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Adaptationism and the Adaptive Landscape. Genomic imprinting, mathematical models, and notions of optimality in evolution. Overview. Adaptationism Zoom and Grain in the adaptive landscape Mathematical models of genomic imprinting. Adaptationism.

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Adaptationism and the Adaptive Landscape


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adaptationism and the adaptive landscape

Adaptationism and the Adaptive Landscape

Genomic imprinting, mathematical models, and notions of optimality in evolution

overview
Overview
  • Adaptationism
  • Zoom and Grain in the adaptive landscape
  • Mathematical models of genomic imprinting
adaptationism
Adaptationism
  • Primary role for natural selection in evolution
    • versus drift, historical and developmental constraints, etc.
  • Modern debate framed by the Sociobiology wars (Wilson, Dawkins, Lewontin, Gould, etc.)
  • Continuation with Evolutionary Psychology, but
  • Partial reconciliation in most fields
    • Tests of selection, contemporary systematics
types of adaptationism
Types of adaptationism
  • Empirical
    • Central causal role for selection
  • Explanatory
    • Selection answers the big questions
  • Methodological
    • Selection is a good organizing concept
        • Godfrey-Smith (2001)
the adaptive landscape
The Adaptive Landscape
  • Natural selection is conceived of as a hill-climbing algorithm
caveats
Caveats
  • Units (genotype vs. phenotype, population vs. individual fitness)
  • High dimensionality
  • Topology of the landscape
  • Dependence on other organisms
  • Hill-climbing metaphor implies a deterministic process
zoom level 1
Zoom level 1
  • High level analyses invoke rugged landscapes, which emphasize the role of historical contingency
zoom level 2
Zoom level 2
  • Intermediate levels of analysis focus on local regions with a small number of peaks, emphasizing optimization
zoom level 3
Zoom level 3
  • Low-level analyses reveal the discontinuities in the fitness landscape, emphasizing drift, recombination, etc.
zoom level 31
Zoom level 3
  • Low-level analyses reveal the discontinuities in the fitness landscape, emphasizing drift, recombination, etc.
sickle cell anemia
Sickle-cell anemia
  • HbA / HbA
    • Susceptible
  • HbA / HbS
    • Resistant
  • HbS / HbS
    • Sickle-cell

Resistant

parents

Susceptible

Resistant

Sickle-cell

population genetic timescale
Population-genetic timescale
  • Mendelian segregation recreates sub-optimal phenotypes every generation

HbA / HbS

HbA / HbA

HbS / HbS

~100 generations

mutation timescale
Mutation timescale
  • The mutation giving rise to the HbS allele represents a partial adaptation to malaria

HbA + HbS

HbA

~104 generations

chromosomal rearrangement timescale

HbAS

HbA + HbS

HbA

Chromosomal rearrangement timescale
  • A (hypothetical) rearrangement could give rise to a single chromosome containing both the HbA and HbS alleles. This new allele should sweep to fixation.

~108 generations

immune system evolution timescale
Immune-system evolution timescale
  • In principle, we could ask why our immune system is susceptible to malaria at all.

IgM

IgA

IgG

IgE

HbAS

Ig-

HbA + HbS

HbA

~1010+ generations

genomic imprinting
Genomic Imprinting
  • Non-equivalence of maternal and paternal genomes
  • Normal development in mammals requires both
genomic imprinting1

gene 1

gene 1

gene 2

gene 2

gene 1

gene 1

gene 2

gene 2

Genomic Imprinting

Oogenesis

Spermatogenesis

  • Epigenetic differences result in differences in expression
  • DNA methylation
    • reversible chemical modification of the DNA
slide20

Maternal optimum

Paternal optimum

Fitness increases as more resources are acquired for self

Inclusive fitness

Fitness decreases as cost to siblings becomes too great

Growth factor expression level

Asymmetries in relatedness

slide21

Conflict over resources

Growth-enhancing locus

Unimprinted

gene

Cis-acting

maternal

modifiers

Maternal expression

Maternal optimum

Cis-acting

paternal

modifiers

Paternal optimum

Paternal expression

slide22

Conflict over resources

Growth-suppressing locus

Unimprinted

gene

Cis-acting

maternal

modifiers

Maternal expression

Paternal optimum

Cis-acting

paternal

modifiers

Maternal optimum

Paternal expression

game theoretic stability analysis models of imprinting
Game-theoretic / stability analysis models of imprinting
  • X - expression level
  • Wm - matrilineal fitness
  • Wp - patrilineal fitness
  • U - individual fitness
  • V - fitness of other offspring
  • G - resource demand
  • C - cost of gene expression
  • 2p - fraction of mother’s offspring with the same father

Growth enhancer:

population genetic models
Population-genetic models
  • Two sibs, paternal imprinting
  • A - unimprinted allele
  • a - imprintable allele
  • a = A when maternally inherited
  • a -> (a) when paternally inherited
  • AA = aA
  • a(a) = A(a)
  • Fitness of unimprinted sibs: 1
    • e.g., AA, AA
  • Fitness if both imprinted: 1+u
    • e.g., a(a), A(a)
  • If only one is imprinted:
    • e.g., AA & A(a)
    • Imprinted fitness: 1-sfor A(a)
    • Unimprinted fitness: 1+tfor AA
population genetic models1
Population-genetic models
  • Parameters: allele frequencies, fitnesses, frequency of multiple paternity
        • Spencer, Feldman, and Clark 1998 Genetics
population genetic models2
Population-genetic models
  • Two sibs, paternal imprinting
  • A - unimprinted allele
  • a - imprintable allele
  • a = A when maternally inherited
  • a -> (a) when paternally inherited
  • AA = aA
  • a(a) = A(a)
  • Fitness of unimprinted sibs: 1
    • e.g., AA, AA
  • Fitness if both imprinted: 1+u
    • e.g., a(a), A(a)
  • If only one is imprinted:
    • e.g., AA & A(a)
    • Imprinted fitness: 1-sfor A(a)
    • Unimprinted fitness: 1+tfor AA
  • Monandrous females:
    • a invades A if u > s
    • a stable if u > t/2
  • Polyandrous females:
    • a invades A if s < 0
    • a stable if u > t/2
predictions and contradictions
Predictions and contradictions
  • Game-theoretic
  • Imprinting requires multiple paternity (p < 1/2)
  • Allele favoring lower expression will be completely silenced
    • maternal silencing of growth enhancers
    • paternal silencing of growth suppressors
  • Population-genetic
  • Particular combinations of s, t, and u can produce stable polymorphisms
  • Multiple paternity is not required
  • Maternal silencing for growth enhancers is more likely, but paternal silencing can occur
slide28

Paternally silenced growth enhancer

Growth-enhancing locus

Unimprinted

gene

Cis-acting

maternal

modifiers

Reduced paternal

expression would

be favored from

these points

Maternal expression

Cis-acting

paternal

modifiers

Maternal optimum

Paternal optimum

Paternal expression

key assumption
Key assumption
  • Game-theoretic models assume that the unimprinted expression level is at its optimum before the introduction of an imprinted allele
  • Is this assumption a good one?
  • Gene expression array analyses of population-level variation reveal a high level of variation
  • This implies a good opportunity for selection to find the optimum
separation of timescales in the evolution of imprinting
Separation of timescales in the evolution of imprinting

Imprinting opens up a new dimension in strategy space

Unimprinted alleles are restricted to a subspace in the fitness landscape

If mutations that quantitatively change gene expression are much more common than those that give rise to imprinting, imprinting will always arise in the context of an optimized expression level

take home message
Take-home message
  • Choice of a particular modeling framework implies certain assumptions that can affect your interpretation of your results
  • When smart people doing reasonable things disagree, there is probably something interesting going on