The evolution of conflict and cooperation Lecture in the population biology and population genetics seminar series Tom Wenseleers, 2001

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

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.

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. Mutual (synergistic) benefits Wolves hunt in packs and then share their prey.

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

Game theory Von Neumann & Morgenstern 1944 Theory of Games and Economic Behaviour

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

consequence consequence player 1 player 2 for player 1 player 1 player 2 for player 1 defect cooperateB cooperate cooperate B-C defect cooperateB cooperate cooperate B-C defect defect0 defect defect0 cooperate defect -C cooperate defect -C Prisoner’s dilemma “PAYOFFS”

Hawk-dove game 0 -B B -C DOVEHAWK DOVE HAWK Maynard Smith & Price 1973

Hawk-dove game 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 - SYNERGY

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.

A A A A Z ZZ Z Group with altruists, busily outcompeting all the other groups. Z A Z A Z ZZ Z Z Z Z Z Z ZZ Z The selfish individuals in the group are getting the benefit but paying no cost. In the next generation they’ve increased within the group. And now altruists are extinct even though they’ve helped the group.

An important distinction D N A Organism DNA/Gene: the “Replicator” that actually gets copied in reproduction. 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 ( )

Hamilton’s Rule (1964) Kin selection Hamilton’s Rule (1964) r.B > C Relatedness to partner 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 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 Fisher Trivers & Hare

Relatedness coefficients in an ant colony. X 0,5 0,750,25 CAB ACBCACBC ACAC BCBC A, B Worker generation Social insect colonies

X 0,5 CAB ACBCACBC ACAC BCBC Worker generation Calculating relatedness 1 Relatedness between sisters? Share genes via father with a chance of 1 x 0.5

X 0,5 CAB ACBCACBC ACAC BCBC Worker generation 0.5 Relatedness between sisters? Share genes via mother with a chance of 0.5 x 0.5 Calculating relatedness

X 0,5 0,75 CAB ACBCACBC ACAC BCBC Worker generation 1 0,5 0.5 Sisters share genes via father OR mother, so average chance is 1 x x 0.5 = 0.75 Calculating relatedness

Sex-ratio conflicts Mother queen: equally related to sons and daughters (r F =0.5, r M =0.5) 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) 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 ! Parent-offspring conflict ! Trivers & Hare

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

X 0,5 Calculating relatedness Relatedness W- Q produced male = 0.25 Relatedness W- W produced male = ,5 0,75 0,5 0,25 0,375 0,5 Single mating  no worker policing

X Calculating relatedness Relatedness W- Q produced male = 0.25 Relatedness W- W produced male = (1/3) x (2/3) x = ,375 0,25 0,5 Treble mating , ,5 0,125  worker policing

Empirical evidence Worker reproduction in monandric species (stingless bees, bumble bees, some wasps). Worker policing in honey bees (polyandrous, mating with 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

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! Bourke & Ratnieks 1999

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’) Wenseleers et al Conflict over caste fate

Melipona bees MALES QUEEN PRODUCED SOME WORKER PRODUCED (data are from months with maximum queen production) Prop. of queens produced GLZ, p < PREDICTED ESS HIGH RELATEDNESS LOW RELATEDNESS

stingless bees honey bees Policing of caste fate 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

COOPERATEDRIVE COOPERATE DRIVE -B0 B-C Meiotic drive cf. hawk-dove game

But there are differences GenesOrganisms Option to breednousually independently Fighting strategy poisoningphysical aggression Type of ESSpuremixed

Nucleo-cytoplasmatic conflict Nuclear genes r F =0.5, r M =0.5 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. 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 ? SPORE SOMA CELL EACH CELL WANTS TO BECOME A SPORE ?

>1 clone LOW r 1 clone HIGH r DeAngelo et al An experiment

Strassmann et al. 2001

BB Green beard genes: ultimate selfish genes BbBb BbBb BbBb BbBb BbBb Keller & Ross 1998 Gp-9 allozyme locus

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