Modern Evolutionary Theory

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Modern Evolutionary Theory I. Post-Darwinian
Facts II. Population Genetics III.
Post-Darwinian Theory
2
III. Post-Darwinian Theory A. Mutationist
School (1900-1930) Richard Goldschmidt T. H.
Morgan
large mutations are the major agent of
evolutionary change
3
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, an 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. pseudoobscura
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 population
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.
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Species and Speciation
D. melanogaster
D. simulans
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Species and Speciation I. Species Concepts
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Species and Speciation I. Species Concepts How
we define a species depends on the goal we have
in mind.
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Species and Speciation I. Species Concepts How
we define a species depends on the goal we have
in mind. Are we categorizing existing or fossil
organisms?
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Species and Speciation I. Species Concepts How
we define a species depends on the goal we have
in mind. Are we categorizing existing or fossil
organisms? Are we trying to understand
correlates between populations adapting to
different environments?
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Species and Speciation I. Species Concepts How
we define a species depends on the goal we have
in mind. Are we categorizing existing or fossil
organisms? Are we trying to understand
correlates between populations adapting to
different environments? Are we trying to
reconstruct phylogenies?
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Species and Speciation I. Species Concepts A.
Morphological Species Concept
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Species and Speciation I. Species Concepts A.
Morphological Species Concept -
Categorical/'essential' in a platonic sense
based on morphological similarity to a 'type'
specimen
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Species and Speciation I. Species Concepts A.
Morphological Species Concept -
Categorical/'essential' in a platonic sense
based on morphological similarity to a 'type'
specimen - Useful, but many species are
polymorphic and some sibling species are
indistinguishable morphologically.
H. erato
D. melanogaster (M)
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Species and Speciation I. Species Concepts A.
Morphological Species Concept -
Categorical/'essential' in a platonic sense
based on morphological similarity to a 'type'
specimen - Useful, but many species are
polymorphic and some sibling species are
indistinguishable morphologically. -
Nonetheless, for dead or fossilized specimens,
the phenotype is all we might have to analyze. As
such, there are ways of quantifying the phenotype
and defining "phenetic" species... by quantifying
the within-group phenotypic variation,
statistical analysis can ascertain whether a
novel individual lies within that typical range.
New Species!!
old species
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Species and Speciation I. Species Concepts A.
Morphological Species Concept -
Categorical/'essential' in a platonic sense
based on morphological similarity to a 'type'
specimen - Useful, but many species are
polymorphic and some sibling species are
indistinguishable morphologically. -
Nonetheless, for dead or fossilized specimens,
the phenotype is all we might have to analyze. As
such, there are ways of quantifying the phenotype
and defining "phenetic" species... by quantifying
the within-group phenotypic variation,
statistical analysis can ascertain whether a
novel individual lies within that typical range.
Problem... need a pretty good sample to describe
within-group variation with confidence.
old species
New Species?
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• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942

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• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potentially interbreeding
  populations that are reproductively isolated from
  other such groups"

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• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.

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• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.
• - Limitations

48

• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.
• - Limitations
• - Process may be continuous - where do you
  draw the "line" of isolation?

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• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.
• - Limitations
• - not applicable to asexual species

50

• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.
• - Limitations
• - not applicable to asexual species
• - hybridization occurs in nature, even between
  otherwise 'good' species. Natural variability is
  not strictly discontinuous, so pigeon-holing on
  any grounds will be wrong in some cases. It
  becomes a matter of degree. The best example are
  "Ring Complexes"...series of species which breed
  with neighboring species but the 'end' species do
  not. Salamanders in California, Gulls in
  circumpolar regions.

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• Ring Species

Divergence that correlates with geographical
distance can create interesting patterns on a
spherical globe, or around a geographical feature.
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• Ring Species

Divergence that correlates with geographical
distance can create interesting patterns on a
spherical globe, or around a geographical feature.
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• Ring Species

Divergence that correlates with geographical
distance can create interesting patterns on a
spherical globe, or around a geographical feature.
54

• Species and Speciation
• I. Species Concepts
• A. Morphological Species Concept
• B. Biological Species Concept - Mayr 1942
• "Groups of actually or potetially interbreeding
  populations that are reproductively isolated from
  other such groups"
• - Biological units are genetically defined
  reproductive isolation makes populations
  different from one another, creating new units.
  So, reproductive isolation is the key
  characteristic of a species.
• - Limitations
• - not applicable to asexual species
• - hybridization occurs in nature, even between
  otherwise 'good' species. Natural variability is
  not strictly discontinuous, so pigeon-holing on
  any grounds will be wrong in some cases. It
  becomes a matter of degree. The best example are
  "Ring Complexes"...series of species which breed
  with neighboring species but the 'end' species do
  not. Salamanders in California, Gulls in
  circumpolar regions.
• - Allopatric populations Potential
  interbreeding means that populations that are
  spatially separated and morphologically/geneticall
  y distinct may be in the same species.

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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat)
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation

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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates
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Species and Speciation I. Species Concepts II.
Recognizing Species III. Making Species -
Reproductive Isolation A. Pre-Zygotic
Barriers 1. Geographic Isolation (large scale or
habitat) 2. Temporal Isolation 3. Behavior
Isolation - don't recognize one another as mates
4. Mechanical isolation - genitalia don't fit

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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg

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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation 1. Genomic
Incompatibility - zygote dies
63
Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation 1. Genomic
Incompatibility - zygote dies 2. Hybrid
Inviability - F1 has lower survival
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation 1. Genomic
Incompatibility - zygote dies 2. Hybrid
Inviability - F1 has lower survival 3. Hybrid
Sterility - F1 has reduced reproductive success

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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation A.
Pre-Zygotic Barriers 1. Geographic Isolation
(large scale or habitat) 2. Temporal Isolation
3. Behavior Isolation - don't recognize one
another as mates 4. Mechanical isolation -
genitalia don't fit 5. Gametic Isolation -
gametes transfered but sperm can't fertilize egg
B. Post-Zygotic Isolation 1. Genomic
Incompatibility - zygote dies 2. Hybrid
Inviability - F1 has lower survival 3. Hybrid
Sterility - F1 has reduced reproductive success
4. F2 breakdown - F1's survive but F2's have
incompatible combo's of genes
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation III.
Speciation
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation III.
Speciation Speciation is not a goal, or a
selective product of adaptation. It is simply a
consequence of genetic changes that occurred for
other reasons (selection, drift, mutation, etc.).
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation III.
Speciation A. Modes
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Species and Speciation I. Species Concepts II.
Making Species - Reproductive Isolation III.
Speciation A. Modes 1. Allopatric
Divergence in geographically separate
populations - Vicariance - range divided by new
geographic feature
A
B
C
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III. Speciation A. Modes 1. Allopatric
Divergence in geographically separate
populations - Vicariance - range divided by new
geographic feature - Peripatric - divergence of
a small migrant population
A
B
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III. Speciation A. Modes 1. Allopatric
Divergence in geographically separate
populations - Vicariance - range divided by new
geographic feature - Peripatric - divergence of
a small migrant population 2. Sympatric
Divergence within a single population
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2. Sympatric Divergence within a single
population Maynard Smith (1966) - hypothesized
this was possible if there was disruptive
selection within a population - perhaps as a
specialist herbivore/parasite colonized and
adapted to a new host.
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3. Sympatric Divergence within a single
population Maynard Smith (1966) - hypothesized
this was possible if there was disruptive
selection within a population - perhaps as a
specialist herbivore/parasite colonized and
adapted to a new host. Example Hawthorn/Apple
Maggot Fly (Rhagoletis pomonella)
Hawthorn maggot fly is a native species that
breeds on Hawthorn (Crataegus sp.)
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3. Sympatric Divergence within a single
population Maynard Smith (1966) - hypothesized
this was possible if there was disruptive
selection within a population - perhaps as a
specialist herbivore/parasite colonized and
adapted to a new host. Example Hawthorn/Apple
Maggot Fly (Rhagoletis pomonella)
Europeans brought apples to North America. They
are in the same plant family (Rosaceae) as
Hawthorn.
75
3. Sympatric Divergence within a single
population Maynard Smith (1966) - hypothesized
this was possible if there was disruptive
selection within a population - perhaps as a
specialist herbivore/parasite colonized and
adapted to a new host. Example Hawthorn/Apple
Maggot Fly (Rhagoletis pomonella)
Europeans brought apples to North America. They
are in the same plant family (Rosaceae) as
Hawthorn. In 1864, apple growers noticed
infestation by Apple Maggot flies...which were
actually just "hawthorn flies"...
76
3. Sympatric Divergence within a single
population Maynard Smith (1966) - hypothesized
this was possible if there was disruptive
selection within a population - perhaps as a
specialist herbivore/parasite colonized and
adapted to a new host. Example Hawthorn/Apple
Maggot Fly (Rhagoletis pomonella)
races breed on their own host plant, and have
adapted to the different seasons of fruit
ripening. Only a 4-6 hybridization
rate. Temporal, not geographic, isolation.
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III. Speciation A. Modes B. Speciation
Rate
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III. Speciation A. Modes B. Speciation
Rate Some characteristics should increase the
chances that a species radiates....
79
III. Speciation A. Modes B. Speciation
Rate Some characteristics should increase the
chances that a species radiates.... - high
dispersal capacity (increase geographic
isolation)
80
III. Speciation A. Modes B. Speciation
Rate Some characteristics should increase the
chances that a species radiates.... - high
dispersal capacity (increase geographic
isolation) - small (more likely to become
isolated)
81
III. Speciation A. Modes B. Speciation
Rate Some characteristics should increase the
chances that a species radiates.... - high
dispersal capacity (increase geographic
isolation) - small (more likely to become
isolated) - produce lots of offspring (more
successful colonists)
82
III. Speciation A. Modes B. Speciation
Rate Some characteristics should increase the
chances that a species radiates.... - high
dispersal capacity (increase geographic
isolation) - small (more likely to become
isolated) - produce lots of offspring (more
successful colonists)
INSECTS in general, and Beetles in particular....
they fly (disperse), but poorly (don't
return...isolation). And they are tough as nails,
so they can survive to wherever they disperse.
83
III. Speciation A. Modes B. Speciation
Rate - so speciation can be instantaneous -
speciation can occur as a function of selective
pressure and pop size (drift and selection in
peripatric speciation) - and the rate can be
influenced by the characteristics of the species,
themselves. So, although genetic change may
be constant within genes, the efftc that these
changes have on reproductive isolation can vary
dramatically... speciation is not a
uniformitarian process...