Sexual Selection Bio Ch 50.3

• This material covers biological science, specifically section 50.3, focusing on choosing mates and sexual selection, as found on pages 1107-1108.
• Week 10, Thursday March 20th.
• Textbook pages 1107 onward from "Ultimate Cause of Mate Choice" for an overview of sexual selection evolution
• Cultural biases influence interpretation of society-related topics.

Sexual Selection

• Sexual selection is a selection type where members of the same or opposite sex of a species exhibit preferences.
• It's often considered part of natural selection.
• A fundamental asymmetry of sex is the major argument for sexual selection.

Fundamental Asymmetry of Sex Theory

• In most species, females invest more in their offspring compared to males, representing the fundamental asymmetry of sex.
• Female fitness is limited by resources for eggs and healthier offspring; male fitness by the number of available females.
• Mammals invest further care through gestation, milk production, and nursing.
• Predictions on sexual selection indicate females are choosy about mates while males compete.
  • Alleles increasing male mating success increase in the population, leading to male-male competition.
  • Alleles enhancing male attractiveness to females increase due to female choice.
• Competition happens for male access when females are harder to get.
• Females may accept a male that won against other males.
• Courtship behavior displays can be used by males to attract females, and "attractiveness" leads to female choice.

Intrasexual Selection

• Individuals compete for mates, often in male-male competition.

Operational Sex Ratio (OSR)

• OSR is the ratio of sexually competing males to sexually competing females ready to mate.
• A biased OSR occurs when there are too many sexually competing males or females.

Reproductive Success

• Reproductive success measures fitness and is quantified by offspring number.
• There isn't an average difference between male and female reproductive success since each zygote results from fused gametes. The difference lies in success distribution among each sex, indicated by standard deviation/variation.

Male-Male Competition (Elephant Seals)

• Sexual selection is intense in male elephant seals due to high reproductive success variation.
• Females have lower reproductive success variation.
• 90% of male seals die before reproducing compared to 60% of females.
• A few successful males father 81-100 offspring.
• Sexual selection in elephant seals is male-male competition driven.
• Male elephant seals defend territories, where battle winners monopolize matings.
• Larger males usually win fights.

Reproductive Success Variation Scenarios

• Average reproductive success is the same for both males and females. The variation in their reproductive success rates may vary.
• Males that compete for mates and do not provide offspring care present a different scenario compared to other types of relationships
• Male elephant seals show strong sexual dimorphism.

When Males Compete

• Males may not compete as much and contribute to offspring care instead.
• Blue-footed boobies represent low sexual dimorphism.
• Variance of reproductive success are similar amongst both males and females
• Monogamous mating systems with parental care is high are more similar in their habits
• Selective force is lacking since both parents raise their young together

When Females Compete

• Males provide all parental care while females compete for mates.
• Red-necked phalaropes are an example, females are larger and more colorful while leading courtship behavior, males incubate eggs.

Weaponry and Ornament

• Strong dimorphism results from sexual selection
• Competition for mate leads to strong sexual dimorphism
• The jacana uses a wing spur to fight for a larger territory

Alternative Reproductive Strategies in Male Salmon

• Jack males: mature early, adopt sneak behavior to fertilize eggs (5% of males).
• Hooknose: mature later, engage in severe competition for females (majority of males).
• Sneakers utilize alternative strategies instead of the competion seen in salmon
• Sneakers exist in invertebrates to vertebrates

Topi Antelope (Serengeti-Mara)

• Females are slightly smaller than males.
• Both sexes have horns, with no male parental care.
• During the mating season, females actively compete for mates.
• Competition for mates and mate choice is not clearcut

Forms of Sexual Selection

• Intrasexual selection: individuals compete for mates.
• Intersexual selection: mate choice is observed.

Parental care

• Females prefer males who provide resources
• Well constructed nests and habitats give offspring a higher chance of survival
• Traits that lead to a higher survivability are passed down

Female Choice

• Traits can be puzzling as predators are evolved
• Traits that reduce survivorship
• Females don't get any direct benefits in these cases

Runaway Sexual Selection

• "Runaway sexual selection" dictates elaborate male traits evolve because females prefer them with purely natural selection benefiting them.
• A theory by scientists predicts that females should be choosy, while males compete.
• Alleles increasing male success/attractiveness should proliferate.
• Consequently, male traits show directional selection (bigger size, extreme features).

Runaway Sexual Selection Specifics

• Females with no preference mate equally.
• Females with a preference only mate with preferred males.
• Males with preferred traits produce more offspring.
• Alleles for preference and traits co-occur, spreading preference alleles; sensory systems may drive female choice.
• Runaway selection causes traits to push past the limits of natural selection

Laupala Crickets

• Laupala crickets from the Hawaiian Islands have genes controlling song "pulse rate" tightly linked to female preferences.
• Divergent female preferences in Laupala crickets can cause speciation in allopatry (drift, mutation, selection).
• Divergence in female preferences in sympatry can lead to sympatric speciation.
• Divergent female preferences are likely to play a role in the sympatric speciation events.

Key Takeaways

• Sexual selection violates the Hardy-Weinberg principle by altering allele frequencies, resulting in evolutionary change.
• Natural selection favors traits aiding organism survival in environments.
• Sexual selection acts upon members within the same species.
• Reproductive success is a consequence of intrasexual and intersexual selection.