The Origins of Population Genetics and the Evolutionary Synthesis 1894-1944

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The Origins of Population Genetics and the
Evolutionary Synthesis1894-1944

• Evolution-- Summer 2007

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Three arguments--Argument 1

• Theories of inheritance and evolution are always
  built together
• Evo Devo-- the relatively new field combining
  evolution and ontogenetic development-- must
  overcome the effects of the Evolutionary
  Synthesis.
• A different theory of inheritance was built in
  the early 20th century in order to accommodate
  current understandings of Evolution by Natural
  Selection

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Three arguments--Argument 2

• The Evolutionary Synthesis had two distinct
  phases
• (1900-1931) methodological consolidation to
  produce a genetic description of evolution.
• (1932-1947) (apparent) theoretical and
  disciplinary consolidation. The second phase
  failed to include paleontologyand represents
  larger problems.

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Argument 3

• We are still paying a price for misunderstanding
  the scope of the Synthesis.
• Early mathematical models exclude certain
  questions and levels of evolution
• Non-Mendelian groups, either extinct
  populations or higher taxa
• Any evolutionary process that works above the
  species level
• Elements of Evo-Devo, particularly the
  possibility of a flexible relationship between
  genotype and phenotype.

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Foundations for Population GeneticsDarwins
Adaptation Problem

• Darwin and Incipient Adaptations
• - No explanation for the initial stages of
  adaptations
• - No possibility of assigning adaptive function
  to any given characteristic
• - (Repeated courses of adaptive change)

• Possible solutions
• 1) Heredity supplies small, adaptive variations
  (or ordered variations?)
• 2) Selection always organizes adaptation but
  thats all we can say.
• 3) Selection waits for large, adaptive
  variations

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Foundations of Population GeneticsIncipient
Adaptation and the 1896 Impasse

• William Bateson (1894) Materials For the Study of
  Variation (argument, evidence)
• Francis Galton (1890s) Biometry, sports and
  regression analysis (powerful methodology)
• Debate Between August Weismann and Herbert
  Spencer (1894-1896) (undermining opponents but no
  possibility of a crucial experiment)
• (1900 DeVries, experiment and Mendelian
  inheritance)

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The Synthesis, Part I, 1900-1932
Neo-Darwinism A theory without a method

• Three causal theories
• Neo-Darwinism
• Mutationism/
• Saltationism
• Neo-Lamarckism/
• Orthogenesis
• Two methods
• Biometry
• Mendelism

• All included natural selection. The problem was
  to explain
• 1) variation
• 2) adaptation
• 3) speciation
• -- without a complete or even workable theory
  of inheritance.

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1890-1900 Arguments for Mutation-driven evolution

• Francis Galton
• Populations and swamping
• Interbreeding among sports as an explanation
  for adaptation and speciation

• Hugo DeVries
• - Mendels system as an explanation for the
  behavior of sports in populations
• - Mendelian characters large, and adaptive
• - Evidence and experiment in Oenothera

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Heredity and exclusion(Method and Theory)

• Stable inheritance
• Gametes are impervious to outside influence
• Genes and morphology have in a fixed relationship
  (not a formal part of Weismanns idea)

• Neo-Lamarckism is excluded
• Orthogenesis (ordered, non-Lamarckian variation)
  is excluded
• Are these methodological- or theoretical
  consequences?

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R. A. Fisher, The Correlation between Relatives
on the Supposition of Mendelian Inheritance
(1918)

• A demonstration of method for biometricians the
  heritable causes of human height can be better
  described in Mendelian terms than by regression
  analysis among relatives.
• An argument for describing and investigating
  Darwinian (gradual, selection-driven) evolution
  within an operational description of heredity
  associated with mutationism/saltationism.

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Fisher, 1922-1930 Fitting Method to a
Neo-Darwinian Theory of Evolution

• On the Dominance Ratio (1922) made the
  effects of genes interaction negligible
  evolution can be described in terms of changing
  (single) gene frequencies and in a single
  stochastic equation.
• The Genetical Theory of Natural Selection
  (1930) was built to express Darwins qualitative
  theory in mathematical terms
• Evolution is selection-driven and gradual, but
  only in very large (Mendelian) populations.

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Sewall Wright,1931-1932 Alternative models of
heredity and evolution, and

• Heredity (1914-1921)
• Method of Path Co-efficients multiple regression
  analyses for all genes
• In theory possible to represent the effects of
  gene interaction
• In practice works only in Mendelian populations
  (as in pedigreed stocks)

• Shifting Balance Theory of Evolution
• More attention to population structure
• Evolution can happen at different rates
• Selection values for alleles are changeable, and
  selection may not be the most significant or
  constant force in evolution

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Wrights Evolution in Mendelian Populations
(1931) Agreement and Disagreement with Fisher

• Wrights mathematical results for single alleles
  in different scenarios agreed with Fishers
  results.
• Different heredity, different causal theory,
  agreement of mathematical results why forge that
  agreement?
• Neo-Lamarckism and Mutationism remained common
  and viable enemies of Neo-Darwinism as in 1900,
  the theory needed a method.
• Fisher and Wrights methodological agreement in
  1931 was the extent of their synthesis, and the
  (imperfect) catalyst for the Modern Synthesis.

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Part II, 1932-1947The Synthesis Scale and
Meaning of Wrights Fitness Surface
Diagrammatic representation of the field of gene
combinations in two dimensions instead of many
thousands (italics added) Wright- The roles of
mutation, inbreeding, crossbreeding and
selection in evolution, 1932
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Dobzhansky and Wright by 1937 The beginning of
the Modern Synthesis

• Genetics and the Origin of Species (1937)
• Foundational theoretical text for the Modern
  Synthesis.
• Foundational piece of disciplinary consolidation
  a naturalist applying population thinking to
  the wild.

• Genetics of Natural Populations Series
  (1937-1975)
• Dobzhansky designs experiments with Wrights
  help.
• Translation problems natural- to Mendelian
  populations genetic variability to population
  size

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Dobzhansky to Simpson (1937-1944) Fitness
Surfaces, Scale and population thinking

• Scale Equid genera

• Scale Niches

Simpson- Tempo and Mode in Evolution, 1944
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Part III, Post-synthesis consequences of the
conflating theory and method

• Paleontologys exclusion from the Synthesis
• Simpson as (mere) consistency argument
• Serious challenges to the Modern Synthesis in
  the 1970s and 1980s--
• Punctuated Equilibrium, Hierarchical
  Evolution, and an argument for a genuine
  distinction between micro- and macroevolution
• Adaptationist Programme remains intact, along
  with its philosophical problems.

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Part III, Post-synthesis consequences of the
conflating theory and method

• It may get even worse
• Commitment to a method built in 1931 limits
  investigable questions
• Causal theories of macro-evolution, or those
  attending to higher taxa are excluded.
• Continued and limiting oversimplification of the
  relationship between genotype and phenotype
  resistance to some of the most interesting
  findings in Evo-Devo.

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Part III, Post-synthesis consequences of the
conflating theory and method

• Phenotypes are, no doubt, more appropriate units
  for dealing with selection, whether between
  individuals or groups, but genotypes seem more
  appropriate for mutation or random drift. The
  choice, however, is practically irrelevant in
  connection with pictorial representation of
  changes that occur in populations.
• Sewall Wright- Surfaces of selective value
  revisited, 1988

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Lasting Questions for Evo Devo orThe continuing
value of the Evolutionary Synthesis

• Does the framework of mathematical population
  genetics allow us to study all elements of
  evolution in nature?
• Did Hox genes evolve in a way that population
  genetics can describe?
• Will the framework established during the
  Evolutionary Synthesis allow us to answer
  questions like these?