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THE NATURAL SELECTION
OF STICK-WORMS

or "You, Too, Can Be Selected, Naturally"

Variations on an old theme by
Don Dunton (ENSI '91)
Fred Fisher
Larry Flammer (ENSI 92)

 

EVOLUTION

Variation and Natural Selection
 NEW ARTICLE: Common Misconceptions about Natural Selection. Go to our Evolution Introduction page, scroll down to bottom of page for "A Few Very Common Misconceptions" and a link to the excellent article that exposes a number of widely held misconceptions, with clues for correcting them (June 2009).
CLICK HERE
For several STEM Applications of Natural Selection

 SYNOPSIS

Students play the role of birds, go out on the school lawn, and pick up toothpick "stick worms" which have been previously scattered on the lawn in equal numbers of green-stained and unstained. "Birds" are chased away before the "worm population" drops too low. Back in the classroom, the number of green and non-green "worms" are compared individually and for the whole class. Discussion relates the experience to the elements of natural selection. As presented here, it does not lend itself to demonstrating the effects of selection over multiple generations.

 PRINCIPAL CONCEPT

Traits are usually favored by natural selection only when they result in more reproductively successful offspring.

 ASSOCIATED CONCEPTS

1. One common form taken by natural selection is when individuals blend with their environment, they tend to avoid being seen and caught by their predators, so more of the "blenders" tend to survive, and therefore have the opportunity to reproduce their kind (which also tend to blend).

2. Students have a chance to see selection and competition working from the perspective of both the prey and the predator.

3. Students realize that the competition occurring in natural selection is NOT between the prey and the predator, but rather between members of the same species: which ones can avoid being caught, and which of the predators can catch the most prey. Whatever traits contribute to greater survival tend to be found in the survivors, and therefore more likely to be more common in the next generation.

4. Large sample size is more consistent and reflective of a population than small sample size.

ASSESSABLE OBJECTIVES

   Students will....

1. be expected to participate actively.

2. recognize the distinctions expressed in the concepts.

3. recognize why natural selection provides the best explanation for these data.

4. given similar data, recognize that natural selection provides the best explanation for a mechanism of change of traits in populations

 MATERIALS

About 10000 toothpicks, half of which have been stained green.

Worksheet for each student.

 TIME

One 45-60 minute period.
STUDENT HANDOUTS

   (see end of lesson for the formatted handouts).

Worksheet, one page, with description of the scenario, data tables, and discussion questions. You will probably want to create your own version, incorporating your own name and whatever alliteration you can create. For that reason, this worksheet is also included in the Procedures below, so it can be selected, copied, and pasted into your word processor file for appropriate modification. PDF version is also available (see bottom of page).

 

 

 

TEACHING STRATEGY

1. Well in advance of this experience, prepare the colored toothpicks. Probably the best way is to stain them with a light green cloth dye (be careful...it DOES stain EVERYTHING!); wear gloves and apron. As an alternative, try well-thinned green paint, preferably acrylic paint (to avoid the lingering odor and hazard of fumes). Buy boxes of toothpicks, and note the number per box. Round toothpicks are more durable, but they also cost a bit more, and they may be more hazardous to any barefoot people who might walk on the lawn sometime after the event (even though you try to get them all picked up afterwards). Plan on returning all collected toothpicks before you take the next class out, so figure that each class will have the same population of worms to select from.

2. Prepare student worksheets, one per student.

3. It's fun to announce to your students on the previous day that you will be going on a field trip (always builds anticipation).

4. Just before you take your class out to the "field" (an area of grass, if possible), arrange for a lab assistant, other teacher, or one of your students to randomly scatter both colors of toothpicks evenly around that area.

5. Try to find some old farmer's coveralls, a farmer's hat, and a mock shotgun or pitchfork. Ideally, if you can have your assistant take the students out to the "pasture" and get them started looking for "stick-worms", you can put on the coveralls and hat, grab the gun, and run out to chase the birds off the pasture and back to class before they nearly exhaust the population of stick-worms!

6. Be considerate of your colleagues in classrooms between yours and the pasture. If they are sensitive to "interruptive behavior", try to have the students go and come by a route which will be least disruptive. It's a good idea to talk to the teachers who would be affected by this, and try to get their cooperation for "just this one time" (?) of silliness, pointing out the educational benefits of dramatic involvement in a physical experience. Be sure to caution students to go and return with minimal disruption of other classrooms.

7. This lesson probably fits best in your introductory unit on the nature of evolution, especially where you may be trying to convey a clear understanding of natural selection, as distinctly different from Lamarck's ideas of organisms surviving because the "needed" to, or developed traits "in order to" survive.

8. Either before or after this lesson, it is useful to focus on natural variations in a population. There are many such examples; the "Peanut" lab on this site works very well.

9. It is also important during or after discussion of this lab experience, to recognize and discuss the many other types of selective pressures which can operate on populations (e.g. various breeding behaviors, relative sizes of individuals, length of legs, variations in timing of activity or development, environmental impact on availability of food, protection against extremes of temperature, etc.).

 

 

PROCEDURES

1. Hand out the Worksheet for students to read, while you take roll.

2. Summarize briefly what they will be expected to do, including any cautions (move quietly past classrooms, etc.), and give them the signal to be used to start getting their stick-worms.

3. Send your class to the pasture (see "Teaching Strategy" #5).

4. In your best farmer's garb, run out and chase the birds back to the classroom.

5. Collect students' data to compile class totals. (The use of a pre-structured spreadsheet on a class computer can speed up the data entry and calculations, especially if you want to include the totals from other classes, from this year and/or last year's, in order for your students to see the greater consistency of results from larger sample sizes, vs. greater variability from small [individual] sample sizes).

6. Give them time to complete the answering of discussion questions (individually, or, perhaps, in small discussion groups). If time allows, you can begin some informal discussion of what the data shows. The Questions can be completed as homework.

7. Next day, you can simply go over the discussion questions, expanding and adapting as other questions may arise spontaneously.

8. One interesting development sometimes occurs which you should be ready to handle. There is always the "clever-creative student" (and sometimes a group of them) who may conspire to "skew the results" by purposely going after the green stick-worms, avoiding all or most tan worms. If the data reveals that this has happened, you can use it as an example of a peculiar variant in the predator population which caused them to be unusually attracted to green worms, or they have a special sensitivity to UV radiation, so they are able to distinguish between the stick-worm green and the grass green, so those stick-worms stood out more clearly! You can easily take the discussion in this direction, and it still illustrates the points you want to make about natural selection, but focusing on the predator variants rather than the prey. Conspirators are foiled again!

ASSESSMENT

Observe participation.

Prepare test in which objectives are addressed.

 

 

 

EXTENSIONS

& VARIATIONS   

There are many variations of this lesson. Some are mentioned here:

1. If the season, weather or landscape of your school does not allow you to do this outside, consider getting several squares of Astro-Turf, enough so that you can have groups of 4-5 students (sticky birds) per square (each about 60x60 cm square, or larger, placed on a table top). Use flat toothpicks, broken in half. Provide plastic forceps as "beaks", or insist on students using only their thumb and forefinger to pick up the worms. Have the curtains closed, and turn off the lights just as you announce "START". Don't give them much time (maybe 30-60 secs.) before you call "STOP". Then proceed as with the pasture data. [When you reach an age where you lack the energy to put on coveralls, run out to the pasture, and chase them back, and do this for 5 periods in a row, this variation becomes even more attractive!].

2. Instead of Astro-Turf, use big squares of "very busy" wallpaper (floral design, or other), and prepare lots of hole-punches from stacks of that design, along with equal amounts of hole-punches made from some solid color, or white, or black. Alternatively, simply have students or lab assistants cut out large numbers of tiny polygon chips (triangles, rectangles, etc.) randomly from the wallpaper scraps, and an equal amount (by weight?) of plain paper pieces.

3. A somewhat more elaborate, yet similar exercise, focuses on the selection of birds, based on variations in their beaks. You will find this very entertaining lesson on this site as: "Natural Selection for Better Beaks".

4. One of the weaknesses of these exercises in natural selection is that they do not include the production of subsequent generations by the "survivors". A more generic version of #1 above does include a three-generation scenario. You will find this lesson listed as "The Chips are Down: A Natural Selection Simulation". A simpler "generations" version is titled "Natural Selection of Bean Hunters".

5. For an outdoor simulation which uses three generations, try the "Natural Selection of Bean Hunters" lesson on this site. This one, in a very simple, straightforward way, simulates three generations. It also focuses on the natural selection of both the beans and the bean hunters. Other multi-generation lessons are referenced from that lesson.

6. One of the most creative multi-generation lessons we've seen for teaching natural selection is Bebbledwark World, created by Thomas Atkins (ENSI 92) and Gene Nelson, of Fresno, California. It's a fairly elaborate classroom simulation, requiring considerable preparation (initially), and can take 2-3 days to carry out. If you happen to have an old Apple IIe kicking around, there's an interactive illustrative program to accompany the lesson. This lesson, with all instructions and materials, is now available on the web, at its own website, at Bebbledwark World. (When you click on this, you will leave the ENSI web site. To return, just click on the "Back" button of your browser.)

7. Take a look at the handy summary: "Comparing Evolution Mechanisms" near the bottom of the "Introduction to Evolution" page. Darwin's and Lamarck's essential elements are compared, and a few common misconceptions are clarified. Scroll down to download the PDF file of this information.

8. Be sure to read two excellent articles dealing with natural selection in the Natural History magazine (the 12/97-1/98, issue). In the first one ("The Paradox of the Visibly Irrelevant" by Stephen Jay Gould, pp. 12-18, 60-66), the author describes some recent studies of natural selection, and reflects on their relevance in the context of geological time. The last two pages are especially interesting. He also comments on micro-/macro- evolution and punctuated equilibrium in this context.

Also read the article in the same issue by Jonathan Losos and Kevin de Queiroz on "Darwin's Lizards" (pp.34-39), describing the the many species of anole lizards of Jamaica (more discovered every year), and their diverse adaptations to their many niches.

9. If at all possible, be sure to have your students experience the cumulative aspect of natural selection, and how this creates a high probability for new traits to appear which fit the current selection constratins, and prevail. Try the Natural Selection: A Cumulative Process lesson.

 OTHER RESOURCES

National Academy of Sciences. 1998. Teaching About Evolution and the Nature of Science. Activity 3: Investigating Natural Selection. Pages 78-80 (see our Resources page, and click on the link for the first book listed, to see those pages online, or order this very valuable book). This is a well-presented multi-generation lesson, using petri dishes, zip-type bags, pieces of patterned fabric, paper dots, graph paper, colored pencils, and forceps. It also recommends use of computer spreadsheet to facilitate data handling. See the "What's New" area of this site for details on getting this book.

William F. McComas, ed. Investigating Evolutionary Biology in the Laboratory. 1994. NABT. There are several exercises which provide experiences for students to explore natural selection.

 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: Don Dunton, Fred Fisher, Larry Flammer

2. Modified by: Larry Flammer

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

4. Edited / Revised for website by L. Flammer 2/28/99


 The following is a useful Student Handout. If you "select" it then "copy" it (onto your invisible clipboard), you can shift to your word processor, open a new file, and "paste" it onto that page. Later, you can make changes to fit your name and circumstances.
A KEY follows, for teacher use, or for use by students as a self-check.
Below the KEY is a PDF version of the Student Handout, all pre-formatted, ready to use.

 Name_________________________________ Date___________ Per.____

THE NATURAL SELECTION OF STICK-WORMS, or...
"YOU, TOO, CAN BE SELECTED, NATURALLY!"

PROCEDURE: Today, you are all "sticky-birds", members of the species Birdus stickus, which, as every good biologist knows, are great connoisseurs of the common stick-worm, or Stickit tooya in the family Toothpickidae. These worms have a remarkable resemblance to the common toothpick in structure. But make no mistake about it, these skinny sticks of nutrition are actively sought out by all members of your species. Take a good look around you... those famished, beady-eyed birds are your competition. Yes, that's right! It's you or them! Go get 'em, sticky-bird!

You are a migrating bird, and today you are passing over Flammer's fertile farm on your way north for the Winter. That's right, NORTH! You're a weird bird. In the pasture, you spot the stick-worms waiting (sticking around). Lucky for you, whenever a stick-worm sees a sticky-bird, it freezes in fitful fear. This makes it very easy for you to pick it up with a minimum of effort. Although there is only one species of this worm, it is found in two color forms: tan and green (Stickit tooya domesticus, and Stickit tooya chloris, respectively).

Farmer Flammer favors these funny, frolicking stick-worms because they fill the fields with finely filled furrows which favor the flora and fauna of the fertile farm. So watch out! Gorge yourself with these delicacies, but be ready to make a hasty retreat in case farmer Flammer sees you.

When we fly out to the grassy pasture, wait along its side until a signal is given. Then start picking up stick-worms as fast as you can. You won't have much time because farmer Flammer and his friends will chase you nasty birds out of his perfect pasture. When he does, come flying back to your nest as fast as your little wings can carry you, and count your stick-worms by color.

On the work sheet provided below, record your totals and the class totals, as indicated. Then answer the questions.

****************************************
WORKSHEET

DATA:No. of TAN stick-worms caught:...._______ (Total TAN caught by class:....._______)

No. of GREEN stick-worms caught:_______ (Total GREEN caught by class.._______)

Total number caught by you:........._______ (Total worms caught by class:..._______)

QUESTIONS: (answer on back of this sheet)

1. If the total population of worms not eaten today were allowed to be unmolested by the nasty sticky-birds, what would be the results in several years time (assuming the stick-worms were alive and sexy)?

2. Since a change in environment around living organisms can act as a stimulus for natural selection to occur faster, DESCRIBE the environment of the stick-worms and how the environment changed today.

3. Explain the COMPETITION that was set up today: A) between the sticky-birds, and B) between the stick-worms, both in terms of the following: 1) what were they competing for? 2) what survival advantages did some have? and 3) what weaknesses did other have?

4. Why was a time limit placed on the collecting of the worms?

5. What would be the most probable result of this experiment if it were repeated after the lawn was allowed to dry up and turn yellow?

6. How would Lamarck have explained the worms' adaptations over several years?

7. How would Darwin have explained the worms' adaptations over several years?

8. Briefly state Darwin's theory of natural selection.

OTHER ITEMS WORTHY OF DISCUSSION:

9. What reasons can you offer for the differences in the total numbers of worms found?

10. What could cause differences in the ratios (or proportions) collected?

 KEY: sampling of reasonable responses:

1. They would be mostly green worms.

2. They lived in a green-grass world, with no predators, until today.

3. (A) Competition between sticky-birds:
...........1) competing for catching worms for "food";
...........2) better vision, faster, better "beak" (finger) dexterity, more aggressive
...........3) weaker vision, slower, less dexterous, less aggressive, unmotivated (less hungry)
... (B) Competition between the stick-worms:
...........1) competing for survival, escaping predation (not becoming bird food)
...........2) close color match to environment (grass), smaller, vertical position
...........3) contrast with background color, larger, horizontal position

4. So the birds were forced to get what could be seen most easily, didn't have time to search for hard-to-see worms, which would eventually happen as the worm population decreased.

5. Probably more green worms would be found; the natural tan color would blend better with the dry grass color.

6. Lamarck: More worms would be green because they needed to be green in order to survive being caught.

7. Darwin: More green worms would survive each year, so they would produce more of their kind each successive year. Since green is genetically controlled, their offspring would tend to be green.

8. Natural selection is a process which requires that all traits within a population vary. And, within a given environment, certain traits help survival and/or reproduction better than others, so the owners of those traits tend to survive and reproduce more often. Since those traits are already inheritable, their offspring will tend to have those traits. If the environment changes, different traits and trait combinations are similarly favored, so eventually they come to characterize that population (or, eventually, a new species).

9. - easier to see, - different surfaces, - different worm sizes, - different numbers of worms out there, - different degrees of aggressiveness of birds.

10. - different color preferences by the birds, - coincidence, - different ratios of worms in the grass, - one color easier to see, so selected more often.

PDF FILE

 Natural Selection of Stick Worms
Worksheet

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