Modern Evolutionary Theory

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Modern Evolutionary Theory I. Post-Darwinian
Facts II. Population Genetics III.
Post-Darwinian Theory
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) Richard Goldschmidt T. H.
Morgan
large mutations are the major agent of
evolutionary change
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) B. The Modern Synthesis
(1940)
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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle)

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) after one generation of
random mating, and equilibrium is reached in
genotypic frequencies.

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright - plant and animal breeding -
statistical modelling of evolution - Drift -
'Wright' Effect

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright 3. 1930 - R. A. Fisher -
statistician - ANOVA - The Genetical Theory
of Natural Selection

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright 3. 1930 - R. A. Fisher 4. 1932
- J. B. S. Haldane - The Causes of
Evolution - Stressed the importance of
selection over mutation

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright 3. 1930 - R. A. Fisher 4. 1932
- J. B. S. Haldane 5. 1937 - T.
Dobzhansky - pop gen of D. psuedoobscura
inversions - Genetics and the Origin of
Species - 'isolating mechanisms'

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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright 3. 1930 - R. A. Fisher 4. 1932
- J. B. S. Haldane 5. 1937 - T.
Dobzhansky 6. 1942 - Ernst Mayr -
naturalist, not geneticist - influenced
Dobzhansky's interpretations - Systematics and
the Origin of Species - biological species
concept
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Post Darwinian Developments I. Genetics C.
Population Genetics 1. 1908-10 Hardy -
Weinberg - (Castle) 2. 1912-1988 Sewall
Wright 3. 1930 - R. A. Fisher 4. 1932
- J. B. S. Haldane 5. 1937 - T.
Dobzhansky 6. 1942 - Ernst Mayr 7.
1942 - 1950 Huxley, Stebbins, Simpson
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Post Darwinian Developments I. Genetics C.
Population Genetics D. 1940's The Modern
Synthetic Theory of Evolution Sources of
Variation Agents of Change Mutation N.S.
Recombination Drift - crossing
over Migration - independent
assortment Mutation Non-random Mating
VARIATION
look familiar?
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) B. The Modern Synthesis
(1940) C. Mayr's Contributions
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) B. The Modern Synthesis
(1940) C. Mayr's contributions 1. The
Biological Species Concept a. Mayr - Biological
species concept - defined species as "groups of
potentially reproducing organisms separated from
other such groups".
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) B. The Modern Synthesis
(1940) C. Mayr's contributions 1. The
Biological Species Concept 2. Peripatric
Speciation
Evolutionary change should be most dramatic when
the two most powerful agents (drift and
selection) are at work - when small groups of
colonists settle a new habitat
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III. Post-Darwinian Theory A. Mutationist
School (1900-1930) B. The Modern Synthesis
(1940) C. Mayr's contributions D. Eldridge
and Gould - 1972 - Punctuated Equilibrium
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
1. Consider a large, well-adapted population
VARIATION
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
1. Consider a large, well-adapted
population Effects of Selection and Drift are
small - (it's already "well adapted" and it is
large....) little change over time
VARIATION
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
2. There are always small sub-populations
"budding off" along the periphery of a species
range...(Peripatric speciation...)
VARIATION
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
2. Most will go extinct, but some may survive...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
2. These surviving populations will initially be
small, and in a new environment...so the effects
of Selection and Drift should be strong...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
3. These populations will change rapidly in
response...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
3. These populations will change rapidly in
response... and as they adapt (in response to
selection), their populations should increase in
size (because of increasing reproductive success,
by definition).
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
3. As population increases in size, effects of
drift decline... and as a population becomes
better adapted, the effects of selection
decline... so the rate of evolutionary change
declines...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
4. And we have large, well-adapted populations
that will remain static as long as the
environment is stable...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
5. Since small, short-lived populations are less
likely to leave a fossil, the fossil record can
appear 'discontinuous' or 'imperfect'
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
5. Large pop's may leave a fossil....
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
5. Small, short-lived populations probably
won't...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
6. So, the discontinuity in the fossil record is
an expected result of our modern understanding of
how evolution and speciation occur...
VARIATION
X
X
X
TIME
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- 1972 - Eldridge and Gould - Punctuated
Equilibrium
6. both in time (as we see), and in SPACE (as
changing populations are probably NOT in same
place as ancestral species).
VARIATION
X
X
X
TIME
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Modern Evolutionary Theory I. Post-Darwinian
Facts II. Population Genetics III.
Post-Darwinian Theory A. Mutationist School
(1900-1930) B. The Modern Synthesis (1940) C.
Mayr's Contributions D. Punctuated Equilibrium
(1972 - Eldridge and Gould) So, our modern
evolutionary theory PREDICTS that transitional
fossils should be rare, because most evolutionary
change is occurring in small, isolated
populations in new environments. This solves
Darwin's remaining dilemma regarding the
'incompleteness' of the fossil record, and
explains why we don't have intermediates for
every possible lineage.