In-Depth Notes on Sexual Selection

Sexual Selection

• Definition:

  • Sexual selection is a form of natural selection favoring traits that enhance an individual's reproductive success, often involving competition and choice.

• Basic Principle:

  • In sexual reproduction, the dynamic involves cooperation and conflict between sexes.

Anisogamy

• Gamete Production:

  • Males produce many small gametes (sperm).

  • Females produce fewer, larger gametes (eggs).

Bateman's Principle

• Sexual Strategies:

  • Variance in Reproductive Success: Males typically experience higher variance in reproductive success than females due to the number of mates they can obtain.

  • Graphical Representation: The Bateman Gradient illustrates differences in the number of mates and resulting offspring.

  • More mates = More offspring for males.

Sexual Conflict

• Investment Disparity:

  • Differences in gamete investment lead to a conflict in reproductive strategies between sexes.

  • Operational sex ratio impacts mating behaviors and competition strategies.

Mechanisms of Competition for Mates

• Types of Competition:

  • Same-sex Contests: Male-male confrontations for mates.

  • Mate Preference: Selection based on attractive secondary sexual characteristics.

  • Scrambles: Fast searches for mates.

  • Sperm Competition: Competition between sperm from multiple males.

  • Coercion: Forced mating strategies.

  • Antagonistic Coevolution: Evolving traits in response to opposing selective pressures.

Male-Male Competition

• Displays and Fighting:

  • Males will often display physical attributes or fight to win mating opportunities.

  • Directional Selection: Leads to more pronounced traits like size and weaponry (arms race concept).

  • Territoriality: Males defend territories to ensure exclusive access to females.

Mate Choice

• Female Selection:

  • Females often select males based on traits like size or bright coloration that may signal genetic quality.

• Benefits of Choice:

  • Direct Benefits: Resources provided by males enhance female or offspring fitness (nutrition, territory).

  • Indirect Benefits: Selecting mates that produce genetically superior offspring foster long-term fitness.

Indicators of Genetic Quality

• Preference for Superior Traits:

  • Females benefit from choosing mates with traits that indicate good genes.

  • Evidence supports that males in better conditions exhibit more attractive traits and are therefore preferred by females.

Runaway Sexual Selection

• Hypotheses:

  • Focuses on how traits that are attractive to females can become exaggerated over time (e.g., colorful feathers).

Sperm Competition

• Mechanics of Competition:

  • Occurs when multiple males mate with the same female; success hinges on sperm quantity and quality.

  • Sperm Precedence: The first male's sperm may fertilize a female's eggs more effectively.

Exam Tips

• Key Concepts to Understand:

  • Anisogamy, Bateman's principle, types of sexual selection mechanisms, and indicators of mate quality.

• Potential Exam Question:

  • Identify what evidence supports hypotheses about sexual selection (e.g., prey size in nuptial gifts).