2400: Evolution of socialty

cooperation

the process of working tg to the same end

problems:

1. altruism- favoring another’s fitness at the cost of your own

2. free-riders- receiving the benefits from others without cooperate or show altruistic behaviors when they could free-ride instead

3 evolutionary paths to cooperation

1. kinship

2. reciprocity

3. group selection

sociality

sociality is the tendency of individuals to live and interact in groups, forming communities with links

kinship and cooperation

genetic relatedness relates to cooperation and kinship

“blood is thicker than water”

an individuals total fitness (inclusive fitness) can be broken down into:

1. direct fitness- producing your own offspring

2. indirect fitness- help kin produce more offspring

coefficient of relatedness- r

Diploids

• probability of off-spring-parent sharing an allele = ½
  probability of full siblings sharing an allele = 1/2

coefficient of relatedness ( r )

the probability that an allele in one individual has a copy that is identical by descent to another individual

hamilton’s rule

an allele, X, to help a genetic relative will increase in frequency when rb>c

for nat selection to favor assisting family members will depend on how closely related the individuals are

r= coefficient of relatedness

b= benefit of the genetic relative receives

c= cost to the offspring carrying the trait

example of hamiltons rule

long-tailed tits

“helper” birds often forgo reproduction to help relatives raise their offspring by providing food and protection

hatchwell et al:

• marked 100 breeding birds

• genotyped 19 loci

• estimated relatedness

• measured the effect of helpers on offspring survival

helpers → breeders r = 0.2

helpers → chicks r = 0.16

offspring survival increased

kinship: eusociality

inclusive fitness may explain eusociality

eusociality is defined as “true” social behavior that exhibits:

1. reproductive division of labor

2. cooperative rearing of young

3. overlapping generations

inclusive fitness

the overall ability of a gene to be passed on, including through direct and indirect descendants like nieces and nephews

two theories for evolution of eusociality

1. subsocial model

2. parasocial model

subsocial model

evolved from species in which immature offspring remained their nest to receive maternal care

group of related individuals that don’t interbreed

“staying tg”

parasocial model

evolved from an ancestor whose social system had unrelated breeders coming tg to raise offspring communally

“coming tg”

testing eusociality evolution in snapping shrimp (synalpheus)

species exhibit various social behaviors

• single pair raises offspring

• communal breeding (unrelated inds raising all young)

• true eusocial = single “queen” and many nonbreeding inds

why has eusociality evolved nine independent times in hymenopterans?

hypnotera are haplodiploid species: all males are haploid, all females are diploid

females are more related to their sisters (0.75) than their own offspring (0.5)

haplodiploid species tend to skew the sex ratio to 3:1

female workers provide food and protection are three times as related to their sisters (r= 0.75) than brothers (r= 0.25)

• selection favors 3:1 ratio

fishers model

therefore, eusociality in bees is predicted to be associated with monandrous mating systems

the common ancestor of each eusocial clade is monandrous

fishers model

nat selection favors a 1:1 sex ratio

mothers are equally related to sons or daughters

• r= 0.5

monandrous

females have a single mate

~higher genetic relatedness within the group

polyandrous

females mate with multiple males

~lower genetic relatedness

why do we see eusociality in diploid species like termites and mole rats?

possible explanations:

1. harsh conditions outside the colony and unlikely to disperse

2. food is locally abundant

3. parental care is common

reciprocal altruism

exchanging acts of altruisms can be maintained if both inds benefit

necessary conditions

1. inds must recognize each other

2. interaction among inds are long-term

3. free-rider retaliation

reciprocal altruism example: pied flycatchers mobbing an owl

mobb forces are the owl to leave

all prey inds in the area benefit

flycatchers are placed in a triangular position

own is placed near #1

birds from #2 are prevented from mobbing

1 hour later…

predator is placed near #2 and #3

birds from #1 come to aid #3 but not #2

group selection

a proposed mechanism of evolution that acts on the level of the group, not individual

“for the good of the species/population”

sociality via group selection

group selection

meerkats call to warn relative of a predator

non-relatives benefit as well

groups with many alarm callers benefit by outcompeting neighboring groups with fewer alarm callers

within-group vs between-group selection in ants

desert leaf cutting ant

nests initiated by unrelated queens (cofounders)

brood raiding ants attack nearby colonies and capture pupae and larvae ~ attacked colony usually doesn’t survive

one queen takes the role of forager to bring back food for the rest of the colony

“forager queen” takes risks and benefits

other free-riding queens free-ride

within-group variation

cooperation within nests leads to a greater production of workers

between group variation

more workers → higher probability of surviving brood raiding between groups

kin selection hypothesis

inds will be closely related

beneficiaries do not reciprocate

if eusocial → kin selection

hamiltons rule: rB>C (relatedness*benefit> cost)

reciprocity hypothesis

inds not closely related

reciprocal interactions and memory of previous interaction (exchange of favors)

excludes cheaters

small cost to helping, large benefits to being helped

benefit to RECIPIENT (B)> Cost to donor (C)

group selection hypothesis

between-group selection FAVORS altruism (altruistic groups survive better), although it may not be favored on an ind level

variation among groups, with some groups being more successful

no reciprocity; unrelated

ind cost negative, put positive benefits for the group

benefit to GROUP (B) > Cost to donor (C)

social evolution can also involve conflict

1. conflict among nonkin

2. conflict over parental investment

3. parent-offspring conflict

4. conflict within the genome

cooperation and conflict not mutually exclusive

conflict among nonkin

same sex competition usually occurs for access to territory or access to mates

evolutionary biologists use game theory to analyze social interactions and their fitness consequences

the hawk dove game: why does one ind back down?

“hawk” strategy ~ aggressive

“dove” strategy ~ cautious

c=cost

v=resource

nash equilibrium

a state where no player can improve their outcome by unilaterally changing their strategy

mixed nash equilibrium

frequency of playing hawk

p = v/c

conflict over parental investment

resources provided from parent to offspring

• food

• shelter

• defense

could have sexual xonflict

parent-offspring conflict

sibling conflict

burying beetle (nicrophrous vespilloides)

parents raise offspring on a carcass (usually a small rodent or brid) and feed their offspring after hatching

Darren Rebar et al

• experiment lasted 22 generation

• T1: parents allowed to provide food to their brood

• T2: no food

with parental care = competition

no parental care = cooperation

worth paying the costs when prize is high

parent-offspring conflict

assume a dixed total resource can be used to feed offspring

parents want to distribute resources equitably to all offspring (equally related to all offspring r = 1/2)

offspring are related to siblings by r = ½

• but are related to itself r = 1

• so an offspring will value resources it receives more than if they are distributed to their siblings

conflict within the genome: segregation distortion

mendels law “fair”

segregation distorters (or meiotic drive alleles)

what we expect:

• ½ gametes Sd Rsp-i

• ½ gametes Sd+ Rsp-s

locus: Sd = segregation distorter gene on chromosome 2L

• alleles

  • Sd = active allele

  • Sd+ = inactive allele

Locus: Rsp = responder gene on chromsome 2R

• alleles

  • i = response insensitive

  • s = response sensitive

meiotic drive alleles

the chromosome carrying the specific allele is transmitted to offspring at a higher rate than other chromosomes

segregation distortion in fruit flies

if the Sd allele is on either chromosome, Rsp-s is sensitive to it and will break down in meiosis

99% of the sperm are Rsp-i and 99% are Sd due to linkage

conflict within the genome: parent of origin conflict and genomic imprinting

conflict of interests between males and females begins long before offspring are born

parent-of-origin of chromosomes affects zygote viability

paternally imprinted genes

maternally imprinted genes

haig’s “tug-of-war” model for genomic imprinting

paternally imprinted genes

genes that express when found on the maternal chromosome but silenced when on the paternal chromosome

maternally imprinted genes

genes that express when found on the paternal chromosome but silenced when on the maternal chromosome

haig’s “tug-of-war” of genomic imprinting

this asymmetry between the male (A) and female (B) will select for males to divert more resources to current offspring… regardless of whether it goes beyond the female’s optimum

chart in slides

live birth is a prereq. for genomic imprinting

in live birth species, males can directly affect maternal allocation of resources

phylogenetics reveal Lgf2 imprinting is not present in monotremes

Lgf2

associated with allocating resources for maximal growth rates for developing embryos (maternally imprinted)

ILgf2r

slow embryo growth rate (paternally imprinted)

information and communication

information is easy to share

not all info is shared w/o cost

how do we know honest info is being shared?

1. mind reading vs manipulation

2. costly signaling theory

3. conventional signals

mind reading vs manipulation

honest communication is NOT ensured

receivers attempt to read signs (mind read) that may not be meant as signs, to gain an advantage

costly signaling theory

honest communication IS ensured

if signals are costly and dishonest signals cost more than honest signals, they’re more likely to remain for long periods of time

conventional signals

a signals (color patterns) that isn’t directly tied to the physical quality it represents (aggression); but it’s interpreted by receivers and influences their behavior

honest communication is ensured when the cost of deviation from the conventional signal keeps signalers honest

arbitrary signals without and extravagant cost; signal represents another quality

begging for food

if begging was costly; we predict

1. parents will deliver more food to stronger beggars

2. begging intensity reflects hunger levels

3. begging will be costly

empirical evidence to address predictions

parents feed more when calls increased

begging intensity reflects hunger

begging is costly

paper wasps

increasing brokenness of black patterns indicates fighting ability

inds w higher fighting ability build bigger nests

bigger nests leads to higher survival in harsh winters

Tibbets and Izzo 2010:

• manipulated wasps face using paint

• applied an artificial hormone to increase aggressive behavior

social reinforcements prevent successful dishonest signalers

why do some rely on costly signals while others use conventional signals?

costly

• chicks begging for hunger

• hard for mother to prove accuracy of the signal

conventional

• wasps signaling fighting ability

• signal receiver can instigate a fight and prove accuracy

• expected when communicating verifiable traits

reflection

size of the black plumage spot on horse sparrows was thought to signal fighting ability and social dominance

There is publication bias or “file drawer effect” in the behavioral literature, in which investigators who find a relationship (e.g. between a signaling trait and a behavior) decide to publish and those that find no relationship decide not to prioritize that study for publication