Mechanisms of Evolution
Gradualism vs. Punctuated Equilibrium
How does it happen?
Two theories of how
evolution works:
Gradualism
Punctuated Equilibrium
#1
1. Natural selection gradually changes the average
features of a species by removing the “less fit”
individuals from the population.
2. If this process continues long enough, a single
species may change into a new species.
3. Evolutionary change occurs all the time and
is not concentrated in any one period.
4. “Missing links” in the fossil record are just
places where intermediate forms have not yet
been discovered.
#2
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Gradualism
Two possible ways from gradualism
PHYLETIC TRANSFORMATION
ALLOPATRIC SPECIATION
Phyletic Transformation
* Gradual accumulation of small genetic variations
preserved by natural selection
* A whole population imperceptibly could evolve into
a new species.
* Impossible to draw a clear line between the end of
the first species and the beginning of its descendant
species
* There would be a long period of intermediate forms.
Allopatric Speciation
Geographical or reproductive isolation of a part of the
population would allow it to evolve in a different direction
Possibly more rapidly than the main population.
If the isolated population is small, it might be very difficult to
find fossils of the intermediate stages
Allopatric speciation
of Species W into
species X due to the
isolation of a small
population of Species
W
Species X
Species Z
Species Y
Species Y
Time
Allopatric speciation
of species Y into
species Z due to the
isolation of a small
population of species
Y
Phyletic transition of species W
into Species Y due to the slow
gradual accumulation of mutations
in Species W
Species W
Evolution
Is there an alternative?
Another idea…
THE PUNCTUATED EQUILIBRIUM MODEL
They observed that the fossil record
gives a different picture for the
evolution
Stephen J
Gould
They claim that there were long periods of
stasis (4-10 million years) involving little
evolutionary change
Then occasional rapid formation of
new species
As little as 5,000 - 50,000 years
Niles Eldredge
#5
Punctuated Equilibrium
1. Evolutionary change is concentrated in “speciation
events.”—rapid periods of evolutionary change.
2. Most species remain mostly the same once they
have stabilized. This is called STASIS.
3. Speciation events are normally concentrated in small
populations separated from the main population.
4. Since populations undergoing evolution are
small, they may be missing from the fossil record
providing “missing links”.
Species Y
Species Z
Species X
Stasis
Species W
Stasis
Time
Rapid
speciation
Stasis
Rapid
speciation
Evolution
Rapid
speciation
Genetic Drift…only in specific cases:
The current population is drastically reduced, leaving survivors
with a different overall percentage of traits than the original
larger population.
Summary:
Gradualism: Created by Charles Darwin the theory of
Gradualism holds that large evolutionary changes in species
are the result of many small and ongoing changes and
processes .
Punctuated Equilibrium: Created by Niles and Gould (1972).
Large evolutionary change is attributed to relatively rapid spurts of
change followed by long period of little or no change.
Past Evolutionary Rates: First 3 billion years very gradual
until the Cambrian explosion when the evolutionary rates
soared.
Fossil Record: Cannot definitely prove or dismiss either
theory since it is incomplete.
Current Theory: A combination of both theories including
gradual and spontaneous change.