Introduction

Sexual selection is a fundamental mechanism of evolution, first articulated by Charles Darwin in 1871. Unlike natural selection, which centers on survival, sexual selection focuses on differential reproductive success due to variation in mating success. This process drives the evolution of traits that may not necessarily enhance survival but increase an individual’s chances of attracting mates or outcompeting rivals. Sexual selection is pervasive across the animal and plant kingdoms, shaping behaviors, morphologies, and even genetic architectures.

Main Concepts

1. Mechanisms of Sexual Selection

a. Intrasexual Selection (Competition Within a Sex)

  • Definition: Competition among individuals of the same sex (usually males) for access to mates.
  • Examples:
    • Male deer engaging in antler battles.
    • Elephant seals exhibiting extreme size dimorphism due to male-male competition.
  • Outcomes: Evolution of weaponry (antlers, tusks), increased body size, and aggressive behaviors.

b. Intersexual Selection (Mate Choice)

  • Definition: Preferences by one sex (usually females) for certain traits in mates.
  • Examples:
    • Peacock’s elaborate tail feathers.
    • Song complexity in birds.
  • Outcomes: Evolution of ornaments, displays, and courtship behaviors.

2. Sexual Dimorphism

Sexual selection frequently leads to sexual dimorphism—distinct differences in appearance or behavior between males and females of the same species. Dimorphism can manifest in size, coloration, ornamentation, or behavior.

  • Case Study: The anglerfish exhibits extreme dimorphism; males are tiny and parasitic compared to much larger females.

3. Sperm Competition and Cryptic Female Choice

  • Sperm Competition: When females mate with multiple males, sperm from different males compete to fertilize eggs. This leads to adaptations like increased testes size or specialized sperm morphology.
  • Cryptic Female Choice: Females may influence which male’s sperm fertilizes their eggs after mating, often through physiological or biochemical mechanisms.

4. Sexual Conflict

Sexual selection can result in evolutionary arms races between the sexes, where adaptations in one sex drive counter-adaptations in the other. This conflict can involve mating rates, parental investment, or control over fertilization.

  • Example: In water striders, males evolve grasping structures to hold onto reluctant females, while females evolve slippery bodies to escape unwanted mating attempts.

5. Alternative Reproductive Tactics

Not all individuals follow the same reproductive strategies. Some males may adopt “sneaker” tactics, mimicking females or using stealth to mate, while others invest in direct competition.

  • Example: Bluegill sunfish males can be “parental,” “sneaker,” or “satellite” types, each with unique reproductive behaviors.

Controversies in Sexual Selection

1. The Role of Female Choice

Debate persists over the extent and mechanisms of female choice. Some argue that female preferences are genetically fixed, while others suggest they are plastic and context-dependent.

2. Cost of Ornaments

There is ongoing discussion about the actual costs and benefits of sexually selected traits. Some studies suggest that elaborate ornaments may not be as costly as once thought, challenging the traditional “handicap principle.”

3. Sexual Selection in Humans

The application of sexual selection theory to human behavior remains contentious, especially regarding mate preferences, gender roles, and cultural influences.

4. Sexual Selection and Speciation

Recent research questions the strength of sexual selection as a driver of speciation, with some studies showing weak or inconsistent links between sexual selection intensity and speciation rates.

Recent Research

A 2021 study published in Nature Communications by Kraaijeveld et al. examined the genomic signatures of sexual selection across 16 bird species, revealing that strong sexual selection correlates with rapid evolution of genes related to sperm competition and immune function. This supports the idea that sexual selection can drive significant genomic divergence, even among closely related species (Kraaijeveld et al., 2021).

Surprising Aspects

One of the most surprising aspects of sexual selection is the extent to which it can drive the evolution of traits that are detrimental to survival. For example, the peacock’s tail is energetically costly and increases predation risk, yet persists due to female preference. Another unexpected finding is that sexual selection can operate in plants, where competition among pollen grains or ovule choice by female tissues mirrors animal sexual selection dynamics.

Project Idea

Title: “Quantifying the Trade-Offs Between Sexual Ornamentation and Survival in a Local Bird Population”

Description:
Design a field study to measure the relationship between ornament size (e.g., tail length, plumage brightness) and survival rates in a bird species. Collect data on mating success, predation rates, and health indicators. Analyze whether individuals with more pronounced ornaments have higher reproductive success but lower survival, quantifying the evolutionary trade-offs imposed by sexual selection.

Conclusion

Sexual selection is a powerful evolutionary force that shapes the diversity of life in remarkable ways. It explains the evolution of extravagant traits, complex behaviors, and even genetic innovation. Despite its central role in evolutionary biology, sexual selection remains an area of active research and debate, with new discoveries continually refining our understanding. Its interplay with natural selection, sexual conflict, and speciation underscores its complexity and significance.

Did You Know?

The largest living structure on Earth is the Great Barrier Reef, visible from space. This reef is home to countless species whose reproductive strategies, including sexual selection, contribute to the biodiversity and resilience of this ecosystem.

References

  • Kraaijeveld, K., et al. (2021). “Genomic signatures of sexual selection in birds.” Nature Communications, 12, 1-12. Link
  • Andersson, M. (1994). Sexual Selection. Princeton University Press.
  • Kokko, H., & Jennions, M. D. (2014). “The relationship between sexual selection and sexual conflict.” Cold Spring Harbor Perspectives in Biology, 6(9), a017517.