13. Altruism and sociality

Primitive animals are all the same. There is no individualistic behaviour.

Higher animals evolved individualism. The highest birds and mammals evolved individualistic characters (moods), motions and fears.

Classical population genetic does not predict individualism because it focuses on optimisation and equilibrium states that are the same for all members of a population.

Evolutionary theory has to explain:• Altruism (the help of others despite of own costs)• Cooperation of related and unrelated individuals• The evolution of cheating• Sexual selection (the existence of differentiated sexual behaviour and mating rituals)• Biased sex ratios (the prevalence of either males or females in a population)• The existence of highly altruistic insect societies (eusociality)• The existence of infanticide in many mammals and birds• The existence of homosexuality in many mammals and birds• The appearance of common beliefs and religion in man

C. Richard Dawkins (1941-

The unit of selection and evolution

Nucleotid

Genome

Gene

Cell

Organelle

Species

Population

Individual

Species

Population

Higher taxonomic level

Family

Group

Higher taxonomic level

Unicellular organisms Multicellular organisms

Classical population genetics (Fisher, Haldane, Sewall Wright)

Wynne Edwards (1962)to explain cooperation

The basic unit is the gene as the smallest essential carrier of information

A more liberal view sees any trait inducing carrier of information as a potential unit of evolution. These include genes, individuals, and even groups but not species.

John F. Nash (1928-

The game theory approach

The classical hawk and dove game

½(B-C)

0 ½B

B

Dove

Hawk

Hawk Dove

John Maynard Smith (1920-2004)

The pay-off matrix

Hawk v. Hawk: Each contest has a 50%

chance to win. The net gain is the difference

between benefits and costs of the contest

Dove v. Hawk: The dove will always loose

Hawk v. Dove: The hawk will always win

Dove v. Dove: Each contest has a 50% chance to win. There are no costs

Assume two players: • a hawk that will always fight until injured or until the opponent retreats• a dove that will always retreat.

Contests are associated will potential benefits (B) and potential costs (C).

½(B-C)

0 ½B

B

Dove

Hawk

Hawk Dove

The pay-off matrix The idea behind game theory is now to define equilibrium conditions that define which game (strategy =

behavioural phenotype) will have the highest payoff in the long run.

Maynard Smith defined such equilibria that cannot be beaten by other strategies as evolutionary stable

strategies (ESS). Populations of individuals playing an ESS cannot be

invaded by immigrating individuals or by mutants playing other strategies.

The fitness

0

0

0

W(H) pW(H,H) (1 p)W(H,D) WB Cp (1 p)B W2

B pB/ 2 pC / 2 W

0

0 0

W(D) pW(D,H) (1 p)W(D,D) W(1 p)B / 2 W B/ 2 pB / 2 W

For H to be an ESS W(H) > W(D)

For D to be an ESS W(D) > W(H)

Is H an ESS?

0 0B Cp (1 p)B W (1 p)B / 2 W B pC2

If B > C, H is always an ESS because per definition 0 ≤ p ≤ 1.

Is D an ESS?

If B > C, D is never an ESS

0 0B Cp (1 p)B W (1 p)B/ 2 W B pC2

½(B-C)

0 ½B

B

Dove

Hawk

Hawk Dove

The pay-off matrix What is if costs are higher than benefits C > B?

H :B pCD :B pC

At equilibrium we haveBB pC pC

For C > B an ESS is to play hawk with probability p and dove with probability 1-p.

Even simple games favour mixed strategies.

This is the start of individualistic behaviour.

½(B-C)

0 ½B

B

Dove

Hawk

Hawk Dove

½B-e

½(B-C)

Retaliator

½(B-C) ½B+dRetaliator ½B-¼C+g

The Retaliator game(fight when meeting a hawk and retreat when

meeting a dove)

½(B-C)

0 ½B

B

Dove

Hawk

Hawk Dove

¼B

¾B-¼C

Bourgeois

¼(B-C) ¾BBourgeois ½B

The Bourgeois game(fight when owner, retreat when intruder)

The Bourgeois is the only ESS of this game.Retaliator and a mixed strategy are the two ESS of this game. Realization depends on

the initial frequencies of players.

Local mate competition

In 1967 W. D. Hamilton proposes that in the long run organisms should preferentially invested in the cheaper sex.

The cheaper sex is the one that promises more offspring at equal costs.

Which sex to produce?

The probability that a son reproduces

is high

The probability that a daughter

reproduces is low

M M M F F Fp r C p r C

p: probability to produce a son; r: expected reproductive success, C: cost of reproduction

For a proper choice a female • needs knowledge about the actual sex ratio and • must have the ability to control which sex she produces

Many Hymenoptera and some other insects have these abilities

Mammals and birds perform selective infanticide

Sex ratio is defined as the proportion of males

y = 0.27x-0.43

00.10.20.30.40.50.60.70.80.9

1

0 1 2 3 4 5Offspring second female /

Offspring first female

Sex

ratio

sec

ond

fem

ale

z

Two examples of sex ratio allocation

Secondary parasitism of the parasitoid wasp Nasonia vitripennis parasitoid of blow and flesh flies

Figs and fig wasps

Agaonidae are closely connected to figs. Depositing eggs into the ovaries they pollinate figs.

Males are wingless and mate only with the local clutch

Parasitic wasps

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1

Proportion of fruit parsitized

Sex

ratio

ro

17 species of fig wasp species

(Agaonidae)

Proportion of fruits parasitized

Selective infanticide in man

Selective infanticide in man is found in nearly all cultures.

Often it serves to • stabilize population size

• to adjust sex ratios to marriage probabilities in cases of highly unequal reproductive success• to adjust to a culturally preferred gender (frequently the male gender)

Some reported sex ratios in childhood of preindustrial societies:

Inuit Eskimos: 0.67

Yanomamö Indians: 0.56

Cashinahua, Peru: 0.60

Rajput caste, India: > 0.9

Upper class medieval Florence: 0.57

The sex ratio is the proportion of males: SR = males / (males + females)

The normal cross cultural sex ratio at birth is 105 males to 100 females = 0.512 (range 101 to 107: 100)

Reciprocal altruism

70

80

90

100

110

120

130

0 20 40 60 80Hours

Per

cent

age

of p

refe

edin

g w

eigh

t

z

Weight lost

Time lost

Donor

RecipientWeight gained

Time gained

Exponential vampire bat weight loss function due

to starvation

• Long term association of group members.• Donorship can be predicted from past helping.• Roles of donors and recipients reverse.• Benefits of the recipients outweigh donor costs.• Donors can detect cheaters.

Reciprocal altruism beween non-related individuals needs:

Blood sharing in the vampire bat

• Primary social groups contain 8 to 12 adults with depending young.

• 30% of the blood sharing events involve adults feeding young other than their own.

• Blood sharing intensity depends on the degree of relatedness.

• Blood sharing is often reciprocal.• Cheaters have not been observed.

Benefits outweigh costs

0 1 2 3 4 5

2

3

4

> 5

P

S

Number of adults providing

care

Young fledged

Additional young fledged per helper

Male helpers

Primary helpers are older sons that are yet unable to breed.

They increase their fitness via their younger sisters and due to additional experience.

Secondary helping males are unrelated to the pair they help.

Secondary helpers increase their fitness due to the chance to become the widow’s mate if the breeding male dies.

Cooperative breeding and helpers at the nest

In the pied kingfisher Ceryle rudis primary and secondary helpers at the nest occur.

Helpers occur in many higher bird species and help adults to raise the offspring.

The evolution of cheating or the Prisoner’s dilemma

Assume two prisoners have the alternative either of defect the other or to cooperate. Defection means shorter imprisoning.

Cooperate

Defect

Defect Cooperate

0 0

B(A) 0

0 B(B)

C(A) C(B)

The pay-off matrix

0

d g

e

Cooperate

Defect

Defect Cooperate

g

0

Tit for Tat

0 gTit for Tat g

Now assume an iterative game where the players play many times. What would be the best strategy?

In the long run there are several possible strategies

One EES is Tit for Tat (defect if prior being defected and cooperate if the other prior also cooperated).

The program playedTit for Tat or reciprocal altruism.

The other EES of this game is always defect.

B>C

If both prisoners defect they do worse than if both cooperate. However cheating the other is superior irrespective of what the other makes.

Hence pure cooperation can never evolve.

The prisoners dilemma cannot fully be resolved analytically.

The first software solution was provided by Rapoport in 1980.

Inbreeding

GM1 GF1 GF2A,B C,D G,H

Grandparents

Parents

Childrens

GM2E,F

M F

Ch

The probability that Ch gets allele C is 0.125.

What is the probability for a children to get a certain allele from their grandparents?

GM1 GF1 GF1A,B C,D C,D

GM2E,F

M F

Ch

The probability that Ch gets allele C is 0.25.

P(C)=0.25

P(C)=0.125

P(C)=0.25 P(C)=0.25P(C)=0

P(C)=0.25

GM1 GF1 GF1A,B C,C C,C

GM2E,F

M F

Ch

The probability that Ch gets allele C is 0.5.

P(C)=0.5 P(C)=0.5

P(C)=0.5

GF1 is already inbred The mean probability to get an allele X from one of the members of a lineage is called

the coefficient of inbreeding.

Sewall Wright defined this coefficient as

i

n1 L

l m li 1

r 2 (1 r )

rl→m is the path from l to descendent m

and L the length of path i.

William D. Hamilton (1936-2000)

Inclusive fitness

In the Hawk - Dove game the EES for C > B was

B<pC → pB>C

P was the probability of a trait to occur. This is formally identical with the probability of a gene to occur via descent, it is identical

to the coefficient of inbreeding.

rB CHamilton’s rule of inclusive fitness

A simple example

Assume a new gene A that promotes parental care.

In cockroaches (Phoraspis and Thorax) the young bite

wholes in the mothers thorax to feed from their

haemolymph.

The probability of transmitting A from mother to daughter is 0.5.

Even if the mother would die due to parental care (cost = 1) two additional raised offspring (B = 2) satisfy

Hamilton’s rule.

0.5 = 1 / 2

Parental care should therefore be widespread in animals.

Kin selection and the evolution of sociality

Individualistic life → Sociality → Eusociality (superorganisms)

Members cooperate but retain reproductive

ability

Part of the members loose own reproductivity in favour of

other group members

Most ‘primitive’ animals and plants

Most bacteria and single cell eucaryotes

→ →ColoniesTrue multicellular organisms (Metazoa, Fungi, Plantae

Social spiders, isopods, many insects,many fishes

Higher birds and mammals

→ →

Joined parental care and defence

Cooperative breeding→

Isoptera (autapomorphy)

Some Aphidae and Thripidae

At least 14 independent lineages of Hymenoptera

Eucalyptus ambrosia beetles (Australoplatypus incompertus)

Sponge shrimp (Synalpheus regalis)

Naked mole rats(Heterocephalus glaber and Cryptomys damarensis)

Often intensive common parental care, aunt behaviour, playing groups, and group defence

All termites (Isoptera). They have male and female workers and

different casts.

All ants (Hymenoptera). They have female workers

only and highly differentiated cast systems.

Some eusocial Apidae and Vespidae (Hymenoptera). They have female workers

only.

Some bumble bees and other Apidae species may be either solitary or eusocial depending on environmental conditions.

Two species of mole rats have non-reproducing workers and a

queen. Colonies have up to 300 members.

Some Aphidae and Thripidae (Homoptera) have sterile

soldiers. Sometimes rudimentary parental care.

What favours Hymenoptera to become eusocial?

Hymenoptera are haplo-diploid organisms

Fertilized eggs become females

Unfertilized eggs become males

QueenA,B

KingC

SonA

SonB

DaughterA,C

DaughterB,C

Daughter

King

Queen

Queen King Daughter Son Brother

0.5 0.5 0.75 0.25 0.25

0 1.0 1.0 0 0.5

1.0 0 0.5 0.5 0.25

rB CHamilton’s rule of inclusive fitness C0.5

B

Queen - daughterC0.75B

Queen - sister

Daughter

King

Queen

Queen King Daughter Son Brother

0.5 0.5 0.5 0. 5 0.5

0 1.0 0.5 0.5 0.5

1.0 0 0.5 0.5 0.5

The haplo-diploid system

The diploid-diploid system

Given that costs and benefits of reproducing are similar it pays for a hymenopteran female more to invest in her sisters than in her own brood.

This explains why eusocial Hymenoptera all have sterile female workers and never sterile males.

But be careful

Most of the haplo-diploid Hymenoptera are solitary.

The theory requires that queens a priori invest more in daughters than in sons.

Interestingly, many Hymenoptera are able to decide whether to lay male or female eggs.They are able to control sex ratios

Termites are diplo-diploid

For instance a hymenopteran female helps her sister at the cost of no reproduction.

At equlilibrum the number of surviving offspring should be 2. Hence C = 2

20.75 0.672 1

The sister raises one additional offspring

20.5 0.672 1

Even for one additional offspring of the sister it pays to resign of own offspring

Eusociality and monogamy

From Hughes et al. 2008

Phylogenetic analysis shows that all ancestral eusocial hymenopteran species were monogame.

Polygamy has derived after the transition to eusociality.

Polygamy never occurs in species with totipotent workers.

Today’s reading

The game theory site: http://www.holycross.edu/departments/biology/kprestwi/behavior/ESS/ESS_index_frmset.html

Selfish gene theory: http://en.wikipedia.org/wiki/Gene-centered_view_of_evolution

The evolution of eusociality: http://www.thornelab.umd.edu/Termite_PDFS/EvolutionEusocialityTermites.pdf

Biology and sexual orientation: http://en.wikipedia.org/wiki/Biology_and_sexual_orientationhttp://www.newscientist.com/article/mg20427370.800-homosexual-selection-the-power-of-samesex-liaisons.html

Biased sex ratios in man: http://huli.group.shef.ac.uk/lummaaproceedins1998.pdfand http://www.jstor.org/cgi-bin/jstor/printpage/00664162/di975349/97p0109i/0.pdf?backcontext=page&dowhat=Acrobat&config=jstor&userID=9e4b1f37@umk.pl/01cce4405a00501c7b1f1&0.pdf

and http://en.wikipedia.org/wiki/Gender_imbalance

Figs and fig wasps: http://www.figweb.org/Interaction/index.htm