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Chapter 12

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University of Waterloo
BIOL 359
Jonathan Witt

Chapter 12 – Kin Selection and Social Behavior - When and why do individuals cooperate and why do they help their parents raise their siblings instead of leaving home to rear their own offspring? - Interaction between individuals has 4 possible outcomes o Mutualism: Cooperation, fitness gain for both participants o Altruism: Individual instigating the action pays a fitness cost & individual on the receiving end benefits (Sacrifice on behalf of another, common) o Selfishness: Opposite of altruism, actor gains and the recipient loses o Spite: Fitness losses for both participants (Rare  quickly eliminated by natural selection) 12.1. Kin Selection and the Evolution of Altruism - Altruism is the central paradox of Darwinism (Challenge for the Theory of Evolution) o Darwin’s explanation: Selection would favor traits that result in decreased personal fitness if they increase the survival and reproductive success of close relatives - Inclusive Fitness o Coefficient of relatedness (r): The probability that the homologous alleles in two individuals “actor & recipient” are identical by descent (Closely related to F, the coefficient of inbreeding) o Hamilton’s rule: An allele for altruistic behavior will spread if Br-C>0  B: The benefit to the recipient, measured in units of surviving offspring  C: The cost to the actor, measured in units of surviving offspring o Altruism is more likely to spread when the benefits to the recipient are great, the cost to the actor is low, and the participants are closely related o Individual’s fitness partitioned into 2 components  Direct fitness: Personal reproduction  Indirect fitness: Additional reproduction by relatives that is made possible by an individual’s actions (reproductive success of relative is above and beyond what they would have achieved on their own) o Kin selection: Natural selection favoring the spread of alleles that increase the indirect component of fitness - Alarm Calling in Belding’s Ground Squirrels and Prairie Dogs o Prey that notice the intruder sometimes give loud, high-pitched calls to alert nearby individuals and allow them to flee or dive for cover, but this may also expose the caller to danger o Q1: Are alarm calls genuinely altruistic?  Type I: Attacking hawk, whistling squirrel is captured 2% of the time while non-whistling squirrels are captured 28% of the time (Whistling = selfish)  Type II: Attacking mammals, trilling squirrel is killed 8% of the time while non-trilling squirrels are killed 4% of the time (Trilling = altruistic) o Black-tailed prairie dogs, lives in family groups “coteries”, each occupy a territory within a prairie dog town. Female remain in their birth coterie for life, whereas males disperse when sexually mature  Both male & female prairie dogs are more likely to give alarm calls if their coterie includes genetic kin ( Rate of calling changes according to their proximity to kin) o Alarm-calling behavior is not dispensed randomly, but is nepotistic (self-sacrifice  indirect fitness) - White-Fronted Bee-Eaters o Young that are old enough to breed on their own will instead remain & help their parents rear brothers, sisters, or half-siblings  Common where breeding opportunities are extremely restricted (e.g. saturated habitats) o They breed in nesting chambers excavated in sandy riverbanks (40-450 per colony, subdivided in groups of 3-17, each defends a feeding territory up to 7km away & each may include several sets of parents & offspring) o The coefficient of relatedness with recipient has a strong effect on whether a nonbreeding member of the clan helps (Non-breeders help the most closely related individuals available)  Significant help to the parents, each help = additional 0.47 offspring reared to maturity - Kin Selection in Other Contexts o Cannibalistic Tadpoles:  Tadpoles of spadefoot toads (Spea bombifrons)  2 morphs  Omnivorous: Typical morph, feeds on decaying plant matter  Cannibalistic: Alternative morph, enlarged jaw muscles for big prey, including other spadefoot toad tadpoles  Experiment: 28 cannibalistic tadpoles w/ separate containers, container contains 1 kin & 1 non-kin spadefoot toad tadpoles o Only 6/28 cannibals ate their siblings  Discriminating behavior  Tiger salamanders (2 morphs - typical & cannibal tadpole)  18 cannibal tadpole, separate cages, each shared the cage with 6 kin & 18 non-kin  Some cannibalistic tadpoles discriminated between kin vs. non-kin, others didn’t  Degree of relatedness (r) = ½ o Siblings of discriminating cannibals were 2X as likely to survive as siblings of non-discriminating cannibals  Not eating of siblings is favored by kin selection in tiger salamander tadpoles o Altruistic Sperm  Female wood mice can mate with more than one male in short succession, the sperm from 2 males are in a race to the egg  Male wood mice have large testes  sperm competition  Wood mice sperm: Hooks on their head, can hold on to other sperms by their heads or th th flagella  Train formation (10 to 1000 of individual cells)  Sperms in a train can move 2x as fast as sperms swimming alone  But train must break up before fertilizing an egg, many sperm undergoes acrosome reaction & sacrifices themselves, given that sibling sperms share ½ of their alleles  Clever Coots  Defense against parasitized altruism in aquatic bird – American coot  Conspecific Nest Parasitism: Female coots increase own reproductive success by laying eggs in other female’s nests o Foster parent loses one of its own biological offspring  H: Since coefficient of related ness is 0, coots should have evolved defenses against parasitism o Coots can distinguish own eggs from parasitized egg based on appearance o Both rejecters & acceptors laid a total of 8 eggs, coots can count Box 12.1. Calculating Coefficients of Relatedness - Half-sibling: Parents contribute ½ to each offspring o Probability that a particular allele is transmitted to actor is ½ & to recipient is also ½ o Probability of relatedness = ½ x ½ = 1/4 - Full-sibling: Actor to actor’s mother is ½, actor’s mother to recipient is ½, actor to actor’s father is ½ & actor’s father to recipient is ½ o ¼ + ¼ = ½ - Cousins: Actor’s parent to actor is ½, Actor’s parent to Actor’s aunt/uncle is ½, Actor’s aunt/uncle to recipient is ½ o ½ x ½ x ½ = 1/8 12.2. Evolution of Eusociality - Epitome of altruism: Social insects that do not reproduce at all, but are helpers at the nests of their parents - Eusociality “True sociality”: Social systems with 3 characteristics o Overlap in generations between parents and their offspring o Cooperative brood care o Specialized casts of non-reproductive individuals - Haplodiploidy and Eusocial Hymenoptera o Hymenoptera: A single ant colony, millions of individuals that are not genetically identical o Haplodiploid species: Female are more closely related to their sisters than they are to their own offspring  For hymenoptera, males are haploid and females are diploid, males develop from unfertilized eggs & females develop from fertilized eggs  Sisters share all genes they inherited from their haploid father (1) & half of the genes inherited from their diploid mother “the colony’s queen” (1/2)  R of sisters = 1 x ½ + ½ x ½ = ¾  R of brothers = ½ x ½ = ¼  R of daughters = ½  Hence this system favors the production of sisters over daughters, sons or brothers  Females can maximize their inclusive fitness by acting as workers rather than reproducers (best to invest in the production of sisters rather than their own offspring) o Testing the Haplodiploidy Hypothesis  H: Workers prefer to invest in sisters rather than their own of
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