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Missing Link, The

Classroom Activity

Objective
To collect, analyze, and interpret information about objects in order to classify them into a cladogram.

• copy of "Hardware Organism Key" student handout (PDF or HTML)
• copy of "Cladogram Basics" student handout (PDF or HTML)

• copy of "Nailing Cladistics" student handout (PDF or HTML)
• identical plastic bags with one of each of the materials listed on the "Hardware Organism Key" student handout
• 11- x 17-inch sheet of paper

1. Biological organisms are traditionally classified according to like, or constant, characteristics. However, to show how organisms have evolved over time to be different, scientists sometimes develop a family tree of how they may have evolved, a method known as cladistics. (See Activity Answer for more information.) Students will use common nails, screws, and bolts to simulate the process of applying cladistics to living organisms or fossil life forms. Note: Point out that students' models will differ from how living organisms actually evolve—the inanimate objects they will be using already have a fixed set of traits and do not represent true biological evolutionary relationship that living organisms exhibit.

2. Collect the materials listed. (You may choose your own "organisms" and create your own organism key, if you prefer.)

3. For Part 1, assign students to teams and provide each team with a plastic bag of the materials and a copy of the "Nailing Cladistics" student handout and each student with a copy of the "Hardware Organism Key" student handout. Have each team first classify the organisms using a dichotomous key that organizes organisms by constant characteristics. Discuss each team's results and variations among team decisions.

4. For Part II, provide each student with a copy of the "Cladogram Basics" student handout, and each team with a large sheet of paper for its final cladogram.

5. Have students list the characteristics of the hardware organisms and make a table of all the traits. Have them analyze and use the information in the table to create rough drafts of their cladograms. Have each team prepare a final cladogram.

6. After all teams have finished, display the cladograms. Have each team explain its cladogram and reasons for where objects were placed. Did all teams agree? Discuss similarities and differences.

7. To conclude, have students compare cladistics and more traditional taxonomy. What are the advantages and disadvantages of each?

8. As an extension, have students add other fasteners, such as clamps, to the mix of objects to see how it changes the cladogram.

Cladistics is a way of sorting organisms based on characteristics that were derived from a common ancestor. Cladograms often do not follow the more traditional methods of animal classification. While traditionally dinosaurs might be considered reptiles and birds classified as aves, on a cladogram the two would share the same line. Scientists generally agree that today's birds are evolutionary descendants of the dinosaurs.

Sample Cladogram
The sample represents one possible way the organisms might have evolved; showing evolution by thread count would be another possible way of building the cladogram. In this sample, all organisms are cylindrical, have a head, and have the same pennyweight (10); these traits are known as plesiomorphic (original) and are common to all the organisms. The apomorphic (advanced) characteristics shown below represent the evolutionary characteristics of the organisms.

Some differences might be seen in final team cladograms. Emphasize to the students that to be correct, the evolutionary development must be the basis, that is, the nail came before the screw. Scientists prefer to use the simplest cladogram that gives all the information. Cladograms drawn by scientists evolve as scientists learn more and simplify them.

Characteristics: no (0), yes (1)
1 has cylindrical object with head
2 has partly threaded shaft*
3 has completely threaded shaft
4 has flat bottom
5 has nut
6 has nut fixed to thread

Books

Pojeta, John Jr., and Dale A. Springer. Evolution and the Fossil Record. Alexandria, VA: American Geological Institute, 2001.
Includes information about the fossil record, Darwin's theory, dating the fossil record, examples of evolution, and more. Accessible online at: www.agiweb.org/news/evolution/

Zimmer, Carl. At the Water's Edge: Fish with Fingers, Whales with Legs, and How Life Came Ashore but Then Went Back to Sea. New York: Touchstone Books, 1999.
Describes the latest fossil discoveries, outlines an evolutionary chronology, and gives insights into macroevolution.

Web Sites

NOVA Online—The Missing Link
http://www.pbs.org/nova/link/
Provides program-related articles, interviews, interactive activities, and resources.

Classification
http://www.brooklyn.cuny.edu/bc/ahp/CLAS/CLAS.HP.html
Explains many different reasons for classification and classification schemes, including cladistics.

Evolution—All in the Family
http://www.pbs.org/evolution/change/family/
Offers an interactive way to explore evolutionary relationships by building phylogenic trees and learning about "outgroups."

Evolution & the Nature of Science Institutes: Transitional Fossils
http://www.indiana.edu/~ensiweb/lessons/c.bkgrnd.html
Describes transitional fossils, compares and contrasts features of early "mammal-like" reptiles with those of mammals, explains several reasons for gaps in fossil records, and lists the main findings from the vertebrate fossil record so far.

The "Nailing Cladistics" activity aligns with the following National Science Education Standards:

Grades 5-8

	Science Standard C: Life Science

Diversity and adaptations of organisms

• Millions of species of animals, plants, and microorganisms are alive today. Although different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry.

• Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.

Grades 9-12

	Science Standard C: Life Science

Biological evolution

• The great diversity of organisms is the result of more than 3.5 billion years of evolution that has filled every available niche with life forms.

• The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors.