Dichotomous Keys. Lab Exercise. Contents. Objectives. Introduction

Transcription

1 Lab Exercise Dichotomous Keys Contents Objectives 1 Introduction 1 Activity.1 Key to the Animal Kingdom 4 Activity.2 Creating a Key 6 Resutls Section 7 Objectives - Understand the concept of classification - Classify objects or organisms - Use and develop a dichotomous key Introduction The branch of science, which classifies living organisms based on their similarities, is known as taxonomy. This classification of organisms is an area that many students find difficult on their first exposure. The names used are foreign, the hierarchy of groups gets confusing, frustration occurs when one author classifies an organism one way and another classifies the same organism differently, and the whole process of placing organisms into these various groups remains a mystery. For the most part, the first two problems of learning the names for organisms and sorting out the hierarchy of groups is a matter of putting in the required time to learn (memorize if you prefer) them. It may help to know some of the reasons for deriving the names of the organisms from Latin or Greek roots. First, these are the languages originally used by early taxonomists to name and classify organisms. Second, because these languages are not in common use, the meanings of the word roots remain constant. Third, these scientific names are the same throughout the world. So if one were to find one s self in California, the people there would use the same scientific name for the same organism while the common name may very well differ. Finally, use of these dead languages doesn t bias the classification system toward one particular nationality. For learning the hierarchy of taxonomic groups, creating a mnemonic may be helpful. For example, you may be able to remember that King Philip Can Order Fine Green Socks more easily than Kingdom, Phylum, Class, Order, Family, Genus, and Species. Identification guides, called dichotomous keys, have been developed to help people identify an organism according to its scientific name. The term dichotomous comes from the word dichotomy, meaning a division into two mutually exclusive groups. When using this type of key, the user is asked to systematically select between two alternative characteristics until arriving at the organism s identification. One observes the characteristics of the organism, determines which description best fits that organism, and then is directed to the next set of choices in the key. The next page shows two examples of dichotomous keys. The first example is a key to the five kingdoms of organisms (in this example all of the prokaryotes are considered members of the Kingdom Monera). First, all organisms are divided into two groups based on the presence or absence of a nuclei within their cells. Then, those with nuclei are further divided based on whether or not they have tissues or not and so on until all organisms are placed within a kingdom. 1

2 Key to 5 Kingdoms 1a Cells lack nucleus and other organelles......k. Monera 2a Multicellular organisms with cells forming tissues and organs...3 3a Organisms photosynthetic autotrophs k. Plantae 3b Organisms heterotrophic with internal digestion of food...k. Animalia 1b Cells with nuclei and other organelles...2 Single celled organisms or organisms without tissues and organs...4 4a Body composed of thread-like cells, organisms heterotrophic... K. Fungi 4b Body not composed of threadlike cells, or if thread-like, then photosynthetic...k. Protista 1a. Cells lack nucleus and other organelles...k. Monera 1b. Cells with nuclei and other organelles...2 2a. Multicellular organisms with cells forming tissues and organs...3 3a. Organisms photosynthetic autotrophs....k. Plantae 3b. Organisms heterotrophic with internal digestion of food...k. Animalia. Single celled organisms or organisms without tissues and organs...4 4a. Body composed of thread-like cells, organisms heterotrophic... K. Fungi 4b. Body not composed of thread-like cells, or if thread-like, then photosynthetic...k. Protista 2

3 Key to Playing Cards The second example is a key to identify several playing cards. Only five cards are used in this example but this key could easily be expanded to account for all 52 cards in a poker deck. The five cards: 1a. Face Cards...2 2a. Face with a beard...king of Spades. Face without a beard...queen of Hearts 1b. Numerical Cards...3 3a. Red suit...4 4a. Suit a heart shape...7 of Hearts 4b. Suit not a heart shape...9 of Diamonds 3b. Black suit...10 of Spades This example of the key to playing cards points out an important limitation of dichotomous keys. These keys are only reliable when identifying an organism (or object) that was part of the set of organisms considered in the key s creation. Organisms (or objects) outside that set may be misidentified by the key. In the key to the five cards for example, if one tried using this key to identify the 3 of clubs, this key would identify that card as the 10 of spades. 3

4 Activity.1 Simulation: Key to the Animal Kingdom Below is a copy of the dichotomous key found in the computer simulation. This key allows you to identify the taxonomic group of various animals. To begin the simulation, select the Dichotomous Key module in The Science of Biology section of the BiologyOne DVD. Then click on the arrow in the lower right to start. Look at the picture of the organism to be keyed. If you wish to change to a different organism, click on the Key Another Organism button. In this key, the first choice you are asked to make is whether the organism displays symmetry or not. Click on the choice that best describes this organism. Next, you will then be presented with a second choice of characteristics. Again, select the choice that best fits the organism. Continue this procedure until the organism s identity is reached. Use the glossary in your textbook for assistance with terms you are unfamiliar with. Record your selections in the Results Section. Key to Animal Groups 1a. No apparent symmetry Go to 2 1b. Symmetry apparent Go to 3 2a. Organism lacks shell Phylum Porifera. Shell present Class Gastropoda 3a. No apparent segmentation and no vertebrae present Go to 4 3b. Body apparently segmented or vertebrae present Go to 18 4a. Radial symmetry Go to 5 4b. Bilateral symmetry Go to 12 5a. Symmetry pentamerous (5 ways) Go to 6 5b. Symmetry not pentamerous Go to 10 6a. Body with short or long spines Go to 7 6b. Body without spines Go to 9 7a. Body star-shaped Go to 8 7b. Body not star-shaped Class Echinoidea 8a. Arms narrow and long Class Ophiuroidea 8b. Arms thick and shorter Class Asteroidea 9a. Body cucumber shaped Class Holothuroidea 9b. Body stalk shaped, feather-like arms Class Crinoidea 10a. Medusa body Class Scyphozoa 10b. Polyp body Go to 11 4

5 Key to Animal Groups (cont.) 11a. Few, long tentacles Class Hydrozoa 11b. Many, short tentacles Class Anthozoa 12a. Body covered by shell or with tentacles Go to Body without shell or tentacles Go to 15 13a. Tentacles present Class Cephalopoda 13b. Tentacles absent Go to 14 14a. Shell of 2 parts, halves hinged Class Bivalvia 14b. Shell greater than 2 parts Class Polyplacophora 15a. Body round & worm-like Go to 16 15b. Body flattened Go to 17 16a. Bulb at anterior end of body Phylum Nemertina 16b. Lacks bulb at anterior end Phylum Nematoda 17a. Suckers absent Class Turbellaria 17b. Suckers present Class Trematoda 18a. Paired appendages not apparent Go to 19 18b. Paired appendages apparent Go to 22 19a. Flattened body, all but first segment identical Class Cestoda 19b. Round, worm-like body Go to 20 20a. Vertebrae present Go to 34 20b. Vertebrae absent Go to 21 21a. Suckers absent Class Oligochaeta 21b. Suckers present Class Hirudinea 22a. Body with exoskeleton Go to Body without exoskeleton Go to 28 23a. Body divided into 2 or 3 sections Go to 24 23b. Body divided into more than 3 sections Go to 27 24a. Body divided into 2 sections Go to 25 24b. Body divided into 3 sections Class Insecta 25a. Body with obvious book lungs Class Merostomata 25b. Book lungs not obvious Go to 26 5

6 Key to Animal Groups (cont.) 26a. Antennae absent Class Arachnida 26b. Antennae present Class Crustacea 27a. 1 pair of appendages per segment Class Chilopoda 27b. 2 pair of appendages per segment Class Diplopoda 28a. Backbone absent Go to 29 28b. Backbone present Go to 30 29a. Cylindrical body Subphylum Urochordata 29b. Elongated body Subphylum Cephalochordata 30a. Hair or feathers absent Go to 31 30b. Hair or feathers present Go to 35 31a. Lacking jaws Class Agnatha 31b. Jaws present Go to 32 32a. Fish-like body Go to Not a fish-like body Go to 34 33a. Operculum absent Class Chondrichthyes 33b. Operculum present Class Osteichthyes 34a. Scales absent Class Amphibia 34b. Scales present Class Reptilia 35a. Hair and mammary glands Class Mammalia 35b. Feathers, lacking mammary glands Class Aves Activity.2 Creating a Key In the Results Section you are asked to create a key to a number of shapes. One of the ways to do this is to start by grouping the shapes into two groups based on a characteristic of the shapes (for example: shapes with flat sides vs. shapes without flat sides). Then each subgroup is divided into two more groups based on another characteristic. This continues until each group contains only one member. 6

7 Lab Exercise Name Results Section Activity.1 Key to the Animal Kingdom Common name of organism to be keyed: Record your choices below (i.e. 1b, 3a, 1, etc.) Choice Selected Characteristic Phylum or class of organism: 7

8 Activity.2 Creating a Key In the space below write a dichotomous key to the above shapes. Do not use the name of the shape to describe it, rather, use descriptive phases (i.e. shape has 4 straight sides or equal length). Identify the shapes by their number. 8