A Mini-Lesson

WHAT, IF ANYTHING, IS A ZEBRA?

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

Lesson developed by Craig Nelson
Adapted by Larry Flammer

EVOLUTION

 SYNOPSIS

 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.

 MATERIALS

 STUDENT HANDOUTS

(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.

 TEACHING STRATEGY

 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. 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).

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

 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:

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

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?

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).

1. C

2. Psychozoa; because both have the largest brains; They aren't, because dolphins are more closely related by descent (as indicated by many details of their anatomy, etc.) to whales, while people are clearly more closely related to apes (again as indicated by many details of their anatomy, molecular biology, etc.)

3. Because we KNOW they are related to their parents!

4. 3; (see p. 361): Burchell's, Grevy's, and mountain zebra

5. true horses, asses, and donkeys

6. A method for establishing the pattern of branching for sets of related species.

7. A branch in an evolutionary tree.

8. Two lineages sharing a common ancestor, from which no other lineage has sprung.

9. chimps and gorillas

10. a cladistic pattern; a chart of branching relationships

11. Orangutans are more distant from chimps and gorillas than people are.

12. Features present only in members of immediate lineage, unique and newly evolved (see p.359); must avoid primitive characters here (see p.358); ["synapomorphies" as a single answer here is insufficient] alone.

13. Features present in a distant common ancestor.

14. chromosome inversions (see p. 359)

15. skeletal features, mostly in the skull

16. no; because one zebra (the mountain zebra) is more like true horses than other zebras

17. yes; because all zebras have fewer than 50 chromosome pairs (all other Equus spp. have more than 50 chromosome pairs).

18. Some fishes, e.g. lungfish, are closer to prehistoric amphibians, etc. than to other fish!

19. A theory of classification based on overall similarities, based on large number of traits, expressed numerically; computerized

20. Real relationships are very complex; also, these classification schemes assume a constant rate of change, but rates can vary greatly.