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Lecture Topic : Speciation
I. Importance
What are species?
How do they evolve?
II. Levels of Variation ~ microevolution
A) Variation within locally breeding populations
(=demes)
1) Environmental variation
2) Genetic variation
3) Genetic x Environment interaction = Adaptation
Levels of Variation (cont.)
B) Variation between populations or demes.
1) Environment
2) Genetic (common environment)
a. Selection
b) Drift
Balance between selection and drift.
c) Reciprocal exchange of individuals = migration
Population differentiation -->
No population differentiation -->
Levels of Variation (cont.)
C) Variation among GROUPS of populations
1. races
2. sub-species
3. often geographically isolated ~ lack of migration
between populations
4. incipient species??
Levels of Variation (cont.)
D) Development of ecotypes (plants) and ecological
races (animals) are produced by a
G x E interaction ~ adaptation
cline = trait exhibits gradual change along a gradient
1920s Turreson
Defined term ecotype from plant work
1940-1970s Clausen, Keck, and Heisey: tested if
ecotypic differentiation exists:
1) Reciprocal Transplant
2) Common Garden
Figure Clinal variation in a plant
Levels of Variation (cont.)
E) Variation between species
1) individuals appear very different
2) often reproductively isolated
3) individuals of a species appear more similar to one
another than to other species.
III. A. History of Definition
1) Type Concept
a. species is a special creation
b. variation around true type
"whether breed true"
III. History of Definition (cont.)
John Ray
First person who asked "what is a species?"
He noted there exist individuals that produce
similar looking offspring.....
Linneaus –early 1700’s
Species basic unit of organizing diversity with
binomial nomenclature Genus species
Thought elemental species from creator
because often garden varieties need constant
work to maintain (since artificially selected).
III. B. Modern Usage of term
1940 Biological Species Concept:
Evolutionary biologist Ernst Mayr = A species is defined
as groups of actively or potentially cross fertilizing
individuals which are isolated from other such groups.
Alternate Defn.: Individuals whose genes can recombine with
one another.
Alternate Defn.:Similar individuals which breed mostly among
themselves and which change as a unit through time.
Phenotype?
Breeding Behavior?
Evolutionary Unit?
Six Concepts of Species Compared
Figure Biological species concept is based on interfertility rather than physical similarity
Figure Galapagos tortoise
Figure Long-distance dispersal- Role of gene flow
Figure Long-distance dispersal
Problems with defining a species
• Are species reproductive units?
• Is a species a unit of evolution?
• V. Origin of Species
• Microevolution = Macroevolution is the SAME PROCESS: An
issue of scale and time!
1) Anagenesis = Slow and directional changes that
transform a species.
2) Cladogenesis = Splitting of species (divergence) to
form a new species or many species.
• Clade = group of species derived from a single common
ancestral species.
Figure Two patterns of speciation
Figure The biological species concept is based on interfertility rather than physical similarity
Modes of Speciation
• 1) Sympatric Speciation
• Evolution of reproductive isolation within a
local population (=deme)
• Likely infrequent but can occur.
• Species A and Species B
• Scale: 10 m square population
ABA
BAB
BAAB
BAB
A BA
Figure Two modes of speciation
Figure Sympatric speciation by polyploidy in plants: Allopolyploidy
Figure Sympatric speciation by polyploidy in plants: Autopolyploidy
Modes of Speciation
• 2) Allopatric Speciation~Geographic
Speciation
• Evolution of reproductive isolation within a local
population (=deme)
• Likely infrequent but can occur.
• Species A and Species B
• Scale: Mt. Range >1000 Km
BB
AA
Figure Two modes of speciation
Modes of Speciation
• 2) Allopatric Speciation (cont.) ~Geographic
Speciation
• geographical separation
• differences in habitat  populations exposed to
different selection pressures ->increasing
divergence over time
• OR peripheral or splinter populations at the edges
of the species range.
• Examples:
Figure Has speciation occurred during geographic isolation?
Figure Allopatric speciation of antelope squirrels in the Grand Canyon
Isolating Mechanisms or Reproductive
Barriers
Post-Zygotic Barriers (initial contact)
1) zygotes do not develop normally or die = reduced hybrid
viability
2) if zygotes develop normally they are sterile = reduced
hybrid fertility
3) Less fit or vigorous F1 progeny produced, AxB mating < fit
then AxA or BxB = heterozygote disadvantage
4) Hybrid breakdown= F1 hybrids okay but F2 and F3 etc.
hybrid progeny produced are less fit then AxA and BxB.
Isolating Mechanisms or Reproductive
Barriers
Pre-Zygotic Barriers (secondary contact)
1) Mechanisms
a) Ecological Isolation = 2 potentially interbreeding species
found in same area but different habitats.
Ex., tiger and lion
b) Temporal Isolation = 2 species mate at different times
during the year.
Ex., flowering time,
Isolating Mechanisms or
Reproductive Barriers
c) Behavioral Isolation = often species specific, courtship
behaviors etc.
Ex., Blue footed boobies
d) Mechanical Isolation = structural differences between
species prevent copulation
Ex., orchid pollinators
Figure Courtship ritual as a behavioral barrier between species
The role of sexual selection in the radiation of Hawaiian Drosophila species
Figure A summary of reproductive barriers between closely related species
End for Fall 2008………
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