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A Mini-Lesson



An Essay by Stephen Jay Gould
in Hen's Teeth and Horse's Toes, 1984.

Lesson developed by Craig Nelson
Adapted by Larry Flammer




The essay (and reading guide) addresses the issue of cladistics, and some of the problems encountered in the science of Systematics.


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.


 The book of essays: Hen's Teeth and Horse's Toes, by Stephen Jay Gould, 1984, W.W. Norton & Co. This is available at discount from Amazon.


   (see end of lesson for the formatted handouts).

1. Copies of essay: What, If Anything, Is A Zebra?for each student (see Materials for source).

2. Copies of Reading Guide for each student (PDF). For reasonable responses, send request (with title of what is desired, using your school email address) to the webmaster.


This reading assignment and discussion would be an excellent extension of your Classification unit, especially if you introduce the concept of cladistics (see "Making Cladograms" lesson on this site). The reading level may be a bit challenging, so it might work best in an honors or AP Biology course. The KEY can be used by teacher and/or students to check answers.


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. Get the excellent article "Why Cladistics?" (Gaffney, E.S., L. Dingus, and M.K. Smith, Natural History Magazine, 6/95, pp. 33-35.) from your public or university library, and have students read and discuss it. See the Mini-Lesson on this site for sample Reading Guides for this article.

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

6. 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.

7. See "Reconstructing Trees: Cladistics" (on Understanding Evolution website).


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:

2. Modified by:

3. ENSI / SENSI original developed by: Craig Nelson

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

5. Edited / Revised for website by L. Flammer 3/06/98

6. Some revisions: 1/29/14

 The following is a useful worksheet for students to complete while reading the article, to help focus and direct their reading.

Click here for PDF version of this Reading Guide


  Name________________________________ S.N.____ Article Copy #_____ Date________ Per____

An Essay by Stephen Jay Gould (Paleontologist) in Hen's Teeth and Horse's Toes, 1994.

Read the article, and answer the following questions (on this sheet, in ink), based on the article. Be sure to ask any questions which occur to you as you read (e.g. meanings of certain words, etc.) on back of this sheet.

____ 1. How are rabbits and rodents related? A) rabbits are rodents; B) rabbits are closely related to rodents; C) rabbits are not closely related to rodents; D) rabbits are not related to rodents at all

2. If animals were classified according to brain size, humans and dolphins would be classified as "________".



Why aren't they?

3. Why aren't children with Down's Syndrome considered to be more closely related to each other (due to many striking similarities) than to their parents?


4. How many living species of zebras are there?______ What are their common names?


5. The genus Equus includes zebras, ___________, ___________, and ____________

6. What is "Cladistics"?

7. What is a clade?

8. What are "sister groups"?

9. What is OUR sister group?

10. What is a "cladogram"?

11. Why are orangutans, chimps, and gorillas (the "Great Apes") not a true genealogical unit?


12. What are "shared derived characters"?

13. What are "primitive characters"?

14. What are the clearest shared derived characters which chimps and gorillas share?


15. What does Bennett base her cladistic analysis of Equus on?

16. According to Bennett, are zebras a genealogical unit?______Why?


17. According to chromosome analysis, are zebras a genealogical group?______Why?


18. Why is there "no such thing as a fish"?

19. What is "phenetics"?

20. Why do phenetics and cladistics sometimes fail to produce identical lineages? (answer on back).

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