing haploid gametes via meiosis:

  • Meiosis I: Diploid -> two haploid gametes.

  • Meiosis II: Gametes fuse, restoring diploidy in the zygote.

Trends of Asexual Lineages in Evolution

• Asexual lineages generally evolved from sexually reproducing ancestors.

• Asexual lineages often face shorter evolutionary lifespans.

• Exploration needed on why asexual reproduction may evolve.

Costs of Sexual Reproduction

• Twofold cost of sex: Asexual females pass on more genetic copies than sexual females.

• Sex can disrupt beneficial gene combinations.

• Costs of energy associated with mate searching and courting.

• Increased risk of disease and parasite transmission through sexual acts.

Advantages of Sexual Reproduction

• Elimination of harmful mutations.

• Generation of genetic variation, leading to improved adaptability.

Muller's Ratchet

• Describes accumulation of deleterious mutations in asexual populations over evolutionary time.

• Recombination provides a mechanism to combine advantageous traits, reducing mutation accumulation.

Fisher-Muller Hypothesis

• Sex accelerates adaptive evolution by facilitating the combination of beneficial mutations from different individuals.

  • Beneficial mutations can stabilize evolutionary advantages through genetic diversity available in sexual reproduction.

Red Queen Hypothesis

• Beyond point of existence, organisms must continually adapt or face extinction.

• Explanation for consistent evolutionary pressures favoring sexual reproduction.

Empirical Evidence

• Example in Yeast: Sexual strains demonstrate quicker fitness improvement in harsh environments compared to asexual strains.

• The rapid generation time of prokaryotes contrasts with the evolutionary advantages of sexual reproduction in eukaryotes.

Sexual Reproduction Leading to Sexual Selection

• Anisogamy: Sexual reproduction involving differently sized gametes (egg and sperm).

• Eggs, being nutrient-rich, create a competitive atmosphere among males, potentially leading to male-male competition and female mating preferences.

Types of Sexual Selection

1. Intersexual Selection

   • One sex (usually females) chooses mates based on traits that indicate genetic quality.

   • Strategies include:

     • Direct benefits (resources, protection).

     • Good genes indicators (healthy displays).

     • Fisherian runaway selection (preference leading to exaggerated traits).

2. Intrasexual Selection

   • Direct competition among individuals of the same sex for access to mates.

Handicap Principle

• Costly traits or behaviors can signal mate quality and fitness, as only the fittest survive such handicaps.

Sensory Bias Hypothesis

• A species' mating preferences can be shaped by existing biases or associations that enhance reproductive success.

Summary

• Sexual reproduction introduces genetic diversity, fosters adaptability, and triggers sexual selection, despite its inherent costs compared to asexual reproduction. Understanding these dynamics is crucial for studying evolutionary biology and ecology.