Exercise 19.2

What Does the Fossil Record Say about the Tempo of Evolution?

(This exercise is based on Hunt, G. 2007. The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages. PNAS 104: 18404–18408.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

Both evolutionary biologists and paleontologists have been intrigued by the nature of the patterns of evolution revealed in the fossil record. Not only are the patterns interesting in their own right, but they can also reveal information as to the processes of evolution.

Three broad patterns can describe a sequence of fossils over time. One possibility is directional change over time wherein the changes in size or shape are in a consistent direction. Another possible pattern is that of stasis, where the characters remain little changed over time. A third possibility is a random walk, wherein the characters change over time but not in a consistent direction. The difference between random walks and stasis is that under stasis, the trait change is bounded within certain limits, whereas in the case of a random walk, the trait can change quite substantially over the course of time through the accumulation of random changes.

QUESTIONS

Figure 1 Illustration of each of the following patterns: A = directional change, B = random walk, and C = stasis. (Hunt, G. The Relative Importance of Directional Change, Random Walks, and Stasis in the Evolution of Fossil Lineages. 2007. PNAS 104: 18404–18408. doi:10.1073/iti4707104 — http://www.pnas.org/content/104/47/18404.full. © [2007] National Academy of Sciences, U.S.A.)

Question 1. In which of the three cases is the scale of time the longest?

Question 2. In which of these cases is the ratio of the variation at any given time to the average magnitude of a change the largest?

As in many aspects of biology, no single pattern of evolution can describe all of the fossil record; thus, questions revolve around the relative frequencies of each of these patterns. These questions include: Which pattern predominates throughout the course of the fossil record as a whole? Do certain types of characters show different relative frequencies of these patterns as compared with other characters? Are particular patterns seen more often in certain taxa than in others? To address these questions, Gene Hunt, a paleontologist at the Smithsonian Natural History Museum, compiled an extensive array of fossil character sequences. The compilation included examples of shape, size, and other characters across a wide range of taxa including planktonic microfossils (foraminifera, conodonts, radiolaria), benthic microfossils (foraminifera, ostracodes), and macrofossils (mammals, mollusks, trilobites, teleost fish). Hunt subjected these fossil sequences to a model he had developed that was designed to determine how well each pattern (directional change, stasis, or random walk) was best supported by the data. The details of the model construction are beyond the scope of this exercise. For each fossil sequence, each model received a score from 0 to 1 showing how well it was supported by the data, and these support scores summed to 1. Thus, if a fossil sequence scored 0.42 for stasis and 0.18 for random walk, it would have scored 0.40 for directional change.

Table 1 The number of times each model was the best-supported model by the fossil sequences. The data are broken down with respect to trait type (size, shape, other) as well as by fossil group (planktonic microfossil, benthic microfossil, and macrofossil). Hunt, G. The Relative Importance of Directional Change, Random Walks, and Stasis in the Evolution of Fossil Lineages. 2007. PNAS 104: 18404–18408. doi:10.1073/iti4707104 — http://www.pnas.org/content/104/47/18404.full. © [2007] National Academy of Sciences, U.S.A.)

Question 3. Are trait shape and trait size usually best supported by the same model? If so, which one? If not, explain.

Question 4. In about what percentage of the cases does the directional change model best support fossil sequence change overall? Does this differ much between the size and the shape types of trait change?

Question 5. Of the planktonic microfossils, the benthic microfossils, and the macrofossils, which of the following is most often best supported by the stasis model?

Figure 2 The distributions of support strength for the directional change, random walk, and stasis models for shape traits (in black) and size traits (in gray). (Hunt, G. The Relative Importance of Directional Change, Random Walks, and Stasis in the Evolution of Fossil Lineages. 2007. PNAS 104: 18404–18408. doi:10.1073/iti4707104 — http://www.pnas.org/content/104/47/18404.full. © [2007] National Academy of Sciences, U.S.A.)

Question 6. How is the pattern of support for stasis different between the size and shape traits? Do cases receiving intermediate support for stasis predominate for either the size or the shape traits?

Question 7. Are there more cases when directional selection receives intermediate or high (above 0.2) support for the size traits or for the shape traits?

Question 8. What type(s) of natural selection can result in stasis in the fossil record?

Question 9. Perhaps the most surprising result of Hunt’s study is the rarity of cases where directional selection receives high support. Can you provide a reason why the lack of support for directional change may be an artifact of the time scale of the fossil sequence?