Lecture 6: Units of Selection continued

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Lecture 6 Units of Selection continued

• Most Extreme example of Kin Selection
• EUSOCIALITY
• Eusociality
• Overlap in generations
• Co-operative brood care
• Specialized castes of non-reproductive
  individuals

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Examples

• Hymenoptera
• (bees, ants, wasps)
• Isoptera
• (termites)
• Mammalia
• (naked mole rats)
• How did this evolve?

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Hypothesis 1 Kin Selection

• Plant Galls
• stick around raise your siblings (r 0.5)
• Leave, find mate raise offspring (r 0.5)
• Difference is in
• RISK LEVEL

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Haplodiploidy

• Eusociality is COMMON in haplodiploid hymenoptera
  (evolved 11 times)
• UNCOMMON in diploid ( only 1 time)

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Haplodiploidy

• Diploid (2n) produce ova by MEIOSIS
• females from fertilized eggs
• Haploid (1n) sperm by MITOSIS
• males come from unfertilized eggs

relative
focal
mother
daughter
sister
father
son
brother
individual
female
0.5
0.5
0.75
0.5
0.5
0.25
male
1
1
0.5
-
-
0.5
Favours reproduction of sisters over all others
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Origin of Eusociality

• Problems with Haplodiploid hypothesis
• 1) Isoptera (diploid)
• Sterile workers of both sexes
• Social transmission of gut symbionts
• 2) Many species queens are promiscuous
• - females not most closely related to
  sisters
• 3) Many species that are haplodiploid are not
  eusocial
• So

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Hypothesis 2 Parental Manipulation

• Queen suppresses reproduction of other workers
• Chemical cues
• Physical inhibition
• Lots of experimental support

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Specialized Castes

• Queens Workers

Fertilized egg
Royal Jelly
Queen
Worker
Royal Jelly hormones that affect development of
tissue including ovaries
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Stingers

• Worker barbed, weak attachment
• Purpose defense of colony
• Queen unbarbed, long curved
• Purpose Killing sisters

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Hypothesis 3 Mutualism

• Worker manipulation?
• Cryptic reproductives
• Greater fitness by helping rather than founding a
  new colony.
• Unmated workers lay haploid eggs in many
  species. 

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Most likely

• Eusociality ONLY occurs in species with
• Complex nests
• Larvae cared for extensively
• Thus, females are unlikely to be successful if
  breed on their own.
• Ecological not genetic?
• Best of a bad situation

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Unit of Selection

• Entity that benefits from adaptation
• What about adaptations that benefit genes at the
  expense of other genes in the same individual
• Segregation Distorters
• Paternal Sex Ratio Trait

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Normal Mendelian Segregation

• Alleles segregate in a 11 ratio in gametes
  produced by heterozygotes
• Meiotic Drive non-random segregation of alleles
  into gametes

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Segregation Distorters

• Drosophila spp.
• sd prevents devt of sperm with allele
• sd/ have low fertility
• sd has advantage genes at other loci suffer
• selection for suppressors recombination

sd
90 10
sd/
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Paternal Sex Ratio Trait

• Parasitic wasp (Nasonia vitripennis)
• Father-son transmission
• Impossible because M are haploid?
• PSR on B chromosome
• B small, unusual, nonessential chromosomes that
  don't go through meiosis normally
• high meiotic drive most sperm get B chromosome

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B Chromosome

• all other chromosomes in sperm supercondense so
  lost in mitosis
• sperm carries only B chromosome
• sperm empty of all other genes than PSR
• Ultimate selfish gene copies itself while
• destroying all other genes !!

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Intragenomic Conflicts
some alleles gain fitness at the expense of genes
at other loci

• SOMA
• Mutations die out

• GERM LINES
• Mutations inherited

If somatic lines give rise to germ
cells selection for cell lines
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Unit of Selection is unit that benefits from
adaptation

• But adaptation benefits
• Genes
• Cells
• Individuals
• Kin groups
• Unrelated groups

What unites these? Units that show adaptation
are units that show heritability
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Heritability

• Proportion of variation in a phenotype in a
  population attributable to individual differences
  in genotype
• Related to the genetic phenotypic makeup of a
  population

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Heritability
e.g. Eye colour
HIGH
e.g. Number of eyes
LOW
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Group Selection

• Group fitness cannot be inherited
• Thus, group selection cant work
• Unit of selection is entity whose frequency
• is adjusted by natural selection over
• generations

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Richard Dawkins

• entity that replicates itself with fidelity is
  stable through time is the gene
• organism is vehicle
• gene is replicator
• organisms fitness affects frequency of genes
  over time
• adaptations exist b/c they ? repro of an allele
  relative to other alleles by ? the fitness of the
  vehicle