Sexual Selection, Kin Selection, and Adaptation in Biological Anthropology

Pedagogical Recommendations and Course Context

• Active Learning Strategies: To succeed in ANHB 2215 (Biological Anthropology: Adaptation & Human Variation), students are encouraged to:

  • Attend all lectures and tutorials.

  • Take hand-written notes to facilitate retention.

  • Actively utilize available learning resources.

  • Prepare for and engage in tutorials.

  • Complete required readings, with a specific focus on the ethnography Nisa.

  • Consolidate knowledge using provided question sets.

• Gender and Reproduction: While the primary focus of these lectures and tutorials is sexual reproduction resulting in offspring, the field of biological anthropology also examines gender diversity. Most traditional societies have historically recognized or currently recognize gender diversity beyond the binary.

Darwin’s Problem and the Theory of Sexual Selection

• Evolutionary Paradox: Charles Darwin observed that certain physical and behavioral features appeared to be disadvantageous for survival. Examples include:

  • The heavy and conspicuous tail feathers of a peacock.

  • Large antlers on deer.

  • Lekking (group mating displays).

  • Extravagant courtship displays.

• Darwin's Quote: Darwin famously stated, ‘‘THE SIGHT OF A PEACOCK’S TAIL MAKES ME ILL!’’ because it challenged his early concept of natural selection based strictly on survival.

• Resolution: These features exist because of competition for mates rather than competition for survival. Darwin defined two types of sexual selection:

  1. Intrasexual Selection: Competition within one sex for access to the opposite sex. This typically results in the evolution of increased body size and weaponry such as teeth, horns, antlers, or ‘‘spurs.’’

  2. Intersexual Selection: Differential choice by members of one sex (usually females) for members of the opposite sex based on an innate preference for certain phenotypes.

• General Outcomes: This often manifests as males competing against each other and females choosing specific males.

Differential Parental Investment (Trivers, 1972)

• Foundational Principle: Robert Trivers (1972) proposed that the relative parental investment (PI) of the sexes in their offspring governs the operation of sexual selection.

• Limiting Resources: The sex that invests the most in an offspring becomes a limited resource. This investment limits the reproductive success of the other sex, driving competition for mates.

• Anisogamy (Different Gamete Sizes):

  • Sexual selection began with the evolution of different gamete sizes.

  • To produce a zygote, a gamete must find another gamete and be able to support new life.

  • Small Gametes (Sperm): Motile and specialized for finding other gametes.

  • Large Gametes (Eggs): Static but contain the resources necessary to support life.

  • Disruptive Selection: Producing intermediate-sized gametes is disadvantageous; selection favors the extremes (maneuverable vs. resource-rich).

• Species Variation: In species where males and females invest equally in offspring, sexual selection has very little influence.

Mammalian Investment Patterns and Reproductive Variance

• Extreme Investment in Mammals: Mammalian females exhibit extreme levels of PI compared to males:

  • Gametes: The ovum is the largest cell in the body; the sperm is the smallest.

  • Fertilization: Occurs internally.

  • Gestation: Typically 9 months (in humans) with high energy costs and mortality risks.

  • Lactation: An even more energy-costly phase following birth.

• Behavioral Incentives:

  • Females are selected to invest in offspring quality (small number of offspring).

  • Males are selected to invest in offspring quantity by seeking more mating opportunities.

• Bateman’s Principle (1948): Male reproductive success (RS) is limited by access to eggs, whereas female RS is limited by her ability to produce eggs. This leads to higher variance in male RS.

• Human Reproductive Statistics:

  • Female RS: Mean = $3.6$, Variance = $3.9$. Maximum recorded offspring: $69$.

  • Male RS: Mean = $3.6$, Variance = $12.1$. Maximum recorded offspring: at least $1,042$ (potentially several thousand).

Case Studies in Sexual Selection and Role Reversal

• Emu (Dromaius novaehollandiae): Displays a reversal of traditional avian roles.

  • Shortening days (rather than longer spring days) trigger reproduction.

  • Male Parental Care: The male spends weeks on the nest without food or drink, then performs sole parenting duties for $1.5$ years.

  • Physical Prowess: Emus can reach speeds of $70\,km/h$.

  • Cultural Context: Aboriginal hunters in Queensland (near Warrego and Bogan rivers) lured inquisitive emus by twirling a ball of feathers and rags from a tree.

• Other Examples:

  • Seahorses: Stallions become "broody" and carry the young.

  • Angler Fish: Extreme sexual dimorphism where the male becomes a permanent parasite on the female.

Kin Selection and Hamilton’s Rule

• Theoretical Basis (Hamilton, 1964): Focuses on the ‘‘gene’s eye view.’’ Individuals increase their inclusive fitness by helping relatives who share their genes.

• Coefficient of Relatedness ($r$): Defined as the proportion of alleles in common due to a shared common ancestor, calculated as $0.5^n$ (where $n$ is the number of reproductive events linking individuals).

  • Parent-Offspring: $r = 1/2$

  • Siblings (Average): $r = 1/2$

  • Grandparents/Grandchildren (Average): $r = 1/4$

  • Uncle/Niece (Average): $r = 1/4$

  • First Cousins (Average): $r = 1/8$

• Hamilton’s Rule: A gene for altruism will evolve when:     $rB > C$     Where:

  • $r$ = Relatedness

  • $B$ = Fitness benefit to the recipient

  • $C$ = Fitness cost to the actor

Research on Kin-Biased Behavior

• Mandrills (Mandrill sphinx) and Parasites (Poirotte & Charpentier, 2020): Research examined whether mandrills avoid contagious individuals (parasitized with protozoa).

  • Findings: Mandrills modulate social avoidance based on kinship. Individuals did NOT avoid grooming close maternal kin even when they had high protozoa richness. However, they significantly decreased grooming for non-kin, low-kin, and paternal half-siblings as parasite richness increased.

• Kin Selection in Action: This model explains the evolution of:

  • Food sharing.

  • Cooperative care of young (alloparenting).

  • Delayed reproduction to care for siblings.

  • Group defense against predators.

  • Cooperative behavior when overlapping generations live together.

Parent-Offspring Conflict Theory (Trivers, 1974)

• Core Conflict: Parents and offspring are not genetically identical ($50\%$ related). A mother aims to maximize inclusive fitness by allocating resources across all current and future offspring. An offspring aims to maximize its own inclusive fitness, valuing itself ($100\%$) more than its siblings ($50\%$—or $25\%$ for half-siblings).

• Investment Thresholds: A mother should move investment when the Benefit/Cost ($B/C$) ratio falls below $1$. A child, however, wants investment to continue until the cost is twice as high ($B/C = 0.5$).

• Preeclampsia (Hollegaard et al., 2013): Pregnancy-induced hypertension (PIH) may be an evolved mechanism where fetal genes increase maternal blood pressure to enhance nutrient supply.

  • Trimester 1 PIH: Correlated with significantly reduced overall later-life disease risks and lower first-year mortality for the offspring.

  • Late-Onset PIH/Preeclampsia: Often reflects an unbalanced conflict detrimental to both maternal and offspring health.

The Grandmother Hypothesis

• The Evolutionary Role of Post-Reproductive Women: Humans are unique in that dependents overlap and children cannot feed themselves at weaning. Grandmothers provide critical assistance:

  • Direct help: Childcare.

  • Indirect help: Collecting and processing ‘‘hard-to-handle’’ foods.

• Hadza Hunter-Gatherers (Hawkes et al.): Post-menopausal women spend more time in food acquisition than younger women, effectively provisioning their daughters and grandchildren.

• Finnish Historical Data (Lahdenper$\ddot{a}$ et al., 2004): The presence of a post-reproductive mother at an offspring's first reproduction resulted in:

  • Increased lifetime fecundity of the offspring.

  • Increased lifetime reproductive success.

  • Shorter inter-birth intervals.

  • Increased grandchild survival to age $15$.

  • Earlier age at first reproduction in children.

Conclusion and Review Concepts

• Sexual selection is driven by differential parental investment and affects behavior across mammalian species.

• Kin selection is a powerful force influencing daily life and social structures.

• Key Terms for Review:

  • Natural selection vs. Kin selection vs. Sexual selection.

  • Hamilton’s Rule ($rB > C$).

  • Genotype vs. Phenotype.

  • Ultimate vs. Proximate explanations.

  • Forces of selection acting on populations.