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As Darwin and Wallace were creating the theory of and , 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. popularized this concept in his famous book, Principles of , 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 . 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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The Saylor Foundation Saylor.org Page 1 of 5 is the concept of Catastrophism, which was first proposed by in 1796. Catastrophism states that many geologic features and histories could be explained by catastrophic events that often caused species .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 5 led to the formation of new species. Georges Cuvier Documenting the process of 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 . 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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The Saylor Foundation Saylor.org Page 2 of 5 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 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 (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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The Saylor Foundation Saylor.org Page 3 of 5 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 Science, 2003, http://www.enotes.com/uniformitarianism- reference/uniformitarianism-177611

3PBS, 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 : http://en.wikipedia.org/wiki/File:Punctuated- equilibrium.svg

Foraminifera Shells:

http://commons.wikimedia.org/wiki/File:Benthic_foraminifera.jpg

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