THE KEY TO IT ALL ours - Siemens STEM Day

Real World Science Topics:• An exploration of the use of a dichotomous key.• An exploration of how scientists classify organisms.

ObjectiveStudents will gain an understanding of how scientists classify organisms, and how a dichotomous key can be used to identify organisms or other items.

Materials Needed for Each Team of 2-4 students:paper clips of different sizesbuttons and beads of different colors and sizesstring (thin enough to fit through the holes in the beads)one medium-sized rectangular rubber eraser (sold in school supply kits)glue and tapescissorspapermarkers or pens

THE KEY TO IT ALL (3–4 Hours)

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Addresses NGSS Level of Difficulty: 2Grade Range: 6-8

OVERVIEWIn this activity students will create there own unique creatures and then make dichotomous keys that will allow their classmates to identify the creatures.

Topic: Dichotomous Key

Science and Engineering Practices

NGSS Three-Dimensions

Constructing Explanations and

Designing Solutions

• Apply scientific ideasto construct anexplanation for real-world phenomena,examples, or events.

DisciplinaryCore Ideas

Crosscutting Concepts

• Patterns can be used toidentify cause and effectrelationships.

• Anatomical similarities anddifferences between variousorganisms living today andbetween them andorganisms in the fossilrecord, enable thereconstruction ofevolutionary history and theinference of lines ofevolutionary descent.

PatternsLS4.A: Evidence of

Common Ancestry and Diversity

STEPS FOR THE KEY TO IT ALL

1. Warm-Up Activity:Ask students if they have ever played the game 20 questions. If they haven’t, explain thepremise—you try to guess what someone is thinking of by asking up to 20 yes-or-noquestions, narrowing down the possibilities until you have identified what the person isthinking of. Explain that scientists use a similar process to identify organisms (and othernatural items). Note that some students might ask why the scientists don’t just look it upin a book, such as an encyclopedia. If students come up with questions such as these, askthem further questions to get them to think more deeply about the problem (e.g., “Howyou would you know where in the encyclopedia to look?” “What if it’s an animal that noone has ever seen before, and it’s not in an encyclopedia?”). Explain that scientists makeobservations about the appearance and behavior of an organism to help them identify theorganism. They compare those observations to a set of paired statements that describedifferent organisms. The group of paired statements that help them identify theorganism is called a dichotomous key. For example, suppose a scientist were trying toidentify an unknown mammal. The scientist might use a dichotomous key like the onebelow. Copy this dichotomous key onto the board (or make copies of it and hand them outto students). Pick two of the mammals in the key and work with students to guide themthrough using the key to identify each mammal. (This key could be used to identifymammals common to the northeastern United States.)

1. a. The mammal flies. It is a bat.b. The mammal does not fly. Go to step 2.2. a. The mammal has no fur on its tail. Go to step 3.b. The mammal has some fur on its tail. Go to step 4.3. a. The mammal has a short tail. It is a mole.b. The mammal has a long tail. Go to step 5.4. a. The mammal has a black mask across its face. It is a raccoon.b. The mammal does not have a black mask across its face. Go to step 6.5. a. The mammal has a tail that is shaped like a paddle. It is a beaver.b. The mammal has a tail that is long and thin. It is an opossum.6. a. The mammal is brown with a white belly. Go to step 7.b. The mammal is not brown with a white belly. Go to step 8.7. a. The mammal has a long, thin body. It is a weasel.b. The mammal has a small, compact body. It is a mouse.8. a. The mammal has black and white stripes. It is a skunk.b. The mammal does not have black and white stripes. Go to step 9.9. a. The mammal has long ears. It is a rabbit.b. The mammal has short ears. It is a woodchuck.


2. Divide students into eight groups. Hand out copies of the The Key to It All worksheetto students. Explain to students that they will now create their own organisms and adichotomous key to classify them. To ensure that the dichotomous keys do not get toocomplex or confusing, students will be able to vary only certain characteristics of theirorganisms. Each organism’s body will be made of a rubber eraser. Students can vary thefollowing characteristics of the organism:

• number of legs: Students can unbend paper clips and push theminto the erasers to represent legs.

• length of legs: Students can use different sizes of paper clips torepresent different lengths of leg.

• length of tail: Students can glue or tape string to the eraser torepresent a tail; the tail can be long or short, or absent altogether.

• type of tail: If the organism has a tail, students can string beads ontoit or leave it plain.

• type of “skin”: Students can leave the eraser unadorned or glue or tapebuttons to it to represent scales; they can use different colors and sizesof beads to represent different kinds of scales.

3. Have each group present its creature to the class. Lead the class in making a list of thecreature’s unique features on the board. This will help with the development of thedichotomous key. Each group should place its organism on a central table for other groupsto examine. To help the groups keep track of the organisms, assign each organism a nameor label (such as “Fred”, “Wolfle” or “Animal A”). Students can then use these labels tokeep track of the organisms as they classify them.

4. Once all the creatures are presented, each group should develop a dichotomous keyto classify them. This is not as simple as it might at first seem. Students may need help indeveloping the key. The best way to develop a dichotomous key is to put the items beingclassified into two groups that differ by only one characteristic, and then further subdivideeach of those groups based on a single characteristic, and so on. So, for example, studentscould first divide the organisms into groups based on whether they have a tail or not.Within the “has a tail” group, they could further divide the organisms based on whetherthe tail is long or short, and so on. Make sure students keep track of the characteristicsthey used to sort the organisms into each group, and that they keep track of whichorganisms are in which group. Remind students that, each time they divide the organisms,all of the organisms must be placed in one of the two groups. Also remind them that theirdichotomous key must allow them to completely and definitely identify all eight organisms.




5. Have students draw or write out their dichotomous keys on sheets of paper. Studentscan either use a text-based key, such as the one in the Warm-Up activity, or a visual key,such as the example shown below.

has two legs

has more than two

legs

has a plain tail

has a beaded

tail

has plain skin

has buttons on skin

tail does not contain blue beads

tail contains blue beads

has no tail

has a tail

has large buttons on skin

has small buttons on skin

has yellow buttons on skin

has green buttons on skin

Animal A

Animal G

Animal E

Animal F

Animal D

Animal C

Animal H

Animal B


6. Have groups exchange keys. Then, give each group one of the organisms. Make sureno group gets its own organism, and make sure to note which group gets each organism.Then, have the groups use the dichotomous keys to identify which organism they havebeen given. Once all groups have identified their organisms (or have attempted to), leada class discussion about the dichotomous keys. Lead students to realize that any group ofobjects can be classified using many different dichotomous keys. If any groups wereunable to correctly identify their organisms, lead a discussion about where they ran intodifficulty.

7. Wrap-Up Activity: As students are constructing their creatures, you should alsoconstruct a creature. Try to make sure your creature is different from any of the othercreatures. At the end of the activity, show students your creature. Ask them whether theycan use the keys they’ve created to classify your creature. (They will probably be unableto.) Explain that this is one problem with dichotomous keys: although they can help youaccurately identify a creature, they cannot be used to identify creatures that do not fitwithin their “rules.” Ask students for other pros and cons of using a dichotomous key toidentify a species. One pro would be that the key allows you to find a species when youonly have a brief physical description. Another disadvantage would be that you wouldhave to have a very large dichotomous key to identify all of the species in an area, andone missed detail could lead you down the wrong trail. Use students’ keys to illustrate thisproblem. For example, consider the key shown above. If a student examined a two-leggedcreature with a beaded tail and did not notice that it had some blue beads on its tail, thestudent would misidentify the creature.

The Key to It All Extension ActivityDichotomous keys are commonly used to identify trees and shrubs. Interested students could locate a dichotomous key for the trees (or another group of organisms) in their area. Dichotomous keys can be found online and in some printed field guides. Then, students should explore a local wooded area and use the dichotomous key to identify all of the trees they find. They should take notes on how they used the dichotomous key and whether there are characteristics of the trees that the key does not describe. (If it is not possible for students to actually go to a wooded area to examine trees, you can provide them with pictures of trees or other organisms to use. If you provide pictures of trees, make sure the leaves are clearly visible, as most tree dichotomous keys are based on leaf shape, size, and arrangement.)

You may also wish to explain to students that dichotomous keys are not used to identify only living things. They can also be used to identify rocks, landforms, and even human-made objects. Interested students could create dichotomous keys to classify a group of similar objects of their choosing (such as cars, books, or students in the class).



What is a dichotomous key?

A dichotomous key is a type of identification system that relies on a series of paired statements. The statements create mutually exclusive categories. The word dichotomous means “divided into two parts.” Therefore, each level of a dichotomous key contains two statements that can be used to separate items. For example, the first pair of statements in a key to identify an organism might be “has wings/does not have wings.” If the organism does have wings, the user is directed to another pair of statements that further eliminate some of the possible identifications (such as “wings have feathers/wings do not have feathers”). Eventually, all possibilities have been eliminated except one, and the organism is completely identified.

How is a dichotomous key created?

Most dichotomous keys are made by examining the group of items to be classified and identifying key characteristics that make them different from one another. When you make a dichotomous key, you basically divide the group of items in half based on a single characteristic. Then, you divide each half in half again, based on other characteristics. You keep dividing the groups in half based on single characteristics until each “group” consists of only one item.

Why use a dichotomous key?

A dichotomous key is a simple way of identifying anything. At its simplest, a dichotomous key is just an extended and very specific process of elimination. As long as you have good observations about the item in question, a dichotomous key provides an easy way to identify it.

Key Vocabulary:dichotomous key: a set of paired statements that leads to the identification of an item by progressively eliminating possibilities


How do scientists use dichotomous keys in real life?

Many scientists use dichotomous keys to identify plants, animals, and other organisms. They may also use dichotomous keys to identify species, or to determine whether a particular organism has been identified and described before. However, dichotomous keys are not used only to identify organisms. Scientists can also use dichotomous keys to identify rocks, fossils, and other natural objects.

What are some problems with using a dichotomous key?

The most obvious problem is that in order for the key to be useful, the organism must have been discovered by someone else. If you were to find a species that wasn’t on your key, the only thing you would know was that it was probably not known by the person who designed the key. You could not tell that it was a new species, as it may be on another key created at a later date or by another person. You also could not identify the organism. A related problem is that the key is only as good as the observations used to create it. If a person creates a dichotomous key based on incomplete or imprecise descriptions, then the key will not be useful correctly identifying organisms. Many organisms look quite similar, and only small differences separate them. If the dichotomous key is not sufficiently detailed—or if the user of the key does not make precise observations—then the key may not lead to a correct identification.

THE KEY TO IT ALL BACKGROUND INFORMATION

Name Date

Group members ____________________________________ Date _________________________

Draw your group’s creature below.

Draw your dichotomous key here.

Which group’s dichotomous key did your group receive? ________________________

Based on the other group’s dichotomous key, which organism did your group receive?

_____________________________________________________________________

STUDENT HANDOUT FOR THE KEY TO IT ALL


THE KEY TO IT ALL ours - Siemens STEM Day

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