Download Social Evolution

Kin Selection &
Social Evolution
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mutualism
altruism
selfishness
spite
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Mutualism + + : every individual benefits
 group foraging: individuals forage with others which
increases the rate or probability of finding food.
Selfishness + - : actor benefits, recipient pays cost
 territorial behaviour: individuals defend an area against
conspecifics, denying other access to food, nest sites etc.
Altruism - + : actor pays cost, recipient benefits
 cooperative breeding: a breeding pair receives help in
raising offspring from other reproductively mature individuals
who forgo their own reproduction (scrub jays, meerkats,
tamarins)
 sharing food: vampire bats share blood with other bats in
their roost. This is costly to the blood donor but helps the
recipient, because bats die if they do not eat for 2 nights.
Spite - - : every individual pays cost
 unlikely to evolve
The paradox of altruism – why do something that reduces
your own fitness, even if it helps another?
Darwin recognized as a challenge to his theory of evolution by
natural selection.
Hamilton found the solution -- Help your kin.
Why is this a solution?
Take the gene-centered view: kin share genes, so helping kin
also ‘helps’ an individual’s own genes. In other words, an allele
that causes altruistic behavior will spread in a population if
individuals help other individuals who carry that allele
(because that increases the frequency of the allele).
Inclusive fitness. Individual fitness includes both the
individual’s own reproduction (direct fitness) and increases in
its relatives’ reproduction that are caused by the individual’s
help (indirect fitness)
But how closely related do kin need to be?
And how much help should an individual provide?
Hamilton’s rule:
Two ways to think of it
r > c/b
relatedness should exceed the ratio of cost to benefit
c < rb
cost must be less than amount of benefit * relatedness
Generally, this means that altruism is likely to evolve when
 costs are low
 benefits are high
 relatedness is high
Ex: Alarm calling in ground squirrels
Beldings ground squirrels live in colonies with burrows in close
proximity. Females give calls to warn of approaching
predators. This increases their risk of being caught by the
predator, but decreases the risk for others in the colony.
So why risk this danger and call?
 Females in a colony are likely to be closely related because
female offspring stay in their natal colony to breed, while
male offspring disperse to other colonies.
 Female ground squirrels are more likely to call than males.
 Females are more likely to call when their close relatives are
nearby.
So, females may help relatives to avoid predation, thereby
increasing the representation of their alleles in future
generations. This means that alarm calling in the presence of
relatives is favored.
Problem: recognizing kin
How to know if another individual is closely related?
Heuristic rules: Familiar individuals are more likely to be kin.
This often works well, but not always.
 nestmates
Phenotype matching: Individuals with very similiar phenotypes
are more likely to be kin. Traits have to be heritable and highly
variable.
 MHC in mice, colonial sea squirts
Signals to identify kin include vocal, chemical, visual, and
morphological.
Calculating relatedness coefficients
Methods for estimating:
 genetic markers – degree of similiarity in RFLP,
microsatellites, allozymes
Assign maternity and paternity to offspring based on shared
marker alleles.
Can also use for entire social groups or to compare
relatedness among social groups.
Do not need prior information about relationships between
individuals. Often useful in field studies and when relatedness
estimates for many individuals is needed.
 pedigree analysis using path analysis
Calculate the expected proportion of shared alleles based on
known relationships among individuals.
Trace paths of descent from actor to recipient, including all
connections. The probability that shared parents transmitted
the same alleles to both is found by multiplying each segment
of the path connecting individuals. If actor & recipient share
more than one parent, add values for each path to determine
total probability of genes identical by descent (ibd).
Often used by breeders, or in long-term studies where
individuals are marked. Cannot be applied in situations where
pedigree is not known.
The evolution of eusociality
An extreme form of altruism -- Eusociality
Social organization with:
1. overlapping generations
2. cooperative brood care
3. division of labor: specialized castes that are
morphologically differentiated usually including:
 queens reproductives
(including alates &
laying queens)

workers - non
reproductives who
raise brood and
maintain colony

soldiers - non
reproductives who
defend colony
.
Found in:
Social insects: bees, ants, wasps, termites
Naked mole rats
Workers completely forgo their own reproduction and help to
raise their siblings. Their inclusive fitness consists entirely of
indirect fitness. They have no direct fitness.
How could such an extreme form of cooperation evolve?
Haplodiploidy facilitates the evolution of eusociality because a
female is more closely related to her sisters than to her own
offspring. This means she increases her inclusive fitness most
by helping to rear sisters.
Haplodiploidy: females are formed when eggs are fertilized
and males are formed when eggs are unfertilized.
Relatedness coefficients
queen to female offspring
queen to male offspring
worker to worker
worker to own offspring
0.5
0.5
0.75
0.5
But, not all haplodiploid taxa are eusocial, and not all
eusocial taxa are haplodiploid.
 Some wasps are not eusocial.
 Naked mole rats and termites are not haplodiploid.
This means that the evolution of eusociality depends on more
than just relatedness.
Remember that Hamilton’s rule also has terms for benefits and
costs, which are influenced by ecological factors. We need to
consider the magnitude of benefits and costs & the role of
ecology.
Social evolution
We can predict social organization based on ecological factors
and Hamilton’s rule. Consider costs & benefits, relatedness.
Social structure shows wide variation:
 solitary (civet cats)
 mother and offspring w/ males solitary (black bears)
 pairs (many birds)
 small groups of both sexes -- often family groups
(marmosets, killer whales, groove-billed anis)
 female groups w/ males solitary – usually females are
related (elephants) but not always (fallow deer)
 colonies – vary in size from a few to thousands to millions
(barn swallows to puffins to free-tailed bats)
We expect group living when there are benefits for survival or
reproduction, which often depend on ecology. It’s especially
likely among relatives.
Cooperation and conflict
Social life is not without conflict because the interests of
individuals are rarely exactly equal. Helping oneself should be
favored in many cases, because r = 1. Conflict of interests is
apparent when we look closely at social groups.
Ex: Worker - queen conflict in social insects
In a normal diploid population the equilibrium sex ratio (#
females to # males) is 1:1 (or 0.5). Why?
In haplodiploid eusocial insects, this no longer holds. Workers
and queens have a conflict of interest over the best sex ratio
(in reproductive offspring, not workers). What causes this
conflict?
This leads to conflict of interest between queens and workers
over the best sex ratio. Who wins?
Two populations live in very different ecological conditions.
Sex ratios differ.
Ex: Parent - offspring conflict
.
The optimal period of parental care often differs for parents
and offspring. Parents usually benefit by stopping care early.
Offspring often benefit if parents continue care longer.
What leads to this conflict of interest?
What is optimum for parents & why?
What is optimum for offspring & why?
The conflict arises because parents are equally related to all
offspring by r = 0.5 and offspring are related to themselves by
r = 1.0.
Ex: Evolution of cheating & cheater prevention
Cooperation is vulnerable to cheating, because individuals
who cheat should have high fitness. They accrue benefits
without paying the costs.
 Cheating kin recognition: unrelated individuals pretend to be
related to extract help.
 Cheating cooperation: individuals do not provide help but
accept it from others.
Prediction is that the frequency of cheaters should increase.
Is cooperation evolutionarily stable?
Kin recognition is favored to detect cheaters. Individuals may
be inspected to assess relatedness. Like a ‘password’ to allow
entrance into a group.
EX: honeybee guards at the hive entrance
Also ability to recognize and remember individuals should be
favored – remember who has helped.
Aggressively coerce potential cheaters to help, or punish for
not helping.
EX: When they find a rich food source, rhesus monkeys give a
call to alert others to the location of food. Individuals who do
not call are attacked if they’re ‘found out’.
These methods make cheating costly, so it no longer
pays (in the evolutionary sense). This increases the cost to
benefit ratio.
Incorporating social structure into population genetics
Social structure can result in genetic structure within
populations
So far, models have assumed that individuals interact with
others randomly. But in many species this is not the case.
Social organization means that interactions are structured –
behavior directed towards some individuals will be very
different from behavior directed to others.
Not only do traits evolve that specifically facilitate social
interactions, but the results of those interactions can change
the evolutionary dynamics of other traits too.
EX: What happens if all mating is within groups?
Inbreeding is high so genetic variation within groups is very
low, while genetic differences between groups is high.
The way genetic variation is distributed in populations is
altered.