How-To-Do-It Modeling Modes of Evoluton: Comparing Phyletic Gradualism& Punctuated Equilibrium WilliamF. McComas BrianJ. Alters Downloaded from http://online.ucpress.edu/abt/article-pdf/56/6/354/46876/4449851.pdf by guest on 28 September 2021 This activity provides students an The latter pattern explains that the cule is the name of the formation or opportunity to explore the tempo and "gaps" in the fossil record are not layer in which it was found. If the mode of evolution by analyzing data simply missing data that will show up specimen is listed as "Upper Walla- and constructing two evolutionary some day-as maintainedby gradual- cian," it was found in the upper, or trees, one gradualisticand one punc- ists-but are real and must be inter- more recent part, of the layer called tuated. The data are fictitious, as are preted as such. the "WallacianFormation." At the left the creaturesused as illustrations,but of each stratigraphiccolumn are num- are representativeof real data. Intended Audience bers representing the number of thou- * General biology sands of years that it took to form that EvolutionaryPrinciples * Advanced biology particular layer. Following a discus- sion of the issues mentioned Illustrated student here, * Tempo and mode of evolution Materials (for each students should follow the specific in- structionsbelow. * Determinationof speciation group) * Copies of the caminalcules in the Introduction genus Molluscaformisand in the genus Specific Instructions "Paleontologists have discovered Pedivarious(see pp. 358-359). (Differ- 1. Workingwith one genus at a time, two major patterns in life that make it ent colors of paper will be useful.) each student group should arrange difficult to support a totally uniformi- * Copies of geologic columns for the the caminalculeson the appropriate tarian view of life's development" sites where samples were found (see stratigraphic column by placing (Benton 1993,p. 100). These two views pp. 360-361). (If enlarged 135%, each individual in the stratum (lay- are known as "phyletic gradualism" these charts will fit neatly on legal er) in which it was found. and "punctuatedequilibrium." size paper.) 2. Next, the species in the genus Phyletic gradualism is the tradi- * Scissors should be arranged into a logical tional Darwinianview that an intermi- * Graph paper (optional) morphology versus time tree (Fig- nable number of intermediate forms Procedure ure 1). Note: It is best if the stu- have existed, linking together all spe- dents do not see these example cies in each group by gradationsas fine Each student group should have trees priorto constructingtheir own. the caminalcule as our existing varieties(Darwin 1975). photocopies of both 3. Draw the genus evolution tree on a strata Punctuated equilibrium, developed genera and the accompanying morphology versus time axis (Fig- cut out all by Niles Eldredge and Stephen Jay sheets. The students should ure 2). Place the correct time units related Gould (1972),offers a contrastingview the caminalcules, keeping the on the Y-axis.Morphological change that organic evolution is not steady data attached. Each caminalcule pro- will have to be estimated(no units). the and regular but episodic and jerky, vided represents morphological 4. Repeatthe previous steps for Genus of a number of caminalcules with long periods of small changes average Pedivarious.The Genus Pedivarious at a location. interspersed with rapid bursts of found particular tree should look like Figure3. number is located large-scale transformationof species. The "averageof" 5. To understand punctuated equilib- each caminalcule in parenthe- below rium, one must examine it point- ses. For one caminalcule example, by-point with Darwin's view of be representedby an average of might phyletic gradualism. Have the stu- William F. McComas, Ph.D., is Director of finds. This although four information, dents make a comparisonlist of the Science Education Programs and Assis- to the stu- fictitious, is provided help two trees. The two of evo- tant Professor at the Universityof South- there some patterns dent understand that is lution with implicationsfor each ern California, School of Education, WPH within the along morphological variation are contrastedin a tableon p. 335. IOOIE,Los Angeles, CA 90089-0031. Brian specimens found at a given site and J. Alters is a Ph.D. candidate in science 6. Have students define the following that conclusions are based on a range educaflon at the Universityof Southern with referenceto theirproposed trees: California. He can be reached at 3419 of specimens rather than on a single Via Lido, Newport Beach, CA 92663. individual. * Transformation The name listed with each caminal- * Speciation 354 THEAMERICAN BIOLOGY TEACHER, VOLUME 56, NO. 6, SEPTEMBER1994 STRATIGRAPHICSEQUENCE FOR THE GENUS MOLLUSCAFORMIS GENUS MOLLUSCAFORMIS Formation Duration Tiime Owenian 250,000 Lyellian 300,000 | ___ _ Downloaded from http://online.ucpress.edu/abt/article-pdf/56/6/354/46876/4449851.pdf by guest on 28 September 2021 Morphiology Smithium 150,000 Figure 2. Evolutionary tree of the ge- nus Molluscaformiswith morphological KiZ characteristics plotted against time. _ _ Huttonean 100,000 C) equilibrium, divide the class into two groups. Students should read some of the background materials detailing the scientific merit of each evolution- Playfairon 80,000 ary pattern. General Review * Alters, B.J. & McComas, W.F. (1994). * Lewin (1980). Linnean 175,000 Pro * Gould, S.J. (1977, 1991). Con * Whitfield, P. (1993). * Wilson, E.O. (1992). Mayrian 200,000 __l Important Considerations * Fossils may be broken, distorted and/or have parts missing. * Only 10% of geologic time is pre- Wallacian 50,000 ___ _ served in sedimentary layers (Van Andel 1981). * Paleontologists generally decide if fossils are of differing species by comparing them to similar living or- Darwinian 100,000 | ' ganisms. Figure 1. A completed chart showing the placement of members of the genus Debate and/or Discussion Molluscaformisarranged by morphologicalcharacteristics and the layer in which Topics each sample was found. Some phyletic gradualists would state that the nine layers in the Pedi- various evolutionary sequence are not * The geologicalmeaning of fast & * Strata complete (Figure 4). Maybe little or no slow * Morphology. rock was formed in a period between * Evidence Gouldian and Eldredgean, and conse- * The questionof how paleontologists Discussion quently there are no fossils repre- decide if organismsare of different sented from this period. Therefore, species After making the basic comparisons the actual evolution of the genus Pedi- * Lineageof descentwith modification of phyletic gradualismand punctuated various could be gradual! GRADUAUSMAND PUNCTUATEDEQUIUBRIUM 355 STRATIGRAPHIC SEQUENCE FOR THE GENUS PEDI VARIOUS GENUS PEDIVARIOUS Formation Duration Lamarckian 75,000 Time Aristotelian 100,000 Simpsonean 200,000 Downloaded from http://online.ucpress.edu/abt/article-pdf/56/6/354/46876/4449851.pdf by guest on 28 September 2021 Morphology Figure 4. Evolutionary tree of the ge- Mendelian 100.000 nus Pedivarious with morphological characteristics plotted against time. applied basic evolutionary principles Eldredgean 35,000 and designed these creatures to be used in teaching various aspects of evolutionary biology. We have modi- fied two of the caminalcules for use in Gouldiar; 50,000 __ .. the activity presented here. We would also like to thank Susan Lafferty, science education specialist of the Los Angeles County Museum of Natural History, for lending her artis- tic talents by drawing the modified caminalcules. Huxlian 250,000 References Alters, B.J. & McComas, W.F. (1994). Punc- tuated equilibrium: The missing link in evolution education. TheAmerican Biology Teacher,56(6). Raupian 100,000 Benton, M. (1993). Four feet on the ground. In S.J. Gould (Ed.), The bookof life. New York: W.W. Norton and Company. Darwin, C. (1975). Theorigin of species;In- troducedand abridgedby PhilipAppleman. New York: W.W. Norton. Eldredge, N. & Gould, S.J. (1972). Punctu- ated equilibria: An alternative to phyletic gradualism. In T.J. Schopf (Ed.), Models Mullerian 200,000 in paleobiology (pp. 82-115). San Fran- cisco: Freeman, Cooper. Figure 3. A completed chart showing the placement of members of the genus Gould, S.J. & Eldredge, N. (1977). Punctuated Pedivariousarranged by morphologicalcharacteristics and the layer in which each equilibria:The tempo and mode of evolution sample was found. reconsidered. Paleobiology,3, 115-151. Gould, S.J. (1991). Opus 200. Natural His- tory, 8, 12-18. Punctuationalistswould counter by the rocks . .. we think that it has now Lewin, R. (1980). Evolutionary theory un- stating that the gradualistsare arguing become an empiricalfallacy' (p. 115). der fire. Science, 210, 883-887. from lack of evidence. (This would be Van Andel, T. (1981). Consider the incom- a great place to have a discussion pleteness of the fossil record. Nature, 294, Acknowledgments 397-398. about the nature of science, such as: Whitfield,P. (1993).Fron so simplea beginning. What counts as scientificevidence?) As We acknowledge the contributionof New York:Macnillan Publishing Company. Gouldand Eldredge(1977) state, "phyl- the late J.R. Caminof the Universityof Wison, E.O. (1992). The diversity of life. etic gradualismwas an a prioriassertion Kansas who developed the fictitious Cambridge: The Belknap Press of Har- from the start-it was never 'seen' in organisms called "caminalcules."He vard University Press. 356 THEAMERICAN