The evolution of conflict and cooperation

Lecture in the population biology and population genetics seminar series Tom Wenseleers, 2001

Major transitions in evolution

Szathmary & Maynard Smith Gene Genome Prokaryotes Eukaryotes Genomes Unicellularity Individual organisms Genome-alliances (diploidy, sex) Multicellularity Societies

Why cooperate?

Two approaches : game theory: cooperation is only a good strategy when it has mutual or delayed benefits (false altruism) kin selection: cooperation also possible when it has personal costs, but only when interactants are genetically related (true altruism)

Delayed benefits: helpers at the nest

Young individuals stay at home and help their parents raise more offspring, rather than breeding themselves. Many birds (including these fairy wrens) do this.

Or, in group-living animals sometimes help raise other group members’ offspring (ostriches, some primates).

Altruism? Helpers may gain useful experience in raising their own offspring; or they may have hopes of inheriting a valuable breeding territory.

Mutual (synergistic) benefits

Wolves hunt in packs and then share their prey. Is this altruism?

All the wolves get a benefit from pack hunting: they can bring down larger prey. Cooperation is increasing each wolf’s own fitness.

Reciprocal altruism (Trivers)

E.g. blood sharing in vampire bats. Sharing blood does have costs to the donor. But they may hope to get something back: the next night, they might miss out and the neighbor they fed will feed them in return (future benefits).

For reciprocal altruism, you need individual recognition and enough memory of past encounters to eliminate freeloaders.

Game theory

Von Neumann & Morgenstern 1944

Theory of Games and Economic Behaviour

Game theory

• Optimal (rational) behaviour in conflict situations? • “players” (genes, individuals, groups) • may each choose a “strategy” • Each pairs of strategies is associated with a “payoff”

Prisoner’s dilemma

player 1 defect cooperate defect cooperate “PAYOFFS” consequence player 2 for player 1 cooperate cooperate B B -C defect defect 0 -C

Hawk-dove game

Maynard Smith & Price 1973 DOVE HAWK 0 -B B -C

Hawk-dove game

- SYNERGY Fitness player 1 w 1 =B.z

1 -B.z

2 -C.z

1 .z

2 z 1 en z 2 = phenotypes of players 1 & 2 (hawk=1, dove=0) Advantage of playing hawk depends on what the other player does: benefit = B-C.z

2 At equilibrium B-C.z

2 =0, and the ESS is to play hawk with a probability of z*=B/C

Hawk-dove game

Limited degree of cooperation is in this case not altruistic! It avoids mutual destruction!

Cf. “Mutually Assured Destruction” (MAD) in cold war

True altruism

Definition :

Reproductive altruism: An individual behaves in such a way as to enhance the reproduction of another individual, at a cost to its own fitness.

Paradox: how can natural selection ever favour such behaviour (Darwin) ?

True altruism

Sterile workers in social insects: give up all reproduction for the benefit of their mother queen. How can such behaviour be selected?

Mutual or delayed benefits can’t account for this one: sterile workers never get to produce any daughters.

Group selection (Wynne-Edwards)

W-E proposed that individuals in group-living species might altruistically restrict their reproduction to avoid overpopulation and starvation.

The behavior would be favored because groups containing such individuals would survive, while groups without them would starve and go extinct.

In general, an altruist that promotes reproduction of its groupmates might be favored.

But there’s a problem with group selection.

Z A A Z Z A Z A

Group with altruists, busily outcompeting all the other groups.

Z Z Z Z Z A Z A

The selfish individuals in the group are getting the benefit but paying no cost. In the next generation they’ve increased within the group.

Z Z Z Z Z Z Z Z

And now altruists are extinct even though they’ve helped the group.

An important distinction

D N A

DNA/Gene: the “Replicator” that actually gets copied in reproduction.

Organism

Organism: the “Vehicle”, a machine built by the DNA to do the copying.

Replicators & Vehicles (Dawkins)

Replicators that get copied a lot become more common, replacing those that get copied less: that’s just what selection is.

Traits in the vehicles are favored by selection if they help the replicators that code for them get copied.

In other words, you always need to look at gene frequency change, not at ecological success.

W.D. Hamilton (1936-2000)

Kin selection

Hamilton’s Rule (1964)

Relatedness to partner

r.B > C

Benefit to partner Personal cost This rule predicts when a gene for altruism should be selected. Prediction: cooperation at high relatedness, conflict at low relatedness.

Inclusive fitness

Hamilton’s rule leads us to the idea of inclusive fitness :

Fitness is not only based on own reproduction but also depends on the effects on other individuals, weighted by relatedness. Inclusive fitness = direct fitness (own reproduction) + indirect fitness (reproduction of others) x relatedness

Empirical tests

•

Reproductive conflicts in insect societies

- Sex-ratio conflicts - Conflicts over male production - Conflict over caste fate •

Parent-offspring conflict (Trivers)

Sex-ratio theory

Fisher Trivers & Hare Trait that has a + effect on the production of females (F) and a - effect on the production of males (M) : E F .

r F

> E M .

r M

(Hamilton’s rule) E F = mating success of females ~ M E M = mating success of males ~ F ESS F:M sex-ratio = F/M* =

r F

/

r M

Social insect colonies

X C AB A C 0,5 0,75 0,5 0,25 A C B C A, B

Relatedness coefficients in an ant colony.

B C

Worker generation

Calculating relatedness

C 1 X AB A C B C 0,5 A C

Relatedness between sisters?

Share genes via father with a chance of 1 x 0.5

B C

Worker generation

Calculating relatedness

X A C B C 0,5 C AB 0.5

A C

Relatedness between sisters?

Share genes via mother with a chance of 0.5 x 0.5

B C

Worker generation

Calculating relatedness

C 1 X A C B C 0,5 0,5

0,75

AB 0.5

A C

Sisters share genes via father OR mother, so average chance is 1 x 0.5 + 0.5 x 0.5 = 0.75

B C

Worker generation

Sex-ratio conflicts

Trivers & Hare • Mother queen: equally related to sons and daughters (r F =0.5, r M =0.5)  Wants to invest equally in both sexes.

• Workers: 3 x more related to sisters than to brothers (r F =0.75, r M =0.25)  Prefer 3:1 F:M sex-ratio • Parent-offspring conflict !

Fratricide in ants

Often have female biased sex ratios. Indicates that sex allocation is controlled by the workers. Except in slave-making ants: slaves have no genetic stake in the slave-makers sex-ratio. Wood ant

Formica exsecta

: faculatative sex-ratio biasing. Some colonies with single mated queen, others with double mated queen. Workers only eat their brothers in colonies headed by a single mated queen. (Sundström)

Conflicts over male production

• Workers can also produce own sons r w-son =0.5 > r w-brother =0.25  worker reproduction • But: r Q-son =0.5 > r Q-grandson =0.25

 ’queen policing’ • At mating frequencies > 2 a worker is less related to an average worker produced male than to a brother  ’worker policing’ (Ratnieks)

Calculating relatedness

Single mating 0,5 0,25 0,5 X 0,75

Relatedness W Q produced male = 0.25

Relatedness W W produced male = 0.375

0,5 0,375 0,5



no worker policing

Calculating relatedness

1 2 Treble mating 0,5 X 3

Relatedness W Q produced male = 0.25

1 0,25 2 0,5 0,25 3 0,125

Relatedness W W produced male = (1/3) x 0.375

+ (2/3) x 0.125

= 0.21

0,375 1 2 3



worker policing

Empirical evidence

Worker reproduction in monandric species (stingless bees, bumble bees, some wasps).

Worker policing in honey bees (polyandrous, mating with 10-15 males).

Worker policing in honey bees (Ratnieks)



Empirical evidence

Facultative worker policing in

Dolichovespula saxonica

: workers only police in polyandrous nests. (Foster & Ratnieks)

Conflict over caste fate

Conflict over caste fate

Bourke & Ratnieks 1999 Stingless bees Colonies are swarm founded and therefore mainly need workers, just a few queens. But : 20% of all females develop as queens. A clear excess!

Conflict over caste fate

Wenseleers

et al.

2002 Explanation: each larva is more related to own offspring than to sisters’ offspring  larva prefers to become a queen herself  overproduction of queens if self determination is possible Colony doesn’t need so many queens  mother queen and adult workers are selected to prevent excess queen production (‘policing’)

Melipona

bees

LOW RELATEDNESS MALES HIGH RELATEDNESS 0.25

0.2

0.15

0.1

0.05

QUEEN PRODUCED SOME WORKER PRODUCED

GLZ, p < 0.0006

PREDICTED ESS 0 M.

b ee ch ei i M.

fa vo sa M.

q u ad ri fa sc .

(data are from months with maximum queen production)

Policing of caste fate

stingless bees honey bees Self determination Social determination 20% queen production 0.005% queen production

In the ’70 Bob Trivers showed that there are also conflict of interests in the seemingly solid

parent offspring bond.

Parent-offspring conflict

Each offspring would like to favour itself over its siblings (r=0.5) Parent on the other hand would prefer to treat all offspring equally (equally related).

Offspring are selected to be more selfish than their parents should be willing to tolerate!

E.g. intra-uterine conflicts

Major transitions in evolution

All the previous also applies at other levels E.g. conflicts between genes within cells or between cells within multicellular organisms

“intragenomic conflicts”

Intragenomic conflicts

Forms of intragenomic conflict : - between genes on homologous chromosomes over transmission to gametes (meiotic drive) - nucleo-cytoplasmic conflicts over optimal sex allocation - conflicts between cells over who ends up producing the gametes

Meiotic drive cf. hawk-dove game

COOPERATE DRIVE 0 -B B -C

But there are differences

Option to breed independently Genes no Fighting strategy poisoning Type of ESS pure Organisms usually physical aggression mixed

Nucleo-cytoplasmatic conflict

• Nuclear genes

r F

=0.5 ,

r M

=0.5

• Cytoplasmic genes (mitochondria, some bacterial symbionts)

r F

=1,

r M

=0 Enhance their own transmission if they manipulate their host to produce a more female biased sex ratio. Males are a dead end.

Male killing

Selective killing of males.

Increases the survival of sisters in the same brood, who carry copies of the maternal element. Works through kin selection, cf. fratricide. E.g.

Ricketssia

,

Wolbachia

and

Spiroplasma

in ladybird beetle

Cytoplasmic male sterility (CMS)

In approx. 4% of all hermaphrodite plants. Mitochondrial gene that benefits the female function by sterilising the male function.

Nuclear genes are selected to suppress CMS.

Feminisation

Feminisation of genetic males. Presumably works by suppressing the androgenic gland.

Occurs in woodlice (

Wolbachia

)

.

Induction of parthenogenesis

Induction of asexual reproduction, resulting in an all-female brood. Occurs in some parasitoid wasps (

Wolbachia

).

“Maternal sex-ratio”

Manipulates her host (

Nasonia

) to fertilise more eggs than she is selected to.

Nasonia

is haplodiploid, so fertilised eggs develop as females. Exact nature of “maternal sex ratio” is as yet unknown.

Evolution of multicellularity

Slime molds

Dilemma cf. caste conflict

EACH LARVA WANTS TO BECOME A NEW QUEEN EACH CELL WANTS TO BECOME A SPORE

?

?

SPORE SOMA CELL

An experiment

1 clone HIGH r >1 clone LOW r

DeAngelo

et al.

1990

Strassmann

et al.

2001

Green beard genes: ultimate selfish genes

B b B b B b B b BB B b Gp-9 allozyme locus

Keller & Ross 1998

The End