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Kin selection and social behavior. Social interactions. X. Altruism. A challenge to Darwinism. WD Hamilton. GC Williams. Hamilton’s solution. Inclusive fitness. Direct fitness through personal reproductive success. Indirect fitness through reproductive success of relatives.

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A challenge to Darwinism

wd hamilton
WD Hamilton

GC Williams

hamilton s solution
Hamilton’s solution

Inclusive fitness

Direct fitness

through personal reproductive success

Indirect fitness

through reproductive success of relatives

kin selection

Kin selection

Natural selection favoring the spread of genes that increase indirect fitness

hamilton s rule

Hamilton’s rule

genes promoting altruistic behavior spread, if

Br - C > 0


B = fitness benefit to recipient

C = fitness cost to actor

r = coefficient of relatedness

calculating r from pedigrees box 11 1
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 19
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
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



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)

cooperative breeding
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
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 woodpeckers20
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 breeding21
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
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 eaters23
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






  • The most advanced form of reproductive altruism
  • A caste of sterile “workers” lives as helpers in their parents’ nest, for life
  • 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
Haplodiploidy and eusociality
  • Ants, wasps and bees:
    • males are haploid, develop from unfertilized eggs
    • females are diploid, develop from fertilized eggs
haplodiploidy and r
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
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 female biased sex ratios
Haplodiploidy explains female-biased 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
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
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 eusociality36
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 eusociality37
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
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
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
Eusociality by enforced dominance
  • Queens “shove” workers to induce them to WORK HARDER
  • Shoves are directed preferentially at non-kin

data from a captive colony (Reeve and Sherman 1991)

eusociality in a marine animal
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
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 conflict46
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
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

Parent-offspring conflict in the extreme: siblicide

a contrast of siblicide in masked boobies and blue footed boobies

parent offspring conflict in the extreme siblicide49

Parent-offspring conflict in the extreme: siblicide

masked boobies: older chick pushes younger sibling from nest, to die

in the Galápagos, both species lay two-egg clutches

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 siblicide50

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