Group

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Group Individual Selection

• Chapter 4

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Group Individual Selection

• What controls population size?
• Early thoughts vs. prevailing theory

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Group Individual Selection

• Regulation of populations early thoughts.
• Levels below which competition becomes important.
• Avoid wastefulness.
• Development of Group Selection Theory.
• Groups of individuals control their numbers to
  avoid extinction.
• Example smaller clutch size in tropical birds vs.
  temperate birds.

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Group Individual Selection

• Group selection implies that individuals should
  not over-utilize their resources for the good of
  the group.
• Successful groups individuals would not act
  selfishly.
• Selfish groups would overexploit their
  environment and die out.

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Group Individual Selection

• Development of Individual Selection.
• 1966, G.C. Williams
• Adaptations and Natural Selection

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Arguments Against Group Selection

• Mutation
• Cheater scenario (bird that lays extra egg).
• Clutch size based on maximizing the number of
  surviving chicks.

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Arguments Against Group Selection

• Immigration
• Selfish individuals can migrate to new areas.
• Individual selection
• Group selection requires that some groups die out
  faster than others.
• Individuals die out more quickly than groups.
• Individual selection is a more powerful
  evolutionary force.

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Arguments Against Group Selection

• Resource prediction
• Group selection needs a reliable and predictable
  source of food.
• Lack of evidence that animals can predict food
  availability.

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Individual Selection

• Self-Regulation
• Intraspecific competition.
• Individuals strive to command as much resources
  as they can.
• Act in self-interest
• Ex. Male lions that that kill existing cubs when
  they take over pride. Increase their own
  offspring.

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Applied Ecology

• Tragedy of the Commons
• 1968, Garrett Hardin
• Tragedy of the Commons
• Humans and cattle grazing.
• Ex. Carrying capacity on a piece of land - 1000
  cattle .
• 10 ranchers share land, each with a 100 cattle.
• One individual wants to add one cattle more than
  his/her share.
• Maximizes his/her profits at expense of others.
• All of the cattle suffer very little.

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Applied Ecology

• Tragedy of the commons (cont)
• What would happen if all ranchers did this?
• Overgrazing
• Not sustainable
• Benefits of the environment often accrue to the
  individual.
• Cost of using the environment is usually shared
  by the entire population.

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Individual Selection

• Individual selection entails an every one for
  themselves philosophy and seems more likely than
  group selection in nature.

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Individual Selection

• If selfishness is the general rule in nature,
  then we must look for explanations for phenomena
  like altruism and non-breeding castes of
  organisms.
• Fitness genetic contribution of an individual
  to future generations.

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Altruism

• Altruism is defined as the enhancement of the
  fitness of a recipient by acts that reduce the
  evolutionary fitness of the donor.
• Apparent cooperation
• Grooming
• Hunting
• Warning signals

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Altruism

• Caring for copies of ones genes.
• Genes in offspring.
• Coefficient of relatedness r
• Probability of sharing a copy of a particular
  gene.

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Altruism

• Parents to its offspring r 0.5
• Brothers and sisters r 0.5
• Grandparents to grandchildren r 0.25
• Cousins to each other r 0.125

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Altruism

• Implications of relatedness to altruism.
• 1964, W.D. Hamilton
• Importance of passing on ones genes through
  offspring and other related individuals.
• Inclusive fitness genetic contribution of an
  individual, and related individuals that share
  the individuals genes, to future generations.

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Altruism

• Kin selection
• Lowers individual chance of reproduction.
• Raises chances of relatives reproduction.
• Quantifying kin selection.
• rB C gt 0
• r coefficient of relatedness
• B of offspring gained by recipient
• C of offspring sacrificed by donor
• So, your own life is equivalent in genetic value
  to 2 siblings or 8 cousins.

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Altruism

• Individuals may display altruism toward relatives
  because doing so maximizes their inclusive
  fitness.

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Kin Selection

• Selection for behavior that lowers an
  individuals own chance of reproduction, but
  raises that of a relative is called kin selection.

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Kin Selection

• Kin selection example Datana caterpillars
• Aposematic contain colors to warn predators of
  bad taste or poison.
• Predator must kill one to learn.
• Advantage to animals to congregate in kin groups
  death of one may benefit siblings, preserving
  its genes.

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Kin Selection

• Alarms from sentries
• Increased risk of being attacked.
• Animals living near sentry probably relatives.
• Favors kin selection.
• Alternative to kin selection
• Sentries that are forced to live at the fringe.
• Alert for their own safety.
• If sentry is successful, predator may seek new
  area.
• Sentry increases chance of own survival.

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Reciprocal Altruism

• Reciprocal Altruism - altruism between unrelated
  individuals.
• You scratch my back, Ill scratch yours.
• Evidence
• Brooding success correlated to availability of
  helpers.
• Helpers unable to breed because of lack of
  territory.
• Social hunting
• Benefit Bigger prey
• Cost Sharing meat

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Nonbreeding Castes

• Altruism in social insects
• Extreme example of altruism sterile castes in
  social insects.
• Female workers
• Rarely reproduce.
• Assist queen with her offspring (eusociality).
• Soldier castes

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Altruism in Social Insects

• Altruism in social insects may arise from the
  unique genetics of their reproduction.
• Relatedness
• Females are diploid.
• Males are haploid develop from unfertilized
  eggs.
• Sperm formed without meiosis.
• Each sperm is identical.
• Sister relatedness
• Each daughter receives an identical set of genes
  from her father.

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Altruism in Social Insects

• Sister relatedness
• Half of a females genes come from her diploid
  mother.
• Total relatedness of sisters 0.5 from father
  0.25 from mother 0.75.
• Genetic system termed haplodiploidy.

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Altruism in Social Insects

• Sister relatedness
• Workers are more related to their sisters (0.75)
  than they would be to their own offspring (0.5).
• Better for female workers to have more sisters.

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Altruism in Social Insects

• Relatedness and the Queen
• Sons and daughters r 0.5.
• Maximize reproductive potential. 5050 sex ratio.
• Average relatedness for sterile workers would be
  0.5.
• Better for female workers to have more sisters.
• Colonies usually have more females than males.

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Altruism

• Non-haplodiploid colonies
• Termites
• Mole rat from South Africa

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Eusociality in Mammals

• Lifestyles that promote eusociality in mammals
• Individuals are confined to burrows or nests.
• Food is abundant enough to support high
  concentrations of individuals.
• Adults exhibit parental care.
• Mothers can manipulate other individuals.
• Heroism is possible.

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Group Living

• Dense living.
• Promotes intense competition.
• There must be significant advantages to
  compensate for the disadvantage of increased
  competition.

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Group Living

• Group living may reduce predator success in
  several ways
• Many-eyes hypothesis
• Large groups may be more successful at detecting
  predators.
• Prey alerted to attack.
• Ex. Goshawks less successful attacking large
  flocks of pigeons.
• The bigger the flock (more eyes) the more likely
  the prey will be alerted to the presence of a
  predator.
• The scanner has best chance of escape
  discourages cheating.

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Group Living

• Selfish-herd theory
• Predators usually only take one prey per attack.
• The bigger the herd, the lower the probability of
  an individual prey being taken.
• Larger herds are attacked more, but probability
  of being taken would still favor individual.

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Selfish Herd Theory

• Geometry of the selfish herd
• 1971, W.D. Hamilton
• Prey prefer middle of herd to avoid predator.
• Predator has difficulty in tracking large numbers
  of prey.
• Peripheral prey easier to visually isolate.
• More difficult for predator to reach the center
  of herd.
• Large herds are better able to defend themselves.

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Group Living

• Conflicting variables
• Competition for food
• Presence of predator

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Group Living

• Field test of model.
• Tame hawk used to simulate predation.
• As predicted, flock size increased with
  increasing threat of predation.
• Avg. flock decreased from 7.3 to 3.9 when hawk
  was removed.
• Most time spent feeding at intermediate size
  flock.

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Summary

• Group Selection
• Past theory
• Population maintained at equilibrium based on
  group selection.
• Self-regulation of individuals.
• Prevent overexploitation of resources.
• Several flaws mutation, immigration, and
  resource prediction.
• Individual Selection
• More likely

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Summary

• Explanations for altruism
• Kin selection.
• Caste systems of social insects.
• Haplodiploid mating systems.

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Summary

• Occurrence of eusociality and cooperation
• Haplodiploid organisms.
• Non-Haplodiploid organisms.
• Confinement to burrows.
• High food concentrations.
• Parental care of offspring.
• Mothers can manipulate other individuals.
• Opportunity for heroism.

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Summary

• Group Size - Trade offs
• Competition for food
• Protection from predators