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Kin selection and social
behavior
Social interactions
Actor benefits Actor is harmed
Recipient benefits
Coope rative
(mutualism)
Recipient is harmed Selfish (e.g.
parasitism)
Altruistic
X
Spiteful
Altruism
A challenge to Darwinism
WD Hamilton
GC Williams
Hamilton’s solution
Inclusive fitness
Direct fitness
Indirect fitness
through personal
reproductive success
through reproductive
success of relatives
Kin selection
Natural selection favoring the spread of
genes that increase indirect fitness
Hamilton’s rule
genes promoting altruistic behavior
spread, if
Br - C > 0
where
B = fitness benefit to recipient
C = fitness cost to actor
r = coefficient of relatedness
Calculating r from pedigrees
(Box 11.1)
• connect actor (performs
behavior) to recipient by
pathways of descent
• each arrow represents a “step”
or a single generation of gene
transmission
Calculating r from pedigrees
(Box 11.1)
• Probability of gene
transmission at each step =
Mendelian probability = 1/2
• Multiply independent
transmission events
• Sum all possible pathways
Alarm calls
• In several birds and mammals,
individuals call to warn of
approaching predators (or
conspecific “aggressors”)
• Casual observations and some
studies show that the alarm
caller is attacked more often
• Is this altruism?
from Sherman (1977)
Belding’s ground squirrels “whistle” to
warn of attack by hawks, and “trill” to
warn of mammalian predators
(weasels,badgers, or coyotes). Do calls
put the caller at risk?
Whistlers are attacked 2% of the
time, non-whistlers 28% of the time
(per capita)
A selfish behavior
from Sherman (1977)
But trillers are attacked 8% of
the time, and non-trillers are
attacked 4% of the time
Altruism???
from Sherman (1977)
Altruism: calling behavior should vary randomly with age and gender.
But instead, females call much more often…why?
(1) females remain close to their natal burrow, while males disperse farther
(2) females are calling to warn their sisters (r = 0.5)
Females were more likely to call when relatives were
close by than when only non-relatives were
Cooperative breeding
• In a few species of birds (about 3%,
several families), reproductively mature
young do not breed their first year
• Instead they “help” forage for, feed, and
protect the young of their parents
• Helping may reflect limited nesting space,
and difficulty for young to compete for it
Cooperative breeding in
red cockaded woodpeckers
• advanced social system
organized into “groups”
• group is the breeding pair
with 0 to 3 “helpers”
Cooperative breeding in
red cockaded woodpeckers
• Helpers are usually juvenile
male offspring of the pair
from the prior season
• They help incubate eggs (10
-12 days) and raise young
(~26 days)
Cooperative breeding
• Is it a “best of a bad situation”
strategy?
• Does helping increase inclusive
fitness?
• Emlen and Wrege have tested these
ideas in white-fronted bee-eaters
White fronted bee-eaters
• Merops bullockoides is a colonial nester native to
East and Central Africa
• Colonies of 40-450, subdivided into “clans” of 3-17
birds, each clan defending a foraging territory away
from riverbank nests
White fronted bee-eaters
• Most one year-olds are helpers and do not
breed
• Their clan consists of several sets of
parents and offspring
– r varies greatly across clan members
– helpers have a choice of nestlings to
help, each with different r
– an excellent system for testing whether
helping correlates with r
•Emlen and Wrege marked
individuals, determined
pedigrees and scored helping
behavior over 8 years
•Helpers that are related to
offspring (“natal”) help much
more often than unrelated
helpers that join the clan
(“in-laws”)
They generated an “expected” null
distribution of helping frequency (from the
distribution of r in the clans)
They compared this to the
observed helping frequency as a
function of r
Helpers help close relatives
much more often than if
helping was randomly
associated with r
This indicates that helpers can recognize (distinguish?)
related young, and choose to help them
Helping has a huge fitness benefit
• > 1/2 of all young die before fledging
• Fledging success rises rapidly with the
number of helpers per breeding group
0
1
2
3
4
Eusociality
• The most advanced form of reproductive
altruism
• A caste of sterile “workers” lives as helpers in
their parents’ nest, for life
Eusociality
• A special challenge to evolution by
natural selection
• Scattered cases throughout the
animals
– several insect orders
(Hymenoptera and Isoptera)
– naked mole rats (Bathyergidae)
– snapping shrimps (Synalpheus)
Haplodiploidy and
eusociality
- Ants, wasps and bees:
 males are haploid, develop
from unfertilized eggs
 females are diploid, develop
from fertilized eggs
Haplodiploidy and r
- females are related to their
sisters by r = 3/4
 they share all of their father’s genes
(no meiosis), which is 1/2 of their
genomes = (1 X 1/2)
 in the other half of their genomes,
they share 1/2 of the queen’s genes
(her eggs undergo meiosis)
= (1/2 X 1/2)
 r = (1 X 1/2) + (1/2 X 1/2) = 3/4
Kin selection and eusociality
• Females are more closely related to
their sisters (r = 3/4) than they would
be to their own offspring (r = 1/2)!
– Females maximize inclusive fitness
by not reproducing, instead helping
their sister reproduce
– They should invest in sisters, not
daughters or sons (r = 1/2) or
brothers (r = 1/4)
Haplodiploidy explains femalebiased sex ratios
• Since workers are 3 times more related to
sisters than brothers, they should behave so
that a 3:1 female biased sex ratio results
 Female biased sex ratios are widespread in
Hymenoptera
 Sundström et al (1996) showed that in wood
ants, the queen lays a 1:1 sex ratio, but it
becomes highly female-biased at hatching
 Workers must be able to recognize and
destroy male embryos!
A genetic conflict of interests
• This shows the action of a “conflict of interests”
between the queen and workers
– Queen is equally related to sons and
daughters, so she should lay a 1:1 sex ratio
– Workers should favor females
– The workers win!
So does haplodiploidy explain
eusociality?
•
•
Hamilton (1972) said yes
Current data says no:
1. The “3/4” rule relies on all workers having
the same father, but queens mate multiply
•
Honeybee queens mate 17 times before
founding a colony (worker r < 1/3)
So does haplodiploidy explain
eusociality?
•
•
Hamilton (1972) said yes
Current data says no:
2. Multiple “foundresses” establish colonies
in several eusocial species (worker r ≈ 0)
So does haplodiploidy explain
eusociality?
•
•
Hamilton (1972) said yes
Current data says no:
3. Many eusocial species are not
haplodiploid, many haplodiploids are not
eusocial
Phylogeny of the
Hymenoptera
•Shows that
haplodiploidy evolved
early, much prior to
eusociality
•Shows that eusocial
groups (bolded)
evolved multiple
times, independently
Phylogeny of the
Hymenoptera
•Eusociality evolved
along with complex nest
building and prolonged
care of larvae
Naked mole-rats!
Heterocephalus glaber, native to Cape
Horn, Africa, forms colonies of 70-80 close
relatives
Cooperatively dig and defend complex
tunnel systems (up to 2 miles long)
A single queen and 2-3 reproductive males
Non-reproductives build tunnels, care for
young, colony defense as they grow
Eusociality in naked mole-rats
• Colonies are highly inbred
– microsatellite DNA shows
average r = 0.81 between
colony members!
• Conflict—workers would still
be more closely related to
their offspring than their
siblings
• Queens control the colony
through threats and violence
Eusociality by enforced
dominance
data from a captive colony
(Reeve and Sherman 1991)
• Queens “shove” workers to
induce them to WORK
HARDER
• Shoves are directed
preferentially at non-kin
Eusociality in a
marine animal
• Synalpheus regalis
lives in colonies of >
300 individuals per
sponge, one
reproductive female
• Like other eusocial species, S. regalis
– shows gradual metamorphosis
– needs prolonged care of larvae
– forms colonies that are closely related family units
Direct developing larvae of S. regalis remain in their
natal colony
Allozyme data show most non-reproductives to be
full sibs
These workers actively defend the sponge against
intruders
Parent-offspring conflict
• kin selection helps us understand
behaviors between parents and
offspring that are difficult to explain
• the key is a genetic “conflict of interests”
– offspring “resemble themselves”
entirely (their self r = 1)
– parents only share half their genes
with offspring
Parent-offspring conflict
• offspring should act “selfishly” while
parents are equally related (r = 1/2)
to all other offspring
• the conflict will be most obvious
when parental care is very costly
(e.g. birds and mammals)
Weaning conflict
• a common observation:
– offspring continue to solicit nursing
even as they grow and become
independent
– mothers begin to refuse
– aggression results
• an “optimal” mother should save
resources for other offspring
Parent-offspring conflict in the
extreme: siblicide
a contrast of siblicide in masked
boobies and blue footed boobies
Parent-offspring conflict in the
extreme: siblicide
in the Galápagos, both species lay twoegg clutches
masked boobies: older
chick pushes younger
sibling from nest, to die
blue-footed boobies:
siblicide is more
complicated: older chicks
will reduce food intake,
and only kill siblings
during extended food
shortages
Parent-offspring conflict in the
extreme: siblicide
Most confusing is the
fact that masked
booby parents do not
try to stop the
behavior! (kin
selection suggests
that they should)
Reciprocal
transplants show that
masked booby chicks
are more likely to be
killed, and that their
parents are less likely
to intervene