Anthropology 201 Fall 2024 Evolution Of Social Behaviour PDF

13. Evolution of Social Behaviour Anthropology 201 Fall 2024  Objectives Define altruism Understand why ‘group selection’ doesn’t exist – Why it can’t explain the evolution of altruism Understand kin selection – Hamilton’s Rule – Kin biases in beha...

13. Evolution of Social Behaviour Anthropology 201 Fall 2024  Objectives Define altruism Understand why ‘group selection’ doesn’t exist – Why it can’t explain the evolution of altruism Understand kin selection – Hamilton’s Rule – Kin biases in behaviour Understand the conditions for the evolution of reciprocal altruism Reading: Chapter 7 2  The story so far… We have explained evolution of morphology and behaviour in terms of individual reproductive success – e.g. primate reproductive strategies But primates live in groups, interact socially, and often perform altruistic behaviours Altruism: “selfless concern for the welfare of others”, i.e. an act that benefits others at one’s own cost 3  Example: Grooming Virtually all social primates groom other group members Beneficial for recipient à hygiene, relaxation Costly for giver à time consumed away from other tasks like foraging, courting mates 4  Altruism Other examples: – Alarm calls – Coalitions in conflicts – Food sharing – Alloparenting How do we explain “selfless” acts in terms of fitness? – They decrease fitness of the giver by definition – Should be eliminated by natural selection 5 If natural selection favours individually advantageous traits, how can it explain the evolution of altruism? 6  Social Interactions Most primates are highly social Sociality produces high degree of contact among individuals with varied relationships Tempting to think of mutualistic behaviours as benefiting the group as a whole – But ‘group selection’ generally not supported (natural selection acts on individuals) Need theoretical framework for understanding how individuals act in their own (adaptive) interest in a social context – Fields of Sociobiology, Behavioural Ecology, and for specific research areas, Evolutionary Psychology 7  Social Interactions Often analyzed in terms of costs & benefits to the actor and/or recipient Ultimate goal would be to apply costs & benefits to fitness, but this is difficult for each individual behavioural interaction Can classify interactions as: – Selfish - benefits the actor and costly to recipient – Altruistic (opposite) - benefits recipient and costly to the actor – Mutualistic - beneficial to both – Spiteful (opposite) - costly to both 8  Group Selection Once popular mechanism to explain the evolution of selflessness and altruism Applies Darwin’s postulates to groups, not individuals – Competition among groups – Groups vary in ways that affect their survival – Some of this variation is heritable Those behaviours that benefit the group as a whole should increase in frequency? 9  Example: Alarm Calling If one monkey gives an alarm call, other group members benefit If every individual gave the call, all members would be better off than if no call given Since every individual benefits, alarm calling should be favoured by NS? 10  But selection acts on individuals That reasoning confuses effect on group with effect on individual It doesn’t matter to selection that other members benefit, all that matters is the effect on the caller 11  Example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white) Compare fitness of the two Alarm call benefits all recipients to the same extent (+) Relative fitness of callers and non-callers is the same – so relative frequency of callers would not change by NS In fact, caller bears a cost (-), so non-callers have higher fitness 12  Example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white) Non-callers are favoured Relative fitness of callers and non- callers is still the same – all slightly reduced (-) Here however, non-caller does not bear a cost May even have an advantage, i.e. non-callers still have higher fitness NS should favour non-callers even though the group as a whole benefits 13  An impasse? Natural selection favours individually beneficial traits Group selection cannot explain evolution of altruistic behaviours Q: How does altruism evolve? 14  An answer: Kin Selection 1964, W.D.Hamilton Our alarm call example assumes altruists and non-altruists interact with equal frequency Hamilton’s insight: what if some factor causes altruists to associate selectively with other altruists? – e.g., what if the group’s members are related? – Relatedness may facilitate evolution of altruism – What you do can benefit your kin at your expense 15  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings Frequencies of the calling/non- calling genes don’t change, but distribution does In the caller’s group, 5 of 8 siblings have the calling gene (4/8 because of inheritance*, + 1 of the 4 remaining (= population frequency) * For any pair of siblings, 50% chance they share a given allele 16  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings 5/8 > ¼ = frequency of gene for calling is higher in the group than in population at large If call is given, more callers benefit than non-callers Caller raises the average fitness of callers relative to non-callers in that group compared with population at large 17  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings For a non-caller, 7/8 siblings share that gene Only 1 caller in group, less than in population at large Not calling lowers the fitness of non-callers relative to callers in this group compared with population at large 18  Kin Selection When individuals interact selectively with relatives, callers (altruists) more likely to benefit than non-callers (non-altruists) Benefits of calling will favour genes for calling BUT! Calling costly, will be favoured only if benefits sufficiently greater than costs When is this trade-off satisfied? 19  Hamilton’s Rule An altruistic act will be favoured by selection when the following inequality is satisfied: rb > c Where b = sum of fitness benefits to all recipients, c = cost to giver, r = coefficient of relatedness between them 20  Coefficient of relatedness (r) Measures genetic relationship between interacting individuals, or the average probability that they share an allele from a common ancestor Probability that A & B share an allele at a given locus, vs. B & C? Probability that A and B get the same allele from their shared mother = 0.5 * 0.5 = 0.25 Probability that B and C get the A and B are half-sibs, B and C full sibs same allele from their shared mother OR FATHER = 0.25 + 0.25 = 0.5 21 Coefficient of relatedness Relationship r Parent and offspring 0.5 Full siblings 0.5 Half siblings 0.25 First cousins (from full sibs) 0.125 Unrelated individuals 0 r decreases as you become more distantly related to kin 22  Kin Selection Hamilton’s rule (rb > c) has two important predictions: – 1) altruism should be directed towards kin (because r = 0 for unrelated individuals) – 2) closer genetic relatedness allows for more costly altruism e.g. if r = 0.5, then b must be at least (2 x c) to satisfy Hamilton’s rule If r = 0.125, then b must be at least (8 x c) 23  Kin Recognition For kin selection to work, primates must be able to recognize kin – Phenotypic matching = smell or likeness to self – Contextual cues = familiarity, proximity, observe patterns of associations 24  Kin Recognition Easier to recognize maternal kin: using contextual clues, can identify your siblings as those who spend time with mom too Paternal kin: harder to identify, but age may provide clues in polygynous species à age- matched peers likely fathered by same male 25  Kin biases in altruistic behaviours Examples of altruism in primates that provide support for kin selection theory 1. grooming Costly: time-consuming, decreased vigilance Beneficial: hygiene, reinforce bonds 26 1. Grooming more common among kin Rhesus macaques on Cayo Santiago (Puerto Rico) Females groom kin more than non-kin (Hamilton’s 1st prediction) Females groom closer kin more than more distant kin (Hamilton’s 2nd prediction 27  2. Coalitions Primate disputes often between two individuals Sometimes, another individual may come to the support of one of them = coalition – Beneficial to individual who receives aid (may win the dispute or avoid injury) – Costly to the ally (time, energy, may get injured) 28 2. Support directed towards kin 29  2. Male coalitions Males do cooperate, even though they also compete for mates Coalitions last longer and are more intense when males are related (e.g. brothers or half-brothers) Examples: – Coalitions to take over groups – Mutual defense of territory Cooperative breeding (polyandry) is an extreme example where some males suffer big declines in RS – But offset by supporting offspring of close male relatives30  Parent–Offspring Conflict Kin selection helps explain why there is conflict between parents and offspring – Offspring shares genes with siblings, but only 0.5 or 0.25 Fitness of future offspring comes at expense of current offspring Mother wants to invest in future offspring Current offspring want mother to continue to invest in them! 31  What about interactions with non-kin? Reciprocal Altruism = altruism can evolve even among non-kin if the behaviour is balanced between partners over time – Take turns giving and receiving Requirements: – 1. Must have opportunities to interact often – 2. Must be able to keep track of support given & received – 3. Must provide support only to those who help (satisfies the selective association condition for altruism to evolve) Reciprocity, not kinship, drives this type of altruism “Tit for tat” – same or different currency can be used in exchange (e.g. grooming for meat sharing) 32 Example: Coalition recruitment in vervets Tape-recorded recruitment calls in pairs with or without prior grooming Rates similar for kin Non-kin more likely to respond if recently groomed by caller Note difference in exchange currency! 33 Example: Food sharing in chimpanzees Chimps will sometimes share food Most successful at getting food if you recently groomed the possessor 34  Take-home Message Altruism can evolve through natural selection (not group selection), so long as altruists are more likely to associate with other altruists (whether kin or unrelated) = nonrandom social interactions Will increase Inclusive Fitness Relative genetic contribution of the individual plus the contribution of close relatives 35