Sexual Selection Notes

Sexual Dimorphism

  • Sexual dimorphism refers to the differences in appearance between males and females.

  • Natural selection cannot fully explain all male/female differences.

Darwin's Problem: Secondary Sexual Characteristics

  • Primary sexual characteristics: Traits directly linked to rearing offspring (e.g., testes, ova, mammary glands).

  • Secondary sexual characteristics: Traits not directly linked to rearing offspring (e.g., horns, bright feathers).

  • These traits pose questions about their existence, evolution in one sex, and driving factors.

Natural Selection vs. Sexual Selection

  • Natural Selection: Differential reproductive success due to variation in survival and fecundity.

  • Sexual Selection: Differential reproductive success due to variation in success at obtaining mates.

Parental Investment

  • Sexual selection acts differently on each sex due to differences in parental investment.

  • Parental investment: Energy and time expended constructing and caring for offspring.

  • Mating is generally more costly for females (large investment in eggs) than males (small investment in sperm).

  • Tradeoffs exist between current reproduction (gametes) and future reproduction (parent health).

  • Males and females may differ in gamete production, gestation, and parental care.

Asymmetries in Parental Investment

  • Gamete Production: Females invest significantly more energy in egg production than males do in sperm production.

  • Gestation and Parental Care: In mammals, females typically carry offspring and provide post-birth care.

Bateman's Gradient

  • Bateman’s gradient = slope of the line, measures the strength of sexual selection between reproductive success vs. mating success.

  • Steeper slope indicates stronger sexual selection.

  • Traits that help males find mates will be under strong sexual selection.

Competition vs. Choice

  • If gametes are costly, limited, and large, and investment is large, sexual selection is weak.

  • If gametes are cheap, numerous, and small, and investment is small, sexual selection is strong.

  • This leads to competition among males and choice by females.

Intrasexual Selection

  • Sexual selection driven by interactions within a sex (e.g., male-male competition).

  • Leads to evolution of weapons, armor, large body size, tactical cleverness, and specific sperm characteristics.

  • Mechanisms: Contest competition, alternative mating strategies, scramble competition, sperm competition.

Intrasexual Selection: Contest Competition

  • Occurs when males fight for access to females or territories.

  • Larger males often win more fights and copulate with more females.

Intrasexual Selection: Alternative Mating Strategies

  • If a class of males fails to mate with the typical strategy, they evolve an alternative strategy to gain mating success.

Intrasexual Selection: Scramble Competition

  • Occurs when resources (receptive females) cannot be monopolized.

  • Males race to reach mates, common in explosive breeders.

Intrasexual Selection: Sperm Competition

  • If a female mates with multiple males, the male whose sperm wins the race to the egg has higher reproductive success.

  • Examples: prolonged copulation, copulatory plugs, sperm removal appendages.

Intersexual Selection

  • Sexual selection driven by interactions between sexes, often females choosing mates.

  • Leads to evolution of courtship displays and ornaments.

Intersexual Selection: Direct Benefit

  • Females gain immediate benefits (survival or increased fecundity) from choosing certain males.

  • Males provide nuptial gifts for females during mating; larger gifts mean more food and sperm transfer.

Intersexual Selection: Good Genes

  • Females choose mates based on traits correlated with “good genes” passed to offspring.

  • Female preference is under indirect selection.

  • Trait must be phenotypically plastic and condition-dependent.

Intersexual Selection: Runaway Selection

  1. Females choose arbitrary traits.

  2. Positive association between male trait and female preference.

  3. Females that mate inherit the trait.

Intersexual Selection: Sensory Bias

  • Female preference arises through bias favoring one type of male signal.

  • Sensory bias influences the evolution of the male trait.

  • Female preference may evolve before the male trait.