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BIO102
Subunits 5.4.1 and 5.4.2
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.i 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 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.ii 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.
Charles Lyell
Uniformitarianism contrasts with the more
contemporary alternative hypothesis called
Punctuated Equilibrium. Niles Eldredge and
Stephen Jay Gould popularized this theory in 1972.iii
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 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.iv 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.v
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
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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
gradual winding path and an end result of multiple
species at the tips.
long
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 Lohmannvi
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).
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
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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.
i
M. Pidwirny and S. Jones, Concept of Uniformitarianism, 2010,
http://www.physicalgeography.net/fundamentals/10c.html.
ii
G. Cengage, World of Earth Science, 2003,
http://www.enotes.com/uniformitarianism-reference/uniformitarianism-177611
iii
PBS, Punctuated Equilibrium, n. d.,
http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
iv
D. O’Neil, Pre-Darwinian Theories, 2012,
http://anthro.palomar.edu/evolve/evolve_1.htm
v
PBS, Punctuated Equilibrium, n. d.,
http://www.pbs.org/wgbh/evolution/library/03/5/l_035_01.html
vi
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.
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:Punctuated-equilibrium.svg
Foraminifera Shells:
http://commons.wikimedia.org/wiki/File:Benthic_foraminifera.jpg
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