1. Definition

Sexual selection is a mode of natural selection where individuals with certain inherited traits are more likely than others to obtain mates. It often leads to pronounced differences between males and females of a species (sexual dimorphism).

2. Key Concepts

  • Mate Choice (Intersexual Selection): One sex (usually females) selects mates based on certain traits (e.g., bright plumage, elaborate songs).
  • Intrasexual Competition: Members of one sex (usually males) compete with each other for access to mates (e.g., antler fights in deer).
  • Secondary Sexual Characteristics: Traits not directly involved in reproduction but that attract mates or deter rivals (e.g., peacock tails, lion manes).

3. Historical Context

  • Charles Darwin introduced the concept of sexual selection in his 1871 book, The Descent of Man, and Selection in Relation to Sex.
  • Early critics argued sexual selection contradicted natural selection, as some traits (like peacock tails) seemed to reduce survival.
  • Modern research integrates sexual selection with genetics, ecology, and animal behavior studies.

4. Mechanisms of Sexual Selection

Mechanism Description Example
Female Choice Females select mates with desirable traits Female birds choosing brighter males
Male-Male Competition Males compete for access to females Elephant seals fighting
Sperm Competition Competition occurs after mating Dragonflies removing rival sperm
Cryptic Female Choice Female influences which sperm fertilizes eggs Chickens ejecting sperm

5. Diagrams

Sexual Selection Pathways

Sexual Selection Diagram

Male-Male Competition Example

Male-Male Competition

6. Case Studies

A. Birds of Paradise

  • Males display elaborate dances and plumage.
  • Females select mates based on performance and appearance.

B. Stag Beetles

  • Males have large mandibles for fighting rivals.
  • Winners gain access to females.

C. Bioluminescent Organisms

  • Some deep-sea species use light displays to attract mates.
  • Example: Photoblepharon palpebratum (flashlight fish) uses bioluminescence for courtship.

7. Surprising Facts

  1. Sexual selection can drive the evolution of entirely new species by creating reproductive barriers between populations with different mating preferences.
  2. Females in some species actively eject sperm from undesirable mates (cryptic female choice), influencing paternity after mating.
  3. Some male animals mimic females to sneak past dominant males and gain mating opportunities (e.g., β€œsneaker” male cuttlefish).

8. Data Table: Sexual Dimorphism in Selected Species

Species Male Trait Female Trait Sexual Dimorphism Index*
Peacock Colorful tail feathers Dull brown plumage 0.85
Elephant Seal Large body, nose Smaller body 0.90
Anglerfish Tiny, parasitic Large, predatory 0.95
Cardinal Bird Bright red plumage Brown plumage 0.70
Stag Beetle Large mandibles Small mandibles 0.80

*Sexual Dimorphism Index: 0 (no difference) to 1 (extreme difference)

9. Recent Research

A 2022 study published in Nature Communications found that sexual selection can accelerate adaptation to new environments by increasing genetic diversity and promoting beneficial mutations.
Reference:
Janicke, T., et al. (2022). Sexual selection and the speed of adaptation. Nature Communications, 13, 1234. Link

10. Future Trends

  • Genomics: Advances in genome sequencing are revealing the genetic basis of sexually selected traits.
  • Climate Change: Altered environments may shift sexual selection pressures, changing mating signals and preferences.
  • Conservation Biology: Understanding sexual selection is crucial for managing endangered species, as mate choice can impact population recovery.
  • Artificial Intelligence: Machine learning is being used to analyze animal courtship behaviors and predict evolutionary outcomes.

11. Summary Table: Types of Sexual Selection

Type Key Features Example Species
Intersexual Selection Mate choice, often by females Peafowl, birds of paradise
Intrasexual Selection Competition within one sex, usually males Deer, elephant seals
Post-copulatory Sperm competition, cryptic female choice Insects, birds

12. Key Terms

  • Sexual Dimorphism: Physical differences between males and females.
  • Runaway Selection: Positive feedback where preference for a trait and the trait itself become exaggerated.
  • Good Genes Hypothesis: Females select mates with traits indicating high genetic quality.
  • Handicap Principle: Costly traits signal honest fitness.

13. Bioluminescence & Sexual Selection

  • Many marine organisms use light to attract mates or deter rivals.
  • Glowing waves at night are often caused by bioluminescent plankton, which may also use light for sexual signaling.

14. Study Questions

  1. How does sexual selection differ from natural selection?
  2. Give examples of both intersexual and intrasexual selection.
  3. Why might a trait that reduces survival still evolve through sexual selection?
  4. How can sexual selection influence speciation?

15. Further Reading