social behaviour chapter 19 (& 15: p. 275-276). evolution of social behaviour altruism kin...

Social BehaviourSocial BehaviourChapter 19 (& 15: p. 275-276)Chapter 19 (& 15: p. 275-276)

Evolution of Social BehaviourEvolution of Social Behaviour

AltruismAltruism MutualismMutualism

EvolutionKin Selection Cooperation

Reciprocal altruism

Evolution

Prisoner’s Dilemma model

Alarm Calls

Eusociality

Helpers

Evolution

EnvironmentRead Pg. Haplodiploidy

Advantages of Group-livingAdvantages of Group-living

Protection from predatorsProtection from predators

Improved food search/huntingImproved food search/hunting

Easier location of matesEasier location of mates

Physical warmthPhysical warmth

Resource defenseResource defense

Richer learning environmentRicher learning environment

Increased ability to modify environmentIncreased ability to modify environment

Costs of Group-livingCosts of Group-living

Increased competition for resources Increased competition for resources Increased exposure to disease/parasitesIncreased exposure to disease/parasites Increased conspicuousness to predators/ Increased conspicuousness to predators/

preyprey Interference with reproductionInterference with reproduction

Types of GroupsTypes of Groups

Social vs. Non-Social (may be a continuum)Social vs. Non-Social (may be a continuum) Evolution of simple aggregationsEvolution of simple aggregations How might non-social groups have evolved?How might non-social groups have evolved?

Hamilton’s “Selfish Herd” hypothesis (Ch.15)Hamilton’s “Selfish Herd” hypothesis (Ch.15) Complex social behaviours (altruism)Complex social behaviours (altruism)

The Evolution of Complex The Evolution of Complex Social BehavioursSocial Behaviours

Evolution of AltruismEvolution of Altruism Altruism can increase fitness if individuals helped Altruism can increase fitness if individuals helped

are kin (relatives) = are kin (relatives) = kin selectionkin selection Degree of relatedness (r)Degree of relatedness (r)

r is the probability that alleles sampled from 2 r is the probability that alleles sampled from 2 individuals are identical by descentindividuals are identical by descent

Hamilton’s rule: rB>CHamilton’s rule: rB>C Kin Selection TheoryKin Selection Theory

If you help your relatives to survive and If you help your relatives to survive and reproduce, than you are helping to pass on the reproduce, than you are helping to pass on the genes that both you and your relatives have in genes that both you and your relatives have in common.common.

youyou sib

mom dad

Gr M Gr FGr M Gr F

auntuncle

r=1/2

r=1/2 r=1/2

r=1/2r=1/2

r=1/4

r=1/4

uncle

cous

r=1/8

r=1/2

Degrees of relatedness between:Degrees of relatedness between:You and mom, dad, or sib = ½ = 0.5You and mom, dad, or sib = ½ = 0.5You and uncle = ½ * ½ = ¼ = 0.25 You and uncle = ½ * ½ = ¼ = 0.25 You and grandparents = ½ * ½ = ¼ = You and grandparents = ½ * ½ = ¼ = 0.250.25You and cousin = ½ * ½ * ½ = 1/8 = 0.125You and cousin = ½ * ½ * ½ = 1/8 = 0.125

r=1/2

r=1/2

Kin Selection Theory Kin Selection Theory If you help your relatives to survive and If you help your relatives to survive and

reproduce, than you are helping to pass reproduce, than you are helping to pass on the genes that both you and your on the genes that both you and your relatives have in common. relatives have in common.

e.g. Alarm calls, Eusocialitye.g. Alarm calls, Eusociality

The Evolution of Complex The Evolution of Complex Social BehavioursSocial Behaviours

EusocialityEusociality

Eusociality describes social systems with Eusociality describes social systems with 3 key characteristics3 key characteristics::

(1) overlap in generations between parents (1) overlap in generations between parents and offspringand offspring

(2) cooperative brood care(2) cooperative brood care(3) non-reproductive worker caste(3) non-reproductive worker caste

Eusociality in Social InsectsEusociality in Social Insects

Haplodiploidy results in unusual Haplodiploidy results in unusual coefficients of relatednesscoefficients of relatedness (r)(r)

males develop from unfertilized eggs, males develop from unfertilized eggs, haploid haploid do not have fathers, only mothersdo not have fathers, only mothers

females develop from fertilized eggs, are females develop from fertilized eggs, are diploiddiploid Sisters get the same set of chromosomes from Sisters get the same set of chromosomes from

their father, (their father, (daughter to father r = 1daughter to father r = 1)) but have a 50% chance of getting the same but have a 50% chance of getting the same

allele from their mother (allele from their mother (daughter to mother r = daughter to mother r = 1/21/2))

•The unusual genetic system of social insects The unusual genetic system of social insects makes eusociality a likely consequence of kin makes eusociality a likely consequence of kin selectionselection

•In In haplodiploidyhaplodiploidy, sex is determined by , sex is determined by chromosome numberchromosome number

-males develop from unfertilized eggs, are -males develop from unfertilized eggs, are haploidhaploid

-females develop from fertilized eggs, are -females develop from fertilized eggs, are diploiddiploid

•Sons do not have fathers, only mothersSons do not have fathers, only mothers

•Sisters get the same set of chromosomes from Sisters get the same set of chromosomes from their father, but have a 50% chance of getting their father, but have a 50% chance of getting the same allele from their motherthe same allele from their mother

Sex determination in social insectsSex determination in social insects

Sisters are highly related to each other in haplodiploidySisters are highly related to each other in haplodiploidy

sistersister sistersister

mothermother(diploid)(diploid)

fatherfather(haploid)(haploid)

1/21/2 11

sister Asister A sister Bsister B


X


1/21/21/21/2

Path #1Path #1 .. Path #2 Path #2 ..


Odds that one of sister A’s alleles came from mom = Odds that one of sister A’s alleles came from mom = 1/21/2Odds that mom gave the same allele to sister B = 1/2Odds that mom gave the same allele to sister B = 1/2

-odds of identical-by-descent allele Path 1 =-odds of identical-by-descent allele Path 1 =(½) (½)(½) (½) = ¼= ¼


sister Asister A sister Bsister B


X


1/21/2 11




1/21/21/21/2

Path #1 Path #1 .. Path #2Path #2 ..

Sisters are highly related to each other in haplodiploidySisters are highly related to each other in haplodiploidy

Odds that one of sister A’s alleles came from dad = Odds that one of sister A’s alleles came from dad = 1/21/2Odds that dad gave same allele to sister B = 1 Odds that dad gave same allele to sister B = 1

(his complete haploid genome)(his complete haploid genome)

-odds of identical-by-descent allele Path 2 = -odds of identical-by-descent allele Path 2 = (½) (1)(½) (1) = ½ = ½




X


1/21/2 11



X


1/21/21/21/2

Combined odds of sisters sharing identical alleles:Combined odds of sisters sharing identical alleles: (Path #1 odds) + (Path #2 odds) = (1/4) + (1/2) = (Path #1 odds) + (Path #2 odds) = (1/4) + (1/2) = 3/43/4

Because of this system, females are Because of this system, females are moremore related to related to their their sisterssisters ((rr = ¾) = ¾) than they are to their than they are to their ownown offspringoffspring (r = ½)(r = ½)

sistersister brotherbrother


X


1/21/211



X


1/21/21/21/2

1/21/21/21/2


Sisters are only distantly related to their brothers:Sisters are only distantly related to their brothers: (1/2)(1/2) = (1/2)(1/2) = 1/4 1/4 (only one path links sisters and brothers)(only one path links sisters and brothers)

In haplodiploidy, females maximize their inclusive fitnessIn haplodiploidy, females maximize their inclusive fitness by investing in the production of reproductive sisters, whoby investing in the production of reproductive sisters, who are closer relatives than their own offspring or brothers.are closer relatives than their own offspring or brothers.

HaplodiploidyHaplodiploidy females are more related to their sisters (r females are more related to their sisters (r

= ¾) than they are to their own offspring = ¾) than they are to their own offspring (r = ½)(r = ½)

females are only distantly related to their females are only distantly related to their brothers (r = 1/4) [only one path links brothers (r = 1/4) [only one path links brothers and sisters]brothers and sisters]

In haplodiploidy, females maximize In haplodiploidy, females maximize their inclusive fitness by investing in their inclusive fitness by investing in the production of reproductive the production of reproductive sisters, who are closer relatives than sisters, who are closer relatives than their own offspring or brothers.their own offspring or brothers.

Haplodiploidy predisposes Haplodiploidy predisposes Hymenoptera to become eusocialHymenoptera to become eusocial

But…But…haplodiploidy does not haplodiploidy does not causecause eusocialityeusociality

Because…Because… Not all haplodiploid species have Not all haplodiploid species have

sterile castes e.g. honeybeessterile castes e.g. honeybees Some diploid species have sterile Some diploid species have sterile

castes e.g. diploid termitescastes e.g. diploid termites

HoneybeHoneybeee

• Queen mates with up to 20 males Queen mates with up to 20 males

•Queens mate multiple times Queens mate multiple times

•reduces relatedness among sisters –do not share reduces relatedness among sisters –do not share fatherfather

•r no longer significantr no longer significant

• workers able to discriminate between sister workers able to discriminate between sister that are more or less relatedthat are more or less related

Conflict between queen and Conflict between queen and workersworkers

Conflict of interest between queen and non-reproductive Conflict of interest between queen and non-reproductive workersworkersQueen is equally related to sons and daughters (Queen is equally related to sons and daughters (r r = 1/2)= 1/2) -she will favor a 1:1 sex ratio (equal # of daughters and -she will favor a 1:1 sex ratio (equal # of daughters and sons)sons)Workers have Workers have r r = 3/4 with sisters, but only = 3/4 with sisters, but only r r = 1/4 with = 1/4 with brothersbrothers -their fitness will be maximized when the queen -their fitness will be maximized when the queen produces a produces a 3:1 sex ratio (more daughters than sons)3:1 sex ratio (more daughters than sons)Who wins the conflict?Who wins the conflict? -in one species of ant, the queen laid eggs in a 1:1 ratio, -in one species of ant, the queen laid eggs in a 1:1 ratio, but atbut at hatching the sex ratio was biased towards many more hatching the sex ratio was biased towards many more femalesfemales-workers selectively destroyed male larvae-workers selectively destroyed male larvae-assert their own reproductive agenda over the queen’s-assert their own reproductive agenda over the queen’s

Bumble BeeBumble Bee

•Queen controls sex of egg Queen controls sex of egg (fertilize or not)(fertilize or not)•Workers control sex byWorkers control sex by

• provisioning provisioning •lay male eggslay male eggs

Eusociality without Eusociality without HaplodiploidyHaplodiploidy

The naked mole rat - 2 castes:The naked mole rat - 2 castes: ““workers” Non-reproductive adults: workers” Non-reproductive adults:

dig tunnels, find fooddig tunnels, find food ““non-workers” Reproductive female non-workers” Reproductive female

(queen) and several reproductive males (queen) and several reproductive males (breed, keep young warm)(breed, keep young warm)

Native to Africa, droughts common, live in Native to Africa, droughts common, live in underground tunnels and eat tubers, can underground tunnels and eat tubers, can only dig when wet, need many individuals only dig when wet, need many individuals to dig to find enough food – ECOLOGY to dig to find enough food – ECOLOGY promotes eusociality (Fig. 19.18)promotes eusociality (Fig. 19.18)

Helpers at the NestHelpers at the Nest

In some bird species (e.g., Florida scrub In some bird species (e.g., Florida scrub jays, pied kingfishers), offspring from jays, pied kingfishers), offspring from previous years help their parents – feed previous years help their parents – feed and protect younger siblings instead of and protect younger siblings instead of reproducingreproducing

Beneficial: Number of young fledged drops Beneficial: Number of young fledged drops if helpers removedif helpers removed

Altruism through kin selection?Altruism through kin selection? Or, beneficial to individual (selfish)?Or, beneficial to individual (selfish)?

Helping at the NestHelping at the Nest

Why help vs. have your own offspring?Why help vs. have your own offspring? Habitat “saturated” with breeders (no room)Habitat “saturated” with breeders (no room) May be better to wait for a high quality May be better to wait for a high quality

territory (inherit it), than leave for a low territory (inherit it), than leave for a low quality onequality one

May not be able to leave group location (e.g., May not be able to leave group location (e.g., limited food resources elsewhere)limited food resources elsewhere)

No mates availableNo mates available Life history characteristics: Small clutch size Life history characteristics: Small clutch size

and low adult mortalityand low adult mortality

How Do Helpers Benefit?How Do Helpers Benefit?

Increase inclusive fitness if helping kinIncrease inclusive fitness if helping kin Enhance likelihood of future breeding (gain Enhance likelihood of future breeding (gain

mate if primary male dies - unrelated males)mate if primary male dies - unrelated males) May increase own survivorship (access to May increase own survivorship (access to

resources, lower risk of predation in a group)resources, lower risk of predation in a group) May gain useful reproductive experience (care May gain useful reproductive experience (care

of young) & may be reciprocated in futureof young) & may be reciprocated in future So, kin selection may explain helping So, kin selection may explain helping

through increased inclusive fitness, but through increased inclusive fitness, but many other factorsmany other factors

Evolution of Human Social Evolution of Human Social SystemsSystems

Read Pgs. 348 – 349Read Pgs. 348 – 349 Possible evidence for kin selectionPossible evidence for kin selection

Cooperation in Non-KinCooperation in Non-Kin Types:Types: Reciprocal altruism (reciprocity)Reciprocal altruism (reciprocity)

Recipient benefits, donor’s fitness Recipient benefits, donor’s fitness decreased; later, roles reverseddecreased; later, roles reversed

e.g., vampire bats e.g., vampire bats Model: Prisoner’s DilemmaModel: Prisoner’s Dilemma

Mutualism (and/or symbiosis)Mutualism (and/or symbiosis) Mutually beneficialMutually beneficial e.g., symbiotic fishe.g., symbiotic fish

•33% of young bats (<2 yrs) fail to get blood 33% of young bats (<2 yrs) fail to get blood on any particular evening versus just 7% of on any particular evening versus just 7% of adults adults

•chronic threat of starvation among vampire chronic threat of starvation among vampire bats can only survive 3 days without a mealbats can only survive 3 days without a meal

•successful bats regurgitate part of their blood successful bats regurgitate part of their blood meal for group members that were not meal for group members that were not successful – but they do not do this randomly, successful – but they do not do this randomly, they only give to those from whom they have they only give to those from whom they have received blood in the past received blood in the past

Reciprocal Altruism(Wilkinson, G.W. (1984) Reciprocal food sharing in the vampire bat. Nature. 308:181-184)

Altruistic acts dispensed primarily to Altruistic acts dispensed primarily to relatives and frequent roostmatesrelatives and frequent roostmates

•Bats more likely to regurgitate blood meals Bats more likely to regurgitate blood meals to other bats they frequently roost withto other bats they frequently roost with

•Bats more likely to regurgitate blood meals to Bats more likely to regurgitate blood meals to close relativesclose relatives

Benefits of act to recipient (R) exceed cost of act to donor (D)

Why is this altruistic Why is this altruistic behaviour favored?behaviour favored?

PrimatesPrimates

Alliances in primates seen in:Alliances in primates seen in: Grooming behaviourGrooming behaviour Fighting behaviourFighting behaviour

most grooming and fighting alliances most grooming and fighting alliances between close relatives but not alwaysbetween close relatives but not always

•Kin intervene more, both to help if kin are Kin intervene more, both to help if kin are the recipient or if kin are the the aggressors. the recipient or if kin are the the aggressors. •Grooming rates highest between kin.Grooming rates highest between kin.

Rhesus macaques:Rhesus macaques:

Japanese macaques:Japanese macaques:•Agonistic aid was 81% between kin, primarily Agonistic aid was 81% between kin, primarily mothers and grandmothers. mothers and grandmothers. •Kin spend more time together (in proximity) than Kin spend more time together (in proximity) than expected by chance. expected by chance. •Grooming higher between kin than non-kin. Grooming higher between kin than non-kin. •Severe aggression only occurred between non-kin Severe aggression only occurred between non-kin (19 incidents). (Kurland)(19 incidents). (Kurland)

Grooming and food sharing in Grooming and food sharing in chimpanzeeschimpanzees

•Chimpanzees in captive colony more Chimpanzees in captive colony more likely to share food with individuals who likely to share food with individuals who had groomed them in previous 2 hourshad groomed them in previous 2 hours•Resisted approaches by individuals who Resisted approaches by individuals who had not groomed them had not groomed them •Reciprocity depends on history of Reciprocity depends on history of interactions between two individualsinteractions between two individuals

Reciprocal Altruism Reciprocal Altruism

• Kin selection cannot account for Kin selection cannot account for cooperation in non-kin…cooperation in non-kin…

• How could RA in non-kin have evolved?How could RA in non-kin have evolved?• A “cheater” could beg blood, then A “cheater” could beg blood, then

refuse to return the favour = donor refuse to return the favour = donor may die (unless it finds another donor), may die (unless it finds another donor), cheater livescheater lives

• Why don’t individuals evolve to act Why don’t individuals evolve to act selfishly?selfishly?

Reciprocal Altruism (RA)Reciprocal Altruism (RA)

Axelrod & Hamilton (1981), others…Axelrod & Hamilton (1981), others…• ““Game theory” – two players interact Game theory” – two players interact

with goal of maximum individual gainswith goal of maximum individual gains• Model for evolution of reciprocal Model for evolution of reciprocal

altruism = “Prisoner’s Dilemma”altruism = “Prisoner’s Dilemma”• ““Tit-for-tat” strategyTit-for-tat” strategy

READ 338 -339READ 338 -339

Evolution of Reciprocal Evolution of Reciprocal AltruismAltruism

Requires these conditions:Requires these conditions: Longer-lived animals, such that future Longer-lived animals, such that future

opportunity for repayment likely opportunity for repayment likely ((multiple encountersmultiple encounters))

Altruist and recipient must be able to Altruist and recipient must be able to recognizerecognize one another; identify and one another; identify and refrain from helping cheatersrefrain from helping cheaters

Benefit to recipient greater than cost Benefit to recipient greater than cost to altruist (but both individuals benefit to altruist (but both individuals benefit in the long run)in the long run)

Mutualism (a form of Mutualism (a form of symbiosissymbiosis))

Cooperation between 2 differentCooperation between 2 different speciesspecies Both benefit, neither harmed, Both benefit, neither harmed,

therefore, not considered altruism therefore, not considered altruism (fitness of both organisms is (fitness of both organisms is increased)increased)

Clownfish (genus Amphiprion) dwell among Clownfish (genus Amphiprion) dwell among the tentacles of tropical sea anemones. the tentacles of tropical sea anemones.

Protects anemone from anemone-eating fishProtects anemone from anemone-eating fish In turn, stinging tentacles of anemone In turn, stinging tentacles of anemone

protect fish from its predators (a special protect fish from its predators (a special mucus on fish protects it from getting mucus on fish protects it from getting stung).stung).

Honeypot ants feed and care for aphids, Honeypot ants feed and care for aphids, “milk” them for their honeydew secretions “milk” them for their honeydew secretions (by stroking them with antennae)(by stroking them with antennae)

Ants protect aphids, aphids feed antsAnts protect aphids, aphids feed ants Mutualism: Mutualism:

Goby Fish + ShrimpGoby Fish + Shrimp Plover + CrocodilePlover + Crocodile

social behaviour chapter 19 (& 15: p. 275-276). evolution of social behaviour altruism kin...

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