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A Mini-Lesson

 

WHY CLADISTICS?

An Article by
Gaffney, Dingus, and Smith
Natural History Magazine, June 1995

Lesson developed by Larry Flammer

 

EVOLUTION

Classification

 SYNOPSIS

This is an easily understood article which explains what cladistics is, why it is useful, how it is applied, and its limitations.

 CONCEPTS

1. Biological classification is intimately associated with evolution.

2. There are many problems encountered in classification, evidence that the living world is a work in progress; evolution provides an explanation for those problems.

3. Cladistics is one of several ways to show relationships, each addressing different problems in different ways.

 MATERIALS

 The article in Natural History Magazine, June 1995, pp.33-35, including illustrations: "Why Cladistics?" by Eugene S. Gaffney, Lowell Dingus, and Miranda K. Smith. Check your school or local library for this issue.

If you can't find this issue, contact the webmaster, give name of article, and request copy.

 STUDENT HANDOUTS
 

 (see end of lesson for the formatted handout).

1. Copies of the article: "Why Cladistics?" for each student (see Materials)

2. Copies of Reading Guide for each student (PDF)
- a. Long Version (27 questions); (see end of this page)
- b. Short Version (5 questions)
- For appropriate responses, request by name from the webmaster using your school email address.

 TEACHING STRATEGY

 This reading assignment (with a reading guide) and discussion of the reading guide, would be an excellent extension of your Classification unit, especially if you introduce the concept of cladistics (see Making Cladograms lesson on this site).

 RESOURCES

1. See "The De-riving Force of Cladogenesis" by Andrew J. Petto, on this site. It is a clear and concise explanation of the concepts and terms of cladistics. This could be copied and printed as a student handout. (Click on the "Back" button of your browser to return here.)

2. The UCMP (University of California Museum of Paleontology) has an excellent presentation of cladistics, phylogenies, and modern systematics (what, why, when, and how). Take a look at it: (<http://www.ucmp.berkeley.edu/exhibit/phylogeny.html>).

3. This might also be a good time to introduce "Tree-Thinking" as it pertains to ancestry and biological relationships. For example, a common misconception is that ancestral relationships are often thought to be lineal - straight lines back through time. In reality, such relationships are more like a branching tree. Click Here to see how a cartoon can help understanding.

4. See the excellent online tutorial by the UCMP: "What did T. rex taste like?" It makes an excellent introduction to classification, phylogenetic trees, and cladistics. This could be given as a homework assignment (online).

5. Get the essay by Stephen Jay Gould "What, If Anything, Is A Zebra?", found in the author's book Hen's Teeth and Horses' Toes, 1994. W.W. Norton & Co. (available at discount from NCSE books). Have students read and discuss it. See the Zebra Mini-Lesson on this site for a sample Reading Guide / Worksheet and key for this article.

6. For an excellent tutorial to introduce phylogenetic (evolutionary) trees, see our review of an article in the American Biology Teacher.

7. See the Cladistics is a Zip-Baggie lesson. It uses a series of nested plastic bags as a 3-dimensional Venn diagram to illustrate the hierarchical grouping of organisms based on their shared derived characters, thus forming the basis of a cladogram.

 ATTRIBUTION

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: Larry Flammer, 7/95

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

3. Edited / Revised for website by L. Flammer 3/24/98

4. Revisions: 1/29/2014.


 The following is a useful worksheet for students to complete while reading the article, to help focus and direct their reading. A KEY follows, for teacher use, or for use by students as a self-check. These pages are also available in PDF format for easy downloading and printing using the FREE Adobe Reader (scroll to bottom of this screen).

 WORKSHEET: "Why Cladistics?"
An Article in Natural History Magazine, 6/95, by Gaffney, Dingus, and Smith

DIRECTIONS: Read the article, and answer all questions on separate sheet, in ink

1. Why is cladistics so popular right now?

2. What IS cladistics?

 

3. What kinds of problems does cladistics help us to solve?

 

4. In what way does cladistics differ from older methods?

 

5. What would a "hierarchy of nested groups" look like?

 

6. What is one of the shared derived characteristics by which the "dinosaur" group is included in the larger "vertebrate" group?

 

7. What is each group in an evolutionary tree called?__________; By what is it defined?

 

8. If cladistics is not a perfect method, why is it used?

 

9. If we relied on relative geological age, what relationship between dromaeosaurs and birds would be indicated?

 

10. What would cladistic analysis indicate regarding that relationship (#8)?

 

11. If cladistics does not specify ancestors and descendants, what does it do?

 

12. If geological age by itself is not a reliable basis for indicating relationships, what IS it used for?

 

13. Which features do we use to determine the evolutionary relationships between living animals?

 

14. Which features do we use to determine the evolutionary relationships between fossil animals?

 

15. What is wrong with using "armor" as an indicator of relationships in dinosaurs?

 

16. What is wrong with using "four limbs" as an indicator of relationships in dinosaurs?

 

17. How many shared derived characters are usually used, in real studies, to show evolutionary relationships?

 

18. What is done if patterns of character distribution conflict with each other?

 

19. What is the graphic depiction of the evolutionary relationships called?

 

20. Which cladogram version is considered the "best" (out of several which could be made for a particular group of organisms)?

 

21. Once the "best" cladogram is determined, what happens to it?

 

22. What are "primitive" features? Why?

 

23. What are "derived" or "advanced" features? Why?

 

24. The character "hole-in-hip-socket" is found only in what vertebrate groups?

 

25. Within what group is that feature (hole-in-hip-socket) a derived feature? Why"?

 

26. Within what group is that feature (hole-in-hip-socket) a primitive feature? Why"?

27. Based on the information in this article, build a Venn Diagram, then build a simplified cladogram (similar to the one done in the “Making Cladograms” lesson).

PDF FILES

 Why Cladistics: Reading Guide, Long (27 items)

Why Cladistics: Reading Guide, Short (5 items)

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