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Speciation and Extinction
Allopatric Speciation (with spatial separation)
Sympatric Speciation (no spatial separation
Allopatric
speciation
• Involves a
physical
separation
• Separation leads
to different
selective
pressure
Allopatric separation  speciation
• This is like going away,
then coming back and
finding everyone has
changed
• (except it is over
generations of time)
• If directional selection is
large enough the
organisms will not be
able to breed
Sympatric Speciation
• Populations are not physically
or geographically isolated from
each other; they live in the same
place.
– Gene flow between the two
populations is interrupted through
mechanisms other than simply being
physically separated
Disruptive selection, eliminate
offspring of “mixed” breeds, and
bring about sympatric speciation
Sympatric, Ecological Speciation
• Organisms may
live quite near
each other, but in
different “micro”
portions of the
habitat
Spadefood toads; species are connected to
different soil types
• Organisms live near each other, but select
different “micro” portions of the habitat
Temporal Isolation
• Species mate at
different times
• Plants often have
different flowering
seasons
• Closely related
fireflies often mate
at different times of
the night
Behavioral Isolation
•Movie link
• Differences in courtship behavior, or in
chemical signals or vocalizations
• Differences in color morphology
Mechanical isolation
• Sexual organs of closely related sympatric
species are incompatible
Gamete Mortality
• Pre-zygotic mechanism
• Note: Gametes are specialized, and
usually perform well in the reproductive
tract of the opposite sex
• Pollen transferred to the stigma of another
species will not result in fertilization
Post Zygotic Isolation
• The zygote may die shortly after
fertilization
• Low hybrid viability
• Hybrid sterility
• Low hybrid fitness
Hybrid Sterility
• a mule is the offspring of a
male donkey and a female
horse
• Donkeys have 62
chromosomes,
• Horses have 64.
• Mules have 63
chromosomes which
cannot evenly divide.
Host-Race
• A population of a
species that becomes
reproductively isolated
from other populations
due to adaptation to a
specific host
• This “gallmaker” seems
to be undergoing
ecological speciation
• Populations have
adapted to two
goldenrod species
S. gigantean.
Female Goldenrod
Gallmaker
Polyploidization can produce
instant sympatric speciation
Hybridization
• Members of one
species mate with
members of another
species.
• Hybrids may be as
fit, or more fit than
the parental species
Non-geographical isolation
Pre-mating
Pre zygotic
Post zygotic
ecological
gamete
incompatibility
hybrid
inviability
behavioral
sperm/pollen
specificity
hybrid sterility
mechanical
hybrid fitness
Nature abhors a vacuum
• There have been 5 documented periods of
extinction
• Each of these has been followed by major
bursts of speciation
• KT Asteroid
Species Radiations
• Burgess shale
• From time to time, the world has
experienced a wide scale proliferation and
radiation of life
• Essentially all the major body plans
evolved in the Cambrian “explosion”,
documented in the Burgess shale
The geological history of Earth
• Geological evolution (full video)
• Sixty-five million years ago: The
Cretaceous period and Extinction
Gradualism
• Phyletic
gradualism :
The
assumption
that “missing
links are due
to failure to
find an
intermediate
fossil
Punctuated Equilibrium
• Large
populations are
relatively static
• Splinter small
populations
may evolve
quickly – and
leave no
evidence
• Small
isolated
populations
may appear
abruptly in
the fossil
record
Retail Extinction
• Parallel evolution:
“repeats” are more
likely explanations
for the similarity of
ichthyosaurs &
dolphins
• Extinction really
does happen
Balance of Life
• There exists a
balance between
the process of
speciation and
extinction