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A Mini-Lesson

 

CONTRIVANCES:
Orchids and the Panda's Thumb

Craig Nelson

EVOLUTION

Adaptations, Imperfections, and Contrivances

 SYNOPSIS

Students are assigned to read and discuss selected and edited excerpts from the essays of Stephen Jay Gould on the subject of contrivances.

 CONCEPTS

1. Many features of modern organisms reflect the structure of their ancestors in ways that are not adaptive.

2. One of the best indicators of evolution is not the examples of "perfect adaptations", but of the relatively imperfect contrivances, structures modified and used for functions quite different than the ancestral functions of those same structures.

 MATERIALS

1. Selected essays of Stephen Jay Gould: Excerpted and edited for use in this lesson:
-----"The Panda's Thumb" from his book, The Panda's Thumb: More Reflections in Natural History. W.W. Norton & Company. 1980
-----"The Problems of Perfection, or How Can a Clam Mount a Fish on Its Rear End?" from his book, Ever Since Darwin: Reflections in Natural History. W.W. Norton & Company. 1977

2. Questions for reading and discussion.

 STUDENT HANDOUTS
 

 (see end of lesson for the formatted handouts).

1. Readings on Contrivances (3 readings, 4 pages)

2. Sheet with questions for reading and discussion.

 

 

TEACHING STRATEGY

Before doing this lesson, consider introducing your students to the concepts of contrivances and imperfections by doing the Blocks and Screws lesson.

1. Give your students the two "Panda" readings (on pandas, and on orchids) to read for homework, and ask them to answer questions 1 and 2 on the Reading/Discussion Guide.

2. In class the next day, have your students break up into small discussion groups to go over their homework answers and to discuss together questions 3 and 4, for about 15 minutes.

3. As a class, discuss what they came up with in their groups. Lead the discussion into the concepts of atavisms and preadaptations as other examples of imperfections that are explained by evolutionary theory.

4. Question #5 would be a good short essay for homework.

5. OPTIONAL: To support your discussion on preadaptation and to provide another example of imperfections and contrivances, assign the reading and questions on Gould's article: "The Problems of Perfection..." from Ever Since Darwin. This deals with the freshwater mussel, Lampsilis, with its "fishy" adaptation.

DEFINITIONS

Preadaptation (or Exaptation): A character that was adaptive under a prior set of conditions and later provides the initial stage for evolution of a new adaptation under a different set of conditions. Examples: bird's flight feathers (from feathery scales on certain dinosaurs, where they served the function of insulation); the vertebrate eye (from a series of light-sensitive organs); barnacle "glue" (from their glue to attach eggs). Also, the process by which this comes about.

Vestigial structure: an anatomical structure found in all or most normal individuals of an extant species; typically very small in size, and with apparently little or no important function now. Such parts typically would be found in ancestors of this species, but as larger and clearly functional structures. An example would be the ear-wagging muscles in humans.

Atavism (Atavistic structure): a vestigial structure found in only a small fraction of the normal members of an extant species (e.g. the rudimentary thigh bone found in about 5% of individual whales, or the extra toes which sometimes appear in horses, enlarged growths from the "splints" of the vestigial toes normally found).

Contrivance: A structure modified and used for a function which is different from the original (or previous) function for that structure in an ancestor. An example would be the radial sesamoid bone in a panda's wrist which is elongated and functions as a novel "thumb" for holding bamboo stems.

Imperfection: A "contrivance" which still retains some of the features of its ancestral source structure, to a greater or lesser degree; clearly not fully or perfectly "adapted" to its new function, but serving adequately. Again, the panda's "thumb" is an example here, as are the many contrivances found in orchid flowers. This term could also be applied to vestigial or atavistic structures.

 

 

EXTENSIONS

& VARIATIONS   

An excellent introduction activity to the concept of contrivances and imperfections is the lesson on Blocks and Screws. Doing this is highly recommended before (or even after) this reading assignment of Gould's article.

An excellent film to show in conjunction with this lesson (and/or your unit on reproduction) is "Sexual Encounters of the Floral Kind." It shows many of the amazing contrivances in orchids that assure pollen transfer by various specific vectors. You can get this film (DVD or VHS) for $89 at Guidance Associates: www.guidanceassociates.com/seenoffikippo.html. Also, Google the title and you'll find a number of "Film Notes" or film worksheets you could use with it.

 The late Stephen Jay Gould offers fascinating anecdotes supporting evolution and the nature of science, yet many of our general level students would find his writing obscure; he writes in a very literary style, with a vocabulary and allusions which many high school students would find challenging. This assignment is intended to help teachers make his excellent discussion of the role of contrivances and imperfections in evolution accessible to more of our students, and provide them with positive experiences reading good literature in biology.

Gould has written many books dealing with evolution, and wrote a monthly column in the Natural History Magazine. See the list of books in the NCSE books selections, where they are available at discount to members.

We encourage you to seek out other articles by Gould which address important issues on evolution and the nature of science. Excerpt and edit them so that his essential points are retained, but in a form which your students would be able to handle without too much trouble. Prepare an appropriate reading/discussion guide similar to the one used here. Use it and modify as necessary, then SHARE IT WITH YOUR FELLOW TEACHERS: send it to us to put on this web site. Be sure to give clear reference to the precise source of the essay.

The use of a scanner with OCR, and a thesaurus (preferably in your word processor) are valuable tools for doing this kind of preparation on a computer. Contact us for information about sending formatted text in a form which we can place on the web easily.

 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: Craig Nelson

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

3. Edited / Revised for website by L. Flammer 9/98

 

PDF FILE

 Contrivances Readings and Questions
5 pages total

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