EE 11- Sexual Selection and Kin Selection: Ecological and Evolutionary Perspectives

Fundamentals of Sexual Selection

• Sexual Selection Definition: A form of natural selection where traits increase reproductive success rather than survival. It explains why some individuals (primarily males) develop extravagant, costly traits.

• Sexual Dimorphism: Phenotypic differences (e.g., color, size, weapons) between males and females of the same species beyond primary genitalia. Examples include Stag beetle, Pea fowl, Mandrill, and Betta fish.

• Parental Investment (Trivers' Theory): Sexual selection stems from unequal investment. Typically, females invest more in offspring, and males produce cheaper sperm.

• Bateman’s Principle: Variance in reproductive success is generally higher in males than in females.

Male and Female Reproductive Strategies

• Male Strategies: Maximize copulations through male-male competition (intrasexual selection), resource provision, direct contest competition, and sperm competition.

• Female Strategies: Focused on offspring investment and mate choice (intersexual selection). Females choose based on:    

•  * Direct Benefits: Resources or investment in offspring.    

•  * Indirect (Genetic) Benefits: "Good genes" or producing attractive sons.

• Zahavi’s Handicap Hypothesis: Secondary sexual traits are honest indicators of quality because they are costly; low-quality males cannot afford the "handicap."

• Fisher’s Runaway Selection Hypothesis: A feedback loop where a male trait and female preference become genetically correlated through non-random assortative mating, leading to exaggerated traits until halted by viability selection.

• Case Study: Jackson’s widowbirds:     * Breeding males grow a glossy black tail (~$20\,cm$) for courtship.    

•  * Andersson (1992) found that males with longer tails ($18\,cm$ controls vs. $14\,cm$ treatments) received significantly more copulations.     

• * Longer tails correlated with better body condition, suggesting an honest signal of "good genes."

Altruism and Reciprocal Altruism

• Altruism: Behavior that is harmful to the actor's immediate fitness but beneficial to the recipient.

• Reciprocal Altruism in Vampire Bats (Wilkinson, 1984):     * Bats share blood meals with close relatives or frequent roost mates.    

•  * Conditions: Donors must recognize and punish cheaters, have repeated interactions, and the benefit to the recipient ($B$) must outweigh the cost to the donor ($C$).     * Weight loss in bats follows a negative exponential decline; a small donation from a well-fed bat significantly extends the life of a starving one.

Kin Selection and Hamilton’s Rule

• Hamilton’s Rule: Altruism is favored when rB > C.     * $r$ = coefficient of relatedness (shared genes).     * $B$ = benefit to the recipient (extra offspring).     * $C$ = cost to the actor (lost offspring).

• Inclusive Fitness: The sum of direct fitness (personal reproduction) and indirect fitness (helping relatives).

• Calculations for Diploids:   

•   * Parent-offspring: $r = 0.5$   

•   * Full siblings: $r = 0.5$    

•  * Grandparent-grandchild: $r = 0.25$   

•   * Half-siblings: $r = 0.25$   

•   * Cousins: $r = 0.125$

• Example Case: A squirrel warning $3$ sisters ($r = 0.5$) at a cost of $0.3$ offspring, where each sister gains $0.4$ survival probability.

•     * Benefit $B = 3 \times 0.4 = 1.2$  

•    * Equation: $0.5 \times 1.2 = 0.6$   

•   * Since 0.6 > 0.3, the behavior is favored.

Alarm Calling in Belding's Ground Squirrels

• Paul Sherman (1985) study on Spermophilus beldingi:    

•  * Whistle (Hawks): Selfish; whistling squirrels are captured $2\%$ of the time compared to $28\%$ for non-whistlers.    

•  * Trill (Mammals): Altruistic; trilling squirrels are killed $8\%$ of the time compared to $4\%$ for non-trillers.

• Kinship Link: Females remain in natal burrows while males disperse. Consequently, females are more likely to perform altruistic alarm calls because they are surrounded by close relatives.

Eusociality and Haplodiploidy

• Eusociality: Social groups with non-reproductive members, common in Hymenopterans (bees, ants, wasps).

• Haplodiploidy: Females are diploid ($2n$) and males are haploid ($n$).     * Full sisters share, on average, $75\%$ ($r = 0.75$) of their alleles, making it more beneficial to raise sisters than their own offspring ($r = 0.5$).

• Ecological Factors: Eusociality is also driven by extreme environmental constraints, such as the construction of elaborate, resource-heavy nests or burrows (e.g., naked mole rats).

detailed

• Sexual Selection Definition: A specialized form of natural selection that operates on certain traits that enhance reproductive success rather than merely survival. It accounts for the evolution of features such as bright plumage, elaborate courtship behaviors, and even combativeness, predominantly seen in males, which can attract mates or deter rivals. Sexual selection helps explain phenomena like why peacocks have extravagant tails and why certain males garner more mating opportunities than others.

• Sexual Dimorphism: Refers to the distinct phenotypic differences between male and female individuals of the same species, particularly beyond the primary sexual characteristics. This may manifest in various ways such as size, color, and morphology. Examples include:
    - Stag Beetle: Males possess larger mandibles compared to females, played a role in combat.
    - Pea Fowl: Males display vibrant feathers to attract females, while females are more camouflaged.
    - Mandrill: Exhibits pronounced color differences, where males display brighter colors compared to females.
    - Betta Fish: Males often have longer fins and more vivid coloration than females, influences mating success.

• Parental Investment (Trivers' Theory): This concept posits that different levels of parental investment between genders lead to different reproductive strategies. Typically, females invest significantly more in offspring via gestation and nursing, while males produce minimal investment through sperm, leading to competition among males for female attention. The greater the investment from one gender, the more selective they tend to be about mates.

• Bateman’s Principle: Suggests that variance in reproductive success tends to be higher among males because they can increase their reproductive success by mating with multiple females, while females' success is limited by their biological capacity to produce and nurture offspring.

Male and Female Reproductive Strategies

• Male Strategies: Males generally maximize reproductive success through various strategies, including but not limited to:
    - Intrasexual Selection: Combat between males, where dominance and physical prowess determine mating success.   - Resource Provisioning: Males may offer resources (food, territory) to entice females.   - Sperm Competition: Males compete post-copulation, ensuring that their sperm successfully fertilizes eggs.

• Female Strategies: Typically focused on their reproductive investment and mate selection, such as:
    - Direct Benefits: Females assess males based on tangible resources paid (like food or protection).   - Indirect (Genetic) Benefits: Preference for males who can provide “good genes” or superior genetic material, enhancing the fitness of offspring.   

• Zahavi’s Handicap Hypothesis: Proposes that secondary sexual traits, which are often extravagant and costly, serve as honest indicators of a male's genetic quality; lower quality males cannot afford to maintain or develop such traits.

• Fisher’s Runaway Selection Hypothesis: Describes a process where male traits and female preferences become strongly genetically correlated through non-random mating. This drive can result in extreme exaggeration of traits, maintained until viability selection intervenes, preventing excessively detrimental characteristics.

• Case Study: Jackson’s Widowbirds:
    - Breeding males grow elongated glossy black tails (~$20 \, cm$) that play a crucial role in courtship displays.
    - A study by Andersson (1992) showed that males with longer tails receive a higher number of copulations (average of $18 \, cm$ tails vs. $14 \, cm$), correlated with better body condition, thus acting as a reliable signal of mate quality.

Altruism and Reciprocal Altruism

• Altruism: A behavior that reduces the actor's reproductive success while benefiting the recipient. Altruistic behaviors can be perplexing in terms of Darwinian evolution, as they seem to contradict the self-preservation principle of natural selection.

• Reciprocal Altruism in Vampire Bats (Wilkinson, 1984): Vampire bats exhibit altruism by sharing blood meals with kin or regular roost mates. The essential conditions for such behavior to evolve include recognition of individuals, repeated interactions, and the benefit to the recipient ($B$) being greater than the cost to the actor ($C$). A well-fed bat’s small donation can significantly enhance a starving bat's chance of survival.

Kin Selection and Hamilton’s Rule

• Hamilton’s Rule: A principle stating that genetic altruism is favored when the formula rB > C is satisfied, where:
    - $r$ = coefficient of relatedness (the fraction of genes shared),
    - $B$ = benefit to the recipient in terms of additional offspring,
    - $C$ = cost incurred by the actor in lost offspring.

• Inclusive Fitness: The overall fitness of an individual, considering both direct reproduction and any contributions made to the reproductive success of relatives.

• Calculations for Diploids: Calculations for genetic relatedness are as follows:
    - Parent-offspring: $r = 0.5$
    - Full siblings: $r = 0.5$
    - Grandparent-grandchild: $r = 0.25$
    - Half-siblings: $r = 0.25$
    - Cousins: $r = 0.125$

• Example Case: A warning behavior seen in squirrels emerging from the study. A squirrel alerts three sisters ($r = 0.5$) at the cost of losing $0.3$ offspring through vigilance, where each sister receives a benefit of increased survival probability of $0.4$.
    - Benefit: $B = 3 imes 0.4 = 1.2$
    - Hamilton's equation yields: $0.5 imes 1.2 = 0.6$, which means since 0.6 > 0.3, the altruistic behavior is evolutionarily favored.

Alarm Calling in Belding's Ground Squirrels

• Paul Sherman (1985) study on Spermophilus beldingi highlights two distinct alarm calls and their implications:
    - Whistle (Hawks): Performed by selfish squirrels, which are captured 2 ext{%} of the time, whereas non-whistling squirrels face a higher capture risk of 28 ext{%}.
    - Trill (Mammals): An altruistic call made to warn others of approaching mammals, resulting in trilling squirrels being killed 8 ext{%} of the time compared to only 4 ext{%} for those not calling.

• Kinship Link: The behavior is explained by the fact that female squirrels tend to remain in their natal burrows, leading to greater chances of altruistic behavior due to proximity to relatives, thus supporting kin selection dynamics.

Eusociality and Haplodiploidy

• Eusociality: A complex social structure characterized by cooperative brood care, overlapping generations, and non-reproductive individuals who assist in the rearing of offspring, notably present in groups like bees, ants, and wasps.

• Haplodiploidy: A unique reproductive system where females are diploid ($2n$) and males are haploid ($n$). This genetic structure influences social behavior because full sisters share, on average, 75 ext{%} of their genes ($r = 0.75$), thus favoring the raising of sisters over individual offspring ($r = 0.5$).

• Ecological Factors: Eusocial behavior is also driven by environmental constraints, for example, species like naked mole-rats construct complex, resource-intensive burrows that necessitate cooperative living for survival and resource acquisition.