Download Uniformitarianism versus Punctuated Equilibrium

Uniformitarianism versus Punctuated Equilibrium
As Darwin and Wallace were creating the theory of evolution and
natural selection, they started to
wonder about how quickly this
process happens and at what
pace. There are two main
hypotheses today regarding the
tempo of evolution, one of which
is called Uniformitarianism.
Charles Lyell popularized this
concept in his famous book,
Principles of Geology, in 1830.1
Uniformitarianism simply states
that the processes and natural
laws that happen today happened
throughout the past and the
universe. In other words, the way
we see things today is the way
Charles Lyell
they have happened throughout
time. Nested within the theory of Uniformitarianism is the concept
of Gradualism. James Hutton first proposed the concept
of Gradualism in 1795.2 This theory states that profound change
is a product of slow but continual changes over time. Gradualism
was incorporated into Uniformitiarianism by Lyell and profoundly
influenced Darwin’s thinking.
Uniformitarianism contrasts with the more contemporary
alternative hypothesis called Punctuated Equilibrium. Niles
Eldredge and Stephen Jay Gould popularized this theory in
1972.3 Punctuated Equilibrium states that in general, species will
go through periods of stasis (no to little evolutionary change)
punctuated by periods of radical change. Nested within this theory
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is the concept of Catastrophism, which
was first proposed by Georges Cuvier
in 1796. Catastrophism states that
many geologic features and life
histories could be explained by
catastrophic events that often caused
species extinctions.4 This theory later
influenced the theory of Punctuated
Equilibrium as
it explains the periods of episodic
change. According to Eldredge and
Gould, these periods of radical change
led to the formation of new species.5
Georges Cuvier
Documenting the process of speciation underlies the importance
(and much of the controversy) surrounding these two hypotheses.
If, in fact, the Uniformitarianism model is correct, then species
evolve over steady periods of time with
gradual changes. Alternatively, if the
theory of Punctuated Equilibrium model
is correct, then species generally
evolve after periods of violent episodic
change. When considering the
ancestral relationships among
organisms and these potential
speciation mechanisms, each
hypothesis can be represented by a
phylogenetic tree.
The top tree represents the hypothesis
of Uniformitarianism (Gradualism), and
the common ancestor to the species is represented, followed by a
long gradual winding path and an end result of multiple species at
the tips.
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Alternatively, the bottom tree represents the hypothesis of
Punctuated Equilibrium whereby the common ancestor is at the
bottom of the figure and the evolutionary path of this species is
punctuated by rapid drastic evolutionary events that led to the
formation of multiple species at the tips of the tree.
Although the fossil record gives evidence for both hypotheses, the
more accept hypothesis today focuses on periods of gradual
change punctuated by events of rapid change. As Darwin was
considering his concept of evolution, the lack of transition fossils
(intermediate species) troubled him. He attributed this to gaps in
the fossil record (which do exist), but over the years the fossil
record has filled in and yet evidence for these periods of rapid
change have appeared.
The strongest support for Punctuated Equilibrium comes from the
smallest of organisms, Foraminifera (Forams). Forams are among
the most common marine
plankton, and they can vary
greatly in shapes and forms.
They have an abundant fossil
record that began over 540
million years ago. In 1983,
Malgream, Berggren, and
Lohmann6 published a study
which focused on the shell
shape of the Globorotalia
tumida lineage of planktonic
Foraminifera. They found that
the shell shape of the
Foraminifera over 10 million
years went through periods of stasis, punctuated by rapid
episodes of change (see the following figure).
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Another consideration when
discussing the mode and tempo of
evolution and speciation is the
concept of relative time.
Contemporary scientists have
evidence for evolutionary process
within a human lifetime (e.g.,
bacteria and plants), yet the fossil
record operates at the level of
millions of years. Therefore, it
should come as no surprise that we
find evidence for what may seem
like rapid evolution in the fossil
record. Although the controversy over the best model for evolution
is far from over, this data should be considered when sorting out
which hypothesis will ultimately prevail.
1
M. Pidwirny and S. Jones, Concept of Uniformitarianism, 2010,
http://www.physicalgeography.net/fundamentals/10c.html.
2
G. Cengage, World of Earth Science, 2003, http://www.enotes.com/uniformitarianismreference/uniformitarianism-177611
3
PBS, Punctuated Equilibrium, n. d.,
http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
4
D. O’Neil, Pre-Darwinian Theories, 2012,
http://anthro.palomar.edu/evolve/evolve_1.htm
5
PBS, Punctuated Equilibrium, n. d.,
http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
6
B. A. Malmgren, W. A. Berggren, and G. P. Lohmann, “Evidence for Punctuated
Gradualism in the Late Neogene Globorotalia Tumida Lineage of Planktonic
Foraminifera,” GeoScience World 9(3, 1983): 377–389.
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Pictures:
Cuvier: http://commons.wikimedia.org/wiki/File:Bolton-cuvier.jpg
Lyell: http://commons.wikimedia.org/wiki/File:Bolton-lyell.jpg
Phyletic Gradualism: http://en.wikipedia.org/wiki/File:Punctuatedequilibrium.svg
Foraminifera Shells:
http://commons.wikimedia.org/wiki/File:Benthic_foraminifera.jpg
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