week 4 lecture notes

Evolution of Sexual Reproduction

• Overview

  • Sexual reproduction includes three major features: recombination, anisogamy, and gender.

  • These features are influenced by disruptive selection, characterized by bimodal distributions in traits.

  • The competition and selection pressures on these aspects lead to evolutionary adaptations across species.

Recombination in Genetic Variation

• Importance of Recombination

  • Recombination allows genes to work together effectively during reproduction.

  • Linked genes tend to cluster on chromosomes, aiding in maintaining advantageous combinations.

  • Recombination helps in reshuffling genes to avoid the negative effects of damaged genes linked together.

• Mechanisms of Recombination

  • Sexual recombination, bacterial conjugation, and crossing over.

  • Even simple organisms like bacteria can engage in gene exchange when environmental pressures increase.

  • Bacteria typically reproduce asexually through division, but can exchange genes when facing changing environments.

Anisogamy and Gamete Differences

• Definition of Anisogamy

  • Anisogamy refers to the production of dissimilar gametes—usually a large, nutrient-rich egg (ova) and a smaller, mobile sperm.

  • This results in contrasting evolutionary strategies between genders.

• Mobility and Parental Investment

  • Sperm are produced in large quantities with low resource costs, while ova are fewer and carry high resource investments for survival.

  • These two types of gametes are subject to conflicting selection pressures, leading to niche specializations.

Gender and Reproductive Strategies

• Hermaphroditism

  • Some organisms, like barnacles, possess both types of gametes, allowing them to reproduce even in low mate-density environments.

  • Hermaphroditism appears where individuals may face limited movement or mate availability, maximizing reproductive success by allowing self-fertilization or partner selection.

• Examples of Gender in Reproduction

  • In seahorses, males carry fertilized eggs, involving more male parental care, contrary to most mammals.

  • In clownfish, individuals start life as males and can change to females depending on social structures, exemplifying the fluidity and adaptability of reproductive roles.

Sexual Selection and Traits

• Darwin's Observations

  • Sexual selection emphasizes traits that attract mates despite the potential disadvantages of those traits (e.g., peacock tail feathers).

  • Traits not contributing to survival can be favored in mate selection if they increase reproductive success.

• Competition and Display

  • Male competition for females leads to pronounced displays, while females may choose mates based on traits signifying quality or fitness.

  • Traits such as elaborate plumage can become indicators of genetic fitness, shaping subsequent generations through sexual selection pressures.

• Runaway Selection

  • The concept where female preference for specific traits leads to a feedback loop reinforcing those traits over generations.

  • As male traits become more pronounced, the associated reproductive success continues to shape the evolutionary pathway of species.

Conclusion: Evolutionary Implications

• Mutually Reinforcing Strategies

  • The relationship between sexual selection and natural selection reveals the complex interplay that shapes biodiversity and reproductive strategies in the animal kingdom.

  • Ultimately, understanding these dynamics provides insights into evolutionary biology and the continuity of species adaptation in diverse environments.