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Modern Evolutionary Theory
I. Post-Darwinian Facts
II. Population Genetics
III. Post-Darwinian Theory
III. Post-Darwinian Theory
A. Mutationist School (1900-1930)
Richard Goldschmidt
T. H. Morgan
large mutations are the major agent of evolutionary change
III. Post-Darwinian Theory
A. Mutationist School (1900-1930)
B. The Modern Synthesis (1940)
Post Darwinian Developments
I. Genetics
C. Population Genetics
1. 1908-10: Hardy - Weinberg - (Castle)
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.
Post Darwinian Developments
I. Genetics
C. Population Genetics
1. 1908-10: Hardy - Weinberg - (Castle)
2. 1912-1988: Sewall Wright
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
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
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
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'
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
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
Post Darwinian Developments
I. Genetics
C. Population Genetics
D. 1940's: The Modern Synthetic Theory of Evolution
Sources of Variation
Recombination
- crossing over
- independent assortment
VARIATION
Mutation
Agents of Change
N.S.****
Drift****
Migration
Mutation
Non-random Mating
look familiar?
III. Post-Darwinian Theory
A. Mutationist School (1900-1930)
B. The Modern Synthesis (1940)
C. Mayr's Contributions
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".
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
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
- 1972 - Eldridge and Gould - Punctuated Equilibrium
VARIATION
1. Consider a large, well-adapted population
TIME
- 1972 - Eldridge and Gould - Punctuated Equilibrium
1. Consider a large, well-adapted population
VARIATION
Effects of Selection and Drift are small - (it's already "well adapted" and
it is large....) little change over time
TIME
- 1972 - Eldridge and Gould - Punctuated Equilibrium
VARIATION
2. There are always small sub-populations "budding off" along the
periphery of a species range...(Peripatric speciation...)
TIME
- 1972 - Eldridge and Gould - Punctuated Equilibrium
2. Most will go extinct, but some may survive...
VARIATION
X
X
X
TIME
- 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
- 1972 - Eldridge and Gould - Punctuated Equilibrium
3. These populations will change rapidly in response...
VARIATION
X
X
X
TIME
- 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
- 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
- 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
- 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
- 1972 - Eldridge and Gould - Punctuated Equilibrium
5. Large pop's may leave a fossil....
VARIATION
X
X
X
TIME
- 1972 - Eldridge and Gould - Punctuated Equilibrium
5. Small, short-lived populations probably won't...
VARIATION
X
X
X
TIME
- 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
- 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
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.