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A Mini-Lesson


Patterns, Trends, and
Rates of Change


Original Author Unknown




Fossil shells of a land snail are arranged by layers of age into a sequence pattern suggesting gradual change, or punctuated equilibria. Variation uses caminalcules in place of fossil shells.


1. Some evolutionary change is rapid and discontinuous, and some change is gradual.

2. Fossil patterns can provide clues to the past.

3. Speciation explains evolutionary branching and diversification.


 Handout with introduction, procedure, and assessment questions

Desktop Fossil Record (fossil shells arranged in layers)


Colored pencils or water-based markers (5 different colors)

Time/Morphology Sheet for fossil layout (Fig.1)

Time/Morphology Graph (Fig.2)

Sample Graphs, showing gradualism and punctuated equilibria (overhead)

Reference Graphs, showing morphological ranges in populations through time (overhead)


 (see end of lesson for the formatted handouts).

Information sheet (introduction, materials, procedure, assessment questions)

Desktop Fossil Record (sheet for making cutouts, or envelope containing cutouts done earlier)

Time/Morphology Sheet (Fig.1)

Time/Morphology Graph (Fig.2)


1. This lesson can be presented near the end of a unit which introduces the essential nature of evolution, or it could be used later in the course, in a more detailed treatment of the elements of speciation. In either case, it does deal with the real existence of two patterns of fossil sequences found in nature, and does provide one of several reasonable explanations for the apparent lack of transitional (intermediate) fossils as found in some cases.

2. Run off materials for handouts and overheads. Optionally, have cutout shells prepared in advance (or saved from previous use), in envelopes. Be sure that the fossils from each layer have a unique color, and that there is a color key identifying which color is from which layer. (This would save "busywork" time and need for scissors and colored pencils). Keep these for reuse period after period, and year after year.

3. Students should work in groups of 3-5.

4. Students are told that an outcropping of rock has recently been exposed on Grand Bahama Island (show them where this is on a map, if you can). The layer numbered one (on the "The Desktop Fossil Record" sheet) was closest to the surface, the other layers are in their corresponding sequence. Their job is to examine the fossils, work on various rearrangements, and attempt to show any patterns, trends, and rates of change over time.

5. On a graph (see figure 2), students should show with lines the pattern, trends, and rates of change they have discovered. They may need help understanding that any shifting left or right (horizontally) as they ascend the graph represents a change of appearance (morphology) over time (vertical).

6. As each group of students is comfortable with its graph, it should be displayed on the board (they are "publishing" their findings for all to see and discuss, just as scientists do).

7. When all groups have completed their graphs and placed them on the board, get the class to discuss similarities, differences, and trends.

8. Questions to help lead discussion:

a. What similarities do you see in the graphs?

b. What differences do you see in the graphs?

c. Is it possible for different scientists to interpret the same data differently? What factors might influence their interpretations differently?

d. What patterns do you see (if any) over time in the morphology of the shells found?

e. Does change take place over a relatively long, or relatively short period of time?

f. Are there any intermediate (or transitional) forms between the more dissimilar fossils?

g. Are there any periods of stability (no change, or "equilibrium") in the fossil record?

h. Evidence of change over time is abundant in the fossil record. It appears that species have actually changed over time, even giving rise to new species. This is called evolution. Can you think of any other explanation for this series of observations, that is consistent with the observations?

Extensions and Variations

1. A simple variation is included. Instead of using the fossil shells, substitute the "Caminalcules" for the "Desktop Fossil Record" sheet (See the "Caminalcules" sheet in PDF format at end of this lesson). Wherever the name "Cerion" is used in the original introduction, substitute "Caminalcules", and wherever "shells" is used in the original, substitute "creatures". Keep in mind that the Caminalcules are imaginary creatures, intended to be used in a classification or phylogeny simulation. [Caminalcules were created by Dr. Joseph H. Camin; please see full citation in the Attributions below.] The beauty of the Cerion version is that their shells were indeed found in sequential layers, as described. They're real fossils!

2. A somewhat more elaborate, but probably more realistic than #1, is what I like to call "A Peek at the Past: Fossil Patterns". This is slightly modified from an activity developed by William F. McComas and Brian J. Alters (1994). This was also published (with a little more detail, and copyable illustrations, in the handbook Investigating Evolutionary Biology in the Laboratory, edited by William F. McComas (1994). My version provides enlarged time scale sheets, which can be placed on the table top, and reduced "caminalcules" (two sets), which can be copied onto paper in two different colors, cut out by students, and placed into envelopes for re-use every period. There are also more detailed instructions, and a set of discussion questions to help students focus in the main points.

3. A different approach, with a somewhat different focus, but still looking at large paleontological patterns, is presented in a recent issue of the American Biology Teacher (April, 1999) by Anton Lawson (see reference below). The focus is more on critical thinking, forming testable hypotheses to challenge three "theories" about the origin of the diversity of life: spontaneous generation, special creation, and evolution. Many of the hypotheses can be tested simply by studying a sampling of fossils representative of the geological column, figuratively "gathered on a field trip into the Grand Canyon". Types and inexpensive sources of fossils you could use for this activity are presented in a companion article in the same issue of the ABT by James Platt (see reference). A sample handout for students, along with tips and strategies for the teacher are clearly presented. Looks like a winner. If you try this activity, please give us your feedback.

4. Use this simple "evolutionary tree" diagram (PDF format) to show:
.......a. how microevolution over time can result in macroevolution, ...
.......b. how classification relates to evolution, and...
.......c. the meaning of evolution.

5. SPECIAL NOTE: Click here to explore many of the different lines of evidence pointing to speciation and macroevolution.

6. MACROEVOLUTION DIAGRAM: See the Macroevolution Diagram and a page of directions for using that diagram on an overhead projector. This nicely shows how accumulated speciations can eventually form all the groups and subgroups of organisms. It also shows how classification is related to evolution. A very nice colorful version of this can be found on page 32 of that most useful resource: Teaching About Evolution and the Nature of Science, by the National Academy of Sciences (1998) (see our Resources section). A particularly interesting alternative diagram is the one Darwin included in The Origin of Species (chapter IV), the only diagram in that book! His discussion there of that diagram should be required reading for any biology teacher. Darwin's Tree makes a great overhead transparency for discussing his concept of evolution by natural selection, as well as how classification reflects that evolution.

7. For a better sense of the actual pattern of fossil evidence over time, strongly suggesting macroevolution of vertebrates (especially in middle school life science or earth science), try our Patterns in Time lesson. In that lesson, students also come to realize that the different vertabrate classes emerged separately over several 100s of millions of years, and did not exist prior to their emergence (as revealed in the fossil record). That lesson also demonstrates the accumulation of modified traits on top of the accumulated traits found in the previously emerged group, showing gradual, additive and mosaic changes over time. All of this provides a strong implication that each group descended from the earlier antecedents through gradual change over time.


McComas, William F. (ed) Investigating Evolutionary Biology in the Laboratory, 1994, National Association of Biology Teachers, pages 131-141.

McComas, William F. and Brian J. Alters. "Modeling Modes of Evolution: Comparing Phyletic Gradualism & Punctuated Equilibrium", The American Biology Teacher, vol.56, no.6, September 1994, pages 354-356).

Gould, Stephen Jay. "Opus 100", in The Flamingo's Smile. 1985. WW Norton & Co. Pages 167-184. This essay discusses the organisms which have been the subject of much of Gould's own research, land snails in the genus Cerion.. Reading this would provide substantial background and insight for the teacher doing the original lesson, and might even be of interest for any highly motivated student who wants to "dig further".

Gould, Stephen Jay. "The Paradox of the Visibly Irrelevant", Natural History magazine (the 12/97-1/98, issue), pp. 12-18, 60-66. You should be sure to read this excellent article. The author describes some recent studies of natural selection (including recent details of his own on Cerion), and reflects on their relevance in the context of geological time. The last two pages are especially interesting. He also comments on micro-/macro- evolution and punctuated equilbrium in this context. Teacher Project: This would be an excellent article to condense and develop a reading guide for it (series of focus questions and clarification comments) for student reading. See "Contrivances: the Panda's Thumb" lesson on this site as an example. If you do this, be sure to send us a copy for inclusion on this website, and credit to you. Gould also references the following article in Science:

Goodfriend, GA, and SJ Gould "Paleontology and Chronology of Two Evolutionary Transitions by Hybridization in the Bahamian Land Snail Cerion," Science, vol.274, 1996. Announced as "Snails Caught in the Act of Evolving".



Lawson, Anton E. "A Scientific Approach to Teaching About Evolution & Special Creation". The American Biology Teacher, vol.61, no.4, April 1999, pages 266-274).

Platt, James E. "Putting Together Fossil Collections for 'Hands-On' Evolution Laboratories". The American Biology Teacher, vol.61, no.4, April 1999, pages 275-281).


Some of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks.

1. Original Source of lesson: Unknown (used in the ENSIs)

2. The "Caminalcules" version uses imaginary creatures created by Dr. Joseph H. Camin (1922-1979), of the Dept. of Entomology at the Univ. of Kansas. They were first published by Dr. Larry Wiedman in 1991 in his Lab Manual for College Level Oceanography (ISBN 0-314-00473-4) and West Publishing. Dr. Wiedman is currently at the Univ. of Saint Francis, 2701 Spring St., Fr. Wayne, IN 47808 (e-mail lwiedman@sf.edu).

3. The Cerion version was Revised in 1992 by Mary Cage, Doug Ramezane, Joyce Roderick, and Joe Schiel. Revised in 1993 by Lucille Williams, Gerald Quissell, Pat Bayonne-Johnson, and Jim Rogers.

4. Reviewed / Edited by: Martin Nickels, Craig Nelson, Jean Beard: 12/15/97

5. Edited / Revised for website by L. Flammer 4/99


 The following sheet provides an introduction, procedure, and questions. A formatted copy is also available in the PDF format further below, along with PDF pages of diagrams and graphs.

Patterns, Trends, and Rates of Change


An outcropping of rock has recently been exposed on Grand Bahama Island. Several layers (strata) of fossils have been exposed. The fossils, arranged in the same layers where they were found, have been replicated on a separate sheet of paper ("The Desktop Fossil Record"). The top layer (closest to the surface) is labeled #1 on your lab sheet. The layers are in order of position as they were found in the wild.

Student Objectives:

You should be able to develop a graph showing possible phylogenetic relationships over time of the organism known as Cerion (a land snail), and demonstrate its pattern of evolution as gradualism, or punctuated equilibria.


Handouts, (and, if needed) scissors and colored pencils (5 different colors, one for each layer).


1. Work cooperatively in groups as assigned by your teacher. Be sure everyone has a part in each task. Give each member of your team a turn at placing shells. Look also for changes in the snails over time. Look for patterns and trends. Try to figure out if these changes were slow and gradual, or happened relatively fast. Discuss pros and cons of any difference of opinion encountered; try to reach consensus.

NOTE: If you are given an envelope labeled "Desktop Fossil Record: Shells", skip to step 4.

2. On the "Desktop Fossil Record" sheet, color the shells in each row a different color (e.g. red for layer #1 shells, blue for layer #2 shells, etc.)

3. Cut out each shell individually (as a small square, keeping the small letter next to each shell).

4. Place the shells in their appropriate layers on the Time/Morphology layout sheet (figure 1): only the first layer shells (top, or closest to the surface) should be placed in layer #1, at the top, and so on, for all layers.

5. Place similar-looking shells so that they tend to be more-or-less aligned in vertical columns, still keeping them in their assigned layers (where they were found). Their appearance is called their morphology, so you are shifting them horizontally according to their morphology.

6. Now for the final placement detail: study the shells in each vertical column. Starting at the bottom and moving up, layer by layer, give each shell a slight shift to left or right if it is a little different than the shell below it. Don't shift it at all if it's virtually identical; shift it more if there is a larger difference. You should begin to see a pattern of lines of shells, with some lines running almost vertically, and other lines at an angling up and to one side or the other. You may find that some lines of shells may show branching. That's ok.

7. On the special graph paper provided (Figure 2 Time/Morphology Graph), draw a few continuous lines to match the pattern of lines of shells as you arranged them on the layer diagram. Be sure to show branching lines if the pattern of shell types had branches.

8. From you graph, determine whether it shows gradual change, or abrupt changes alternating with little or no change (punctuated equilibria). See Figure 3 (Sample Graphs) for examples; these may be shown by your teacher on the overhead.

Assessment: (Answer on back, or in class discussion)

1. What do the lines on your graph represent?

2. What pattern do you see (if any) over time in the morphology of the shells left in the fossil record?

3. Does change take place over a relatively long or relatively short period of time?

4. Are there any intermediate or transitional forms between the dissimilar fossils? If so, in which layers?

5. Are there any periods of stability (little or no change) in the fossil record?


Some of these are fairly large files, so they may be slow loading.

 Handout Page: Intro, Procedure, Questions

Time/Morphology Layout Page, Figure 1

Time/Morphology Graph, Figure 2

Desktop Fossil Record: Shells (for cutouts) (Large, slow loading)

 Sample Graphs (Diagrams) Simple: Grad vs Punc.Equil.

Reference Graphs (3D) Population Range Shifts (VERY large, slow loading)

Desktop Fossil Record: Caminalcules (for cutouts) (Large, slow loading file)


 The following pages are in Adobe Acrobat pdf format in order to maintain their intended layout for easy printing of handouts. Only a "thumbnail" reduced size image of the first page is showing (if more than one page is in that file). For enlarging and copying, (and seeing other pages in that file), you will need to download the free Acrobat Reader from Adobe (unless it's already installed in your system). Then just click on the blue file name above, below, or next to the first page. You may see the "Acrobat Exchange" (Reader application) loading, then the pages will display. You might need to shift-click and drag the lower left corner of the page to enlarge it, or click the magnifying glass on the menu bar.

If this doesn't seem to work, you might need to load and/or enable the PDFViewer plug-in by following one of these protocols:

For Netscape Communicator: EDIT>Preferences>Navigator>Applications (then scroll down to "Portable Document Format (PDFViewer)", click on it, then click OK; if it's not there, click on "New", and add it in).

For Netscape Navigator: OPTIONS>General Preferences>Helpers (scroll to check for PDF on list, add it if it isn't, then click OK to activate it.

For other browsers, or problems with this, check with your browser tech support, Adobe tech support, or, in dire frustration, e-mail me. If nothing else, I will mail you hard copy of the formatted pages desired.

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