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Purpose & Overview: Many evolutionary biologists and paleontologists have debated whether patterns of diversification through geologic time, like those seen in the Sepkoski Phanerozoic diversity curve, are deterministic or a result of stochastic (i.e., random) processes. Said another way, have patterns in evolutionary diversification been caused by intrinsic or extrinsic factors (e.g., newly evolved morphologic innovations or new ecologic opportunities leading to adaptive radiations) or can they be explained simply by random processes? One way to test these two alternative hypotheses is to generate diversity patterns mathematically in random computer simulations and then compare these stochastically generated patterns to real data-based patterns from the fossil record. If the two are sufficiently different (i.e., their means are shown to be unequal statistically), then the random, null hypothesis can be rejected and the alternative, deterministic hypothesis corroborated. This approach, whereby real patterns are compared to randomly simulated ones, is known as "probabilistic paleontology". Accepting a random, null hypothesis has serious implications for the history of life. If the patterns seen through time could be due to random processes exclusively, then our ideas about natural selection and species selection need to be seriously reconsidered. Perhaps these processes do exist, but they would be of little consequence when it came to long term trends in earth's biota. Some paleontologists have suggested that not only are deterministic factors important, but during types of innovation and ecological opportunity radiating clades should be bottom heavy â much more species-rich early in their history. Through subsequent history selection eliminates those species less able to compete while those better-adapted species persist. In such situations the tree of life more resembles a pruned bush with many low branches and few persisting higher up (Gould et al. 1987; Gould 1990). This exercise applies a probabilistic approach to testing causal hypotheses for the pattern of within-clade diversity through the Paleozoic. Metazoans (animals) radiated rapidly in the Cambrian and quickly populated the marine world. Paleontologists have long debated the cause of this radiation. Each person in the class looks at the diversification of 1 or more families of fossils within the Cambrian Fauna. Each student graphically represents within-family diversity data by compiling the ranges of genera through stages of the Paleozoic. Data for ranges of fossil genera are obtained from Sepkoski's compendium (2002) which is accessible through a couple of web sites (see reference list below). The class assumes a priori that randomly generated clade diversity diagrams, when the rates of speciation and extinction are equal, are symmetrical about the mean (Gould et al. 1987). Materials: 1. Laptop computers running Excel. One per student or fewer if students work in small groups. 2. Access to the Sepkoski database (see URLs below). 3. Sample fossils or images of members of the clade for which each student is compiling range data. 4. Geological time scale showing absolute ages by geologic stage. Procedure: 1. Each student or small student group is assigned one of the Cambrian Fauna clades from the list of 24. 2. Using the online Sepkoski database, the genera belonging to that clade and their stratigraphic ranges are obtained. The stage of earliest occurrence of a genus in that clade defines the clade's time of origin; the last occurrence of genus defines the clade's extinction. The time between the clade's origination through its extinction defines the temporal units on the histogram's x-axis. 3. In table form, the number of genera extant in each stage is recorded. This defines the number of occurrences along the histogram's y-axis. 4. The temporal mid-point of each stage defines the bin's central value. The file "Sample Clade Statistics" demonstrates how the descriptive statistics are obtained from these ages. The following files are uploaded as supportive teaching materials: 1. PowerPoint file introducing the exercise and providing the necessary background. Titled: Bottom Heavy Clade Presentation. 2. Handout given to students describing the exercise. Word file titled: Exercise 4 Clade Diversity Exercise. 3. Summary statistics for a collection of Cambrian Fauna clades generated by students. This is intended as sample results and shouldn't be shared with students. Excel file titled: Cambrian Clade Statistics. 4. Handout with a list of 24 Cambrian clades to be included in the analysis using the Sepkoski genus-level compendium. Word file titled: Cambrian Fauna Clades. 5. Excel file showing how descriptive statistics are calculated from the range data. Title: Sample Clade Statistics. 6. File containing genus-level range data for each of the clades included in the above handout. This could be provided to students at the onset or students could acquire these data on their own. Excel file titled: Cambrian Clades in Sepkoski Genus-level Compendium.
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