Systematists rely on keys to help identify known organisms and determine whether they have discovered a new organism entirely. Taxonomic keys are useful tools guiding researchers towards the known name of an organism. However, all taxonomic keys are not created equally. They are often created on a regional level or for a particular group of organisms i. So it is important to pick a key that represents the diversity of the region or group of organisms you are interested in examining.
Dichotomous comes from the Greek root dich -, meaning "two" and temnein , meaning "to cut. At each couplet of a dichotomous key the user is presented with two choices about a specific character present in the group of organisms, a specific character state is described for each lead. Sometimes the characters are quantitative i. Each couplet provides characteristics that become progressively more specific until the final step is reached and identification is made.
Followed correctly, keys will lead you to the correct name of an unknown organism or object. Dichotomous keys can be developed to identify anything in any sort of classification. The keys are generated using interactive computer programs. Polyclave keys use a process of elimination.
The user is presented with a series of choices that describe features of the species they wish to identify. Download Attachment 1.
Download Worksheet 1. Download Lesson Plan. This type of key is not hard to use, but makes more sense when students undertake the process of making one before using it. The instructor will model the process before students begin work on their own keys.
After the demonstration, students will first make a Branching Key using a picture of an organism of their choice. Once the Branching Key is completed, students will convert it into a written dichotomous key.
The end product will be a poster displaying the student pictures, along with their written dichotomous keys. After modeling the process, direct students to take out the ten pictures that they collected for their project. Students will then go through the same process to create a dichotomous key unique to their pictures.
Students will make a poster with a written dichotomous key and pictures on the front and their branching keys rough draft on the back. After grading, students should display their dichotomous keys around the room for all students to observe.
Students will be given different unknown organisms with keys for identification and will work through the identification process. Sources of keys: internet or textbooks. 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. 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. Blackline Master 1 gives students instructions on how to use a dichotomous key in general.
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