How does one go about identifying the species, genus, etc., of a specimen whose identity is unknown (to the person who wishes to identify the specimen)? The most useful tool for such identifications is the dichotomous key. A dichotomous taxonomic key contains information useful in identifying similar kinds of organisms, but is arranged in a way that allows the information to be used quickly.
A dichotomous key is a list of paired statements (each statement is called a lead) that guides you to the identification of a specimen. The paired leads (the pair of leads together is called a couplet) are contrasting descriptions of certain characteristics. In a good key, couplets are written so that you must choose one or the other of the leads as being true for your specimen. Couplet leads should always be mutually exclusive. Here is a bad couplet:
1. Many leaves.1' Few leaves.
How many is "many"? How few is "few"? A better couplet would be:
1. Leaves occur in groups of three or more.1' Leaves occur singly or in groups of no more than two.
This couplet allows no ambiguity. The leads are mutually exclusive. In addition, the leads in a couplet should be parallel; that is, both should ask questions about the same character(s).
To use a dichotomous key, one begins with the first couplet, deciding which statement is true for a specimen. The key may then tell you what taxon you have, or it may direct you to another couplet. Follow the key until you have arrived at a taxon. Here's an example of a key to some kinds of fruit:
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1. Fruits occur singly ....................................................... Go to 3 1' Fruits occur in clusters of two or more ......................... Go to 2 2. Fruits are round ....................................................... Grapes 2' Fruits are elongate ................................................... Bananas 3. Thick skin that separates easily from flesh .............Oranges 3' Thin skin that adheres to flesh .............................. Go to 4 4. More than one seed per fruit ............................ Apples 4' One seed per fruit ............................................ Go to 5 5. Skin covered with velvety hairs .................... Peaches 5' Skin smooth, without hairs ........................... Plums |
Note that the overall organization of the key is very important. One cannot simply read through a key to find characteristics that apply to a specimen. For example, statement 3a above (thick skin that separates easily from meat) is true of both oranges and bananas. Correct identification of bananas depends on couplet number 1, which separates clustering vs. non-clustering fruits.
(A) Using a taxonomic key: identification of conifer trees
Branches and cones from several genera of conifers (cone-bearing trees) are available in the lab. Using the attached key, work with a partner to identify the genus of some of these branches.
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1. Leaves needle-like, much longer than wide 2. Needles grouped into clusters
3´ Needles > 5/cluster, clusters on short offshoots of main branch ..... Cedrus (true cedars) 2´ Needles appearing by themselves 4. Each cone scale with papery 3-pronged bracts below ...... Pseudotsuga (douglas fir) 4´ Cone scales without papery bracts below 5. Base of needles flattened along stem ..................... Sequoia (coast redwood) 5´ Base of needles coming directly out of stem 6. Needles sharp to the touch ............................................. Picea (spruce)6´ Needles soft to the touch .......................................... Tsuga (hemlock) 1´ Leaves scale-like, not much longer than wide 7. Cones with only 3 scales, one central and 2 lateral ................ Calocedrus (incense cedar) 7´ Cones with more than 3 scales
8´ Cones over two inches long ............................................. Sequoiadendron (sequoia) |
(B) Constructing A Taxonomic Key
Working in groups, obtain one specimen of each species, and construct a dichotomous key to the specimens. Do not use body size as a characteristic, since the key must be useful for small specimens of species that do become large. Limit the use of color as a character. Color may fade in preserved specimens, and may vary among individuals. Use morphological characters such as body shape, position of fins, presence or absence of scales or spines in fin supports, etc. The diagram below shows the names of fins and other structures, so that you can use these names in your key. When your key is complete, let the members of another group use it to identify some specimens. Were there mistakes or ambiguities in your key that made it difficult to use? Try out the key written by another group, and find the ambiguities in theirs.