Definition

Sexual selection is a mode of natural selection where certain traits increase an individual’s chance of mating and reproducing, even if these traits do not necessarily enhance survival. It operates through two main mechanisms: intrasexual selection (competition within one sex, usually males) and intersexual selection (mate choice, usually by females).

Key Concepts

1. Mechanisms of Sexual Selection

Intrasexual Selection

  • Analogy: Like a job interview where candidates compete for a single position, individuals of the same sex compete for access to mates.
  • Example: Male elephant seals engage in fierce battles for control of beach territories, with winners gaining access to harems of females.

Intersexual Selection

  • Analogy: Similar to consumers choosing products based on desirable features, individuals select mates based on preferred traits.
  • Example: Female peafowl (peahens) often choose males with the most elaborate and colorful tail feathers.

2. Sexual Dimorphism

  • Definition: Differences in appearance between males and females of the same species, often driven by sexual selection.
  • Example: In birds-of-paradise, males display extravagant plumage and complex dances, while females are more camouflaged.

3. Runaway Selection

  • Process: A feedback loop where a trait becomes exaggerated over generations due to mating preferences.
  • Example: The Irish elk’s massive antlers, which may have evolved due to female preference for larger antlers, despite increased predation risk.

4. Handicap Principle

  • Theory: Costly traits signal genetic fitness because only high-quality individuals can afford them.
  • Example: The bright coloration of guppies increases predation risk but signals health and vitality to potential mates.

Real-World Examples

  • Bioluminescent Organisms: Many marine species, such as certain squid and ostracods, use bioluminescence in courtship displays. For instance, ostracods create intricate light patterns in the water to attract mates, akin to a nighttime fireworks show in the ocean.
  • Bowerbirds: Male bowerbirds build elaborate structures (“bowers”) and decorate them with colorful objects to attract females, paralleling human efforts to decorate homes to impress guests.
  • Stalk-eyed Flies: Males with longer eye stalks are preferred by females. The trait is energetically costly, making it a reliable indicator of fitness.

Key Equations and Models

Bateman’s Principle

  • Equation:
    Reproductive Success (RS) ∝ Number of Mates (NM)
    For many species:
    RS_male ∝ NM_male
    RS_female ≈ constant

Fisher’s Runaway Model

  • Equation:
    Change in trait frequency:
    Δq = s * q * (1 - q)
    Where q is the frequency of the trait, and s is the selection coefficient.

Hamilton-Zuk Hypothesis

  • Equation:
    Trait Expression = Genetic Quality - Parasite Load
    T = G - P
    Where T is trait expression, G is genetic quality, and P is parasite load.

Common Misconceptions

  • Sexual selection only affects males: Both sexes can experience sexual selection, though often in different ways.
  • Sexual selection always leads to larger or more colorful males: In some species, females are larger or more ornamented (e.g., phalaropes).
  • Sexual selection is separate from natural selection: Sexual selection is a subset of natural selection, focusing specifically on traits that increase mating success.
  • All sexually selected traits are beneficial for survival: Many traits favored by sexual selection can actually reduce survival (e.g., long tails in birds increase predation risk).

Emerging Technologies

  • Genomic Sequencing: High-throughput sequencing enables the identification of genes linked to sexually selected traits, revealing the genetic architecture of mate choice and ornamentation.
  • Automated Behavioral Tracking: Machine learning and computer vision allow for precise quantification of courtship and mating behaviors in real-time, even in complex environments.
  • CRISPR/Cas9 Gene Editing: Facilitates experimental manipulation of candidate genes to test their roles in sexual selection, providing causal evidence for genetic hypotheses.
  • Bioluminescence Imaging: Advanced imaging techniques visualize real-time courtship displays in marine environments, uncovering new forms of sexual signaling.

Recent Research

A 2021 study by Wong and Candolin in Nature Reviews Genetics highlights how sexual selection shapes the evolution of genomes, particularly in rapidly changing environments. The authors discuss how climate change and urbanization are altering the dynamics of mate choice and competition, leading to rapid evolutionary responses in sexually selected traits.
Reference:
Wong, B. B. M., & Candolin, U. (2021). Sexual selection in changing environments. Nature Reviews Genetics, 22, 797–812. https://doi.org/10.1038/s41576-021-00392-2

Summary Table

Mechanism Example Species Trait/Behavior Impact on Fitness
Intrasexual Selection Elephant Seal Male combat Access to mates
Intersexual Selection Peafowl Colorful tail Increased mate choice
Handicap Principle Guppy Bright coloration Honest signal of fitness
Bioluminescent Courtship Ostracod Light displays Mate attraction

Key Takeaways

  • Sexual selection drives the evolution of traits that enhance mating success, often at the cost of survival.
  • Both competition and choice are fundamental, and both sexes can be subject to sexual selection.
  • Modern technologies are revolutionizing the study of sexual selection, from genomics to behavioral analysis.
  • Misconceptions persist and should be addressed in STEM education to foster a nuanced understanding.

Further Reading

  • Andersson, M. (1994). Sexual Selection. Princeton University Press.
  • Wong, B. B. M., & Candolin, U. (2021). Sexual selection in changing environments. Nature Reviews Genetics, 22, 797–812.