Using A Dichotomous Key by Cassie Zanca

Transcription

1 Using A Dichotomous Key by Cassie Zanca GOAL The students will explore the benefits of creating and using dichotomous keys as a means of identifying an organism or object. OBJECTIVE The student will use a dichotomous key to identify seashells. BACKGROUND A dichotomous key is a tool that allows the user to determine the identity of items and organisms in the natural world. It is the most widely used form of classification in the biological sciences because it offers the user a quick and easy way of identifying unknown organisms. Keys consist of a series of choices that lead the user to the correct name of a given item. Dichotomous means divided into two parts. That is why dichotomous keys always give two choices in each step. In each step, the user is presented with two statements based on characteristics of the organism. If the user makes the correct choice every time, the name of the organism will be revealed at the end. There are two kinds of descriptions that might be presented to the user of a dichotomous key: qualitative and quantitative descriptions. Qualitative descriptions concern the physical attributes, or qualities, of the item being classified. Examples of qualitative descriptions are such phrases as contains green striations on top surface or feels slick on bottom surface. Quantitative descriptions concern values that correspond with the item being classified. Examples of quantitative descriptions are such phrases as has 10 striations on top surface, has 8 legs, or weighs 5 grams. Knowing the difference between these two types of descriptions can be immensely beneficial for creators and users of dichotomous keys. There are two ways to set up a dichotomous key. One way is to present the two choices together, and the other way is to group by relationships. When the dichotomous key is set up by presenting the two choices together, it is easy to distinguish between them. However, relationships between various characteristics are not emphasized. When the dichotomous key is grouped by relationships, the choices are separated, yet it is easy to see the relationships between them. While this method may prove to be more difficult to construct, many users prefer it because it gives them more information. BENCHMARKS (Louisiana) MIDDLE SCHOOL BENCHMARKS HIGH SCHOOL BENCHMARKS SI-M-A1,A3,A4,A7 SI-M-B4,B6 PS-M-A1,A3 LS-M-A2,A3 LS-M-C1 SI-H-A1,A3,A4,A5 SI-H-B2,B4 PS-H-A1 LS-H-C4,C5 NATIONAL SCIENCE STANDARDS MIDDLE SCHOOL STANDARDS HIGH SCHOOL STANDARDS UC&P- Systems, order & organization UC&P - Systems, order, & organization LS - Diversity & adaptations of organisms PS - Structure & properties of matter 1 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

2 TEACHER PREPARATION Download blackline masters 1,2,3, and 4 (pages 1-3) from the website. There you will find two styles of dichotomous keys and pictures used in the keys. Pick one of the dichotomous key worksheets; both reflect the same dichotomous process. If you are a verbal learner use the key on blackline master 2. If student is a visual learner use blackline master 3. Duplicate as many keys as necessary for each student or student group. It is suggested that if you use this activity many times, you should laminate the pictures of the shells and then cut them apart as sets for future use. Note to teacher: When making copies of the seashell pictures from your blackline master, lighten the density of the copies and if possible use the photo/text option. If you do not lighten the density, the images will be difficult to see. ANSWER KEY See the answer key for the format used in class. Keys for both formats are provided. ALTERNATIVE ASSESSMENT IDEAS Create a concept map from the key. Design a short dichotomous key using a particular item (examples: coke tabs, empty ketchup bottles, paper clips, etc.) and have the students work through the key. Have students work through an online dichotomous key. EXTENSION STARTERS Research ways dichotomous keys are used in other areas of school, society, culture, etc. In geometry class, develop a dichotomous key of a formal proof. Create a collage from the different levels of the key. RESOURCES & WEBLINKS Timme, Stephen, 1991, Association for Biology Laboratory Education website, How to Construct and Use a Dichotomous Key, accessed 02/16/01, Description: An excellent web-based activity on the construction and use of a dichotomous key that also describes the use of a dichotomous key in the field and provides a key for prairie plants. Frontier High School, Red Rock, OK, The Dichotomous Key, accessed 02/16/01, hs/science/hbotkey.htm Description: Provides instructions on the two methods of constructing a dichotomous key as well as several online dichotomous keys. Grade level: High School. Detka, Jon, California State University at Monteray Bay, Designing and Using a Dichotomous Key, accessed 02/ 16/01, Description: Students first construct a simple dichotomous key and then use a basic key to identify some of the native plants and the most unwanted invasive weeds of California. Grade level: 3-5. Santa Cruz Productions, Wastewater Filamentous Bacteria Dichotomous Key, accessed 02/16/01, home1.gte.net/vsjslsk1/gramstainflowchart.htm Description: A completely web-based dichotomous key designed to assist students in identifying wastewater bacteria. 2 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program

3 STUDENT INSTRUCTIONS You will be using a dichotomous key to identify a group of shells or pictures of shells. At the end of the activity, every shell or picture should be accurately identified. Some things to keep in mind when using a dichotomous key: Always read and consider both choices, even if the first one seems to be appropriate. Jumping to conclusions may lead to the wrong classification of the item. Always understand the meaning of the words used in each choice. Define the term. If you are not sure of the meaning, look it up in a dictionary. Never guess, as this could also lead to the wrong classification of the item. When there are measurements given in the choices, use the appropriate measuring tools or adjust them to match your own set of tools. For example, if a key measurement is given in centimeters but your ruler is divided into inches, convert the centimeter measurement into inches. Do not approximate and do not guess. Measure. If you are classifying a living or once-living thing, do not base your conclusion on a single observation. Living things almost always exhibit variability, so it is better to study many specimens in order to be sure that your results are representative of the majority. If you are left with two possible answers, read the description of both and decide which one seems to fit your specimen more precisely. Once you have identified all of your items, do not assume that it is correct. If there is any doubt, recheck the description of the organism to see that it appropriately matches. If it does not, then an error was made somewhere in key development. How to use a dichotomous key: Dichotomous means divided into two parts. That is why dichotomous keys give two choices in each step. In each step, you are presented with two statements based on characteristics of the organism. If you make the correct choice every time, the name of the organism will be revealed at the end. Example: 1. Sort shells by one characteristic; for example, whether the shell is over 3 inches in length or under 3 inches in length. 2. Next, those shells that are over 3 inches will be sorted by a different characteristic; for example, has spiny protrusions or no spiny protrusions. 3. Continue to sort shells by one characteristic until each shell has been identified. 4. With the shells under 3 inches, repeat the same process until all of those shells are identified. 3 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 1

4 Name _ Period Date A Dichotomous Key Activity for Seashells Instructions: Using the images on the card set that your teacher gives you, you should be able to correctly identify where a shell goes in the identification key. Write the number of each shell (noted on the card) on the line provided. I. Univalve A. Outer projections (spikes) _ 1. Spikes at two ends _ a. Shell solid white in color _ b. Shell not solid white _ 2. Spikes at one end only _ a. Shell is longer than it is wide _ (1) Shell has vertical stripes _ (2) Shell has no vertical stripes _ b. Shell is wider than it is long _ B. Spikes not present _ 1. Shell is wider than longer _ a. Shell has speckles _ b. Shell has no speckles _ 2. Shell is longer than wider _ a. Shell is speckled _ (1) Shell is approximately 7 cm from umbo to base _ (2) Shell is approximately 5 cm from umbo to base _ b. speckled _ (1) Shell is striped _ (2) Shell has no stripes _ (a) Curved at pointed end _ (b) No curve at pointed end _ II. Bivalve A. Two shells present (articulated) _ 1. Umbo axial length is approximately 5.0 cm long _ 2. Umbo axial length is less than 5.0 cm long _ B. One shell present _ 1. Shell has pointed umbo area _ a. Shell is smooth _ b. Shell is rough _ 2. Shell has straight umbo area _ 4 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 2

5 Name Period Date A Dichotomous Key Activity for Seashells Instructions: Using the images on the card set that your teacher gives you, you should be able to correctly place a shell in its correct position on the dichotomous key. Once you have correctly identified where the shell goes in the key, write the number of each shell (noted on the card) on the line provided. Spike at two ends _ Shell solid white in color Shell not solid white Outer projections (spikes) Spikes at one end only Shell is longer than it is wide _ Shell has vertical stripes _ Shell has no vertical stripes_ Shell is wider than it is long _ Univalve Shell is wider than it is long Shell is speckles speckles Shell is approximately 7cm from umbo to base Shell is speckles Sea Shells Spikes not Present Shell is approximately 5cm from umbo to base Shell is longer than it is wide speckles Shell is striped Umbo axial length is approx. 5cm long striped Curved at pointed end Two shells present (articulated) Umbo axial length is less than 5cm long No curve at pointed end Bivalve One shell present Shell has pointed umbo area Shell has straight umbo area Shell is striped striped 5 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 3

6 Name Period Date A Dichotomous Key Activity for Seashells Instructions: Using the images on the card set that your teacher gives you, you should be able to correctly place a shell in its correct position on the dichotomous key. Once you have correctly identified where the shell goes in the key, write the number of each shell (noted on the card) on the line provided. I. Univalve 2,3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 16 A. Outer Projections (spikes) 3, 4, 5, 7, 8, Spikes at two ends 8, 10 a) Shell solid white in color 8 b) Shell not solid white Spikes at one end only 3, 4, 7 a) Shell is longer than it is wide 3, 4 (1) Shell has vertical stripes 4 (2) Shell has no vertical stripes 3 b) Shell is wider than it is long 7 B. Spikes not present 2, 5, 6, 9, 11, 14, Shell is wider than longer 2, 16 a) Shell has speckles 16 b) Shell has no speckles 2 2. Shell is longer than wider 5, 6, 9, 11, 14 a) Shell is speckled 6, 11 (1) Shell is approximately 7 cm from umbo to base 6 (2) Shell is approximately 5 cm from umbo to base 11 b) speckled 5, 9, 14 (1) Shell is striped 5 (2) Shell has no stripes 9, 14 (a) Curved at pointed end 14 (b) No curve at pointed end 9 II. Bivalve 1, 12, 13, 15, 17 A. Two shells present (articulated) 12, Umbo axial length is approximately 5.0 cm long Umbo axial length is less than 5.0 cm long 15 B. One shell present 1, 17, Shell has pointed umbo area 1, 13 a) Shell is smooth 13 b) Shell is rough 1 2. Shell has straight umbo area 17 Answer Key 6 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 4

7 Name Period Date A Dichotomous Key Activity for Seashells Instructions: Using the images on the card set that your teacher gives you, you should be able to correctly place a shell in its correct position on the dichotomous key. Once you have correctly identified where the shell goes in the key, write the number of each shell (noted on the card) on the line provided. Answer Spike at two ends 8, 10 Shell solid white in color 8 Shell not solid white 10 Key Outer projections (spikes) 3-5, 7, 8, 10 Spikes at one end only 3, 4, 7 Shell is longer than it is wide 3,4 Shell has vertical stripes 4 Shell has no vertical stripes 3 Shell is wider than it is long 7 Univalve 2-11, 14, 16 Shell is wider than it is long 2,16 Shell is speckles 16 speckles 2 Shell is approximately 7cm from umbo to base 6 Sea Shells Spikes not Present 2, 5, 6, 9, 11, Shell is speckles 6,11 Shell is approximately 5cm from umbo to base 11 Shell is longer than it is wide 5, 6, 9, 11, 14 Shell is striped 5 Two shells present (articulated) 14,15,17 Umbo axial length is approx. 5cm long 14 Umbo axial length is less than 5cm long 15,17 speckles 5, 9, 14 striped 9,14 Curved at pointed end 14 No curve at pointed end 9 Bivalve 1,14,15,17,20 One shell present 1,20 Shell has pointed umbo area 20 Shell has straight umbo area 1 Shell is rough 15 Shell is smooth 17 7 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 5

8 Seashell Pictures for Dichotomous Key Activity Shell 1 Shell 1 Shell 1 Shell 1 Shell 2 Shell 2 Shell 3 Shell 3 Shell 3 Shell 4 Shell 4 Shell 5 Shell 5 Shell 5 Shell 6 Shell 6 Shell 6 8 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 6 - page 1

9 Seashell Pictures for Dichotomous Key Activity Shell 7 Shell 7 Shell 8 Shell 8 Shell 9 Shell 9 Shell 10 Shell 10 Shell 10 Shell 11 Shell 11 Shell 12 Shell 12 Shell 12 Shell 12 Shell 13 Shell 13 9 Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 6 - page 2

10 Seashell Pictures for Dichotomous Key Activity Shell 14 Shell 14 Shell 15 Shell 15 Shell 15 Shell 15 Shell 16 Shell 16 Shell 17 Shell 17 Shell 17 Shell 17 Shell 18 Shell 18 Shell 18 Shell Education on the Halfshell: Dichotomous Key Louisiana Sea Grant College Program Blackline Master 6 - page 3