Lecture 5: Unit of Selection

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Lecture 5 Unit of Selection

• Who/what benefits from adaptation?
• Nucleotide Gene Cell Organism Group
  Species
• What is the unit of selection?
• Can a benefit at one level be detrimental to
  other levels? Conflict?

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Examples

• Segregation distorter genes
• Benefit one gene at expense of others
• Cell lines vs. rest of body
• Must be able to reproduce from gt1 cell line
• Theoretical at the moment

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Reproductive Restraint (Group Benefit)

• NS allele ? in freq if bearer ? fitness vs.
  those w/o the allele
• NS maximization of reproduction
• Why repro restraint? altruism?
• Conflict b/w ind. benefit other levels?

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Interdemic/ Interpopulational Selection

• VC Wynne-Edwards
• Non-breeding in seabirds how explain?
• NS acts on the level of group
• Social behaviour regulates popn density

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Reproductive restraint

• Evidence
• Northern Flicker
• avg 6-8 eggs
• can lay up to 71

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Altruism

• Detriment to ind. fitness for benefit of others

Florida Scrub Jay -young, sexually mature
helpers
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• Infer restraint evolves through
• popn selection not ind. selection
• Why? Unrestrained popn growth depletes resources
  leads to extinction
• Implication selection operates to the detriment
  of the ind. but benefit of the group
• How likely is this really?

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Interdemic Selection Model

• Selfish gene (S) ?? repro rate
• Initial success, ultimate failure

A A A S A A A A
S S S S S S S S S
EXTINCT
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Problem

• Group selection cannot counteract individual
  selection b/c
• Rate of allele freq. ?
• ind. selection gtgtgtgt group selection
• Individual generation time is shorter
• More individuals

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Patch Model (Maynard-Smith)

• Alleles A altruistic S selfish

Overuse resources Extinct unless migrate
Infected by S Goes to fixation
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Group Selection

• Weak force
• Only if migration is very low group extinction
  rates very high
• Group selection may exist, it just cannot
  counteract individual selection

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Traits that benefit other than individual

1. Actually does benefit individual
2. Life history analysis
3. Kin selection

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Life History Trade-offs

• Current vs. Future reproduction
• cost of repro hypothesis

Future Reproduction
Invest now
Future Survival
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Evidence future fecundity

• Collared flycatcher
• Gustaffson Part (1990)
• Manipulated clutch size,
• birds with ? clutches had
• ? clutches for next 3 years

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Number vs. Size of Offspring

• Size often correlates with survival
• Growth vs. Repro etc
• Restraint at time X may benefit ind. at time Y

surviving
N surviving
offspring / size offspring
offspring
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Kin Selection

• Altruism incur cost (c) to bestow benefit (b)
• Contradicts Darwinism Unlikely NS fix altruistic
  allele
• Nepotistic alleles tend to help sibling
• Should feed own offspring or sibling?
• Offspring p(n) 0.5 (meiosis)
• Sibling p(n) 0.5
• Allele says either because they are equal

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Hamiltons Rule of Inclusive Fitness

• If rb gt c or r gt c/b then help
• r degree of relatedness
• b benefit (surviving offspring)
• c cost (lost offspring)
• e.g. Costs c to help sister (r 0.5), each gene
  suffers c but gains b x 0.5
• If b/2 gtc help b/c ? inclusive fitness
• N.B. r probability that two individuals have
  homologous alleles identical by descent

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Inclusive Fitness

• Defn fitness of gene or genotype in ind.
  relatives
• e.g. Cousin b x 0.125 gt c
• As r ? b must ? or c must ? to ? fitness
• I would give my life for 3 brothers or 9
  cousins

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Beldings Ground Squirrel

• Alarm calling
• Cost ? predation risk
• Benefit ???
• Likelihood of calling depends on knowledge of
  relatedness (philopatry?). thus ? inclusive
  fitness

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Spadefoot Toad Cannibalism

• Two morphs omnivore vs. cannibal
• Cannibals tended not to eat their siblings
  (Pfennig 1999)
• Calculated that rb gt c

European Wood Mouse Sperm

• Promiscuous females
• Sperm trains faster than single sperm
• Release before reaching eggmany unsuccessful,
  but improves brothers chances