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BIOL 201: Dichotomous Key Lab • Taxonomy is the science dealing with description, identification,
nomenclature, and classification of living things. • A dichotomous key is a tool that allows users to identify items or
organisms in a systematic and reproducible fashion. • Dichotomous keys may be used in a variety of situations, such as for
identifying rocks and minerals as well as for identifying unknown organisms to some taxonomic level (e.g., species, genus, family, etc.).
• What makes these keys distinctive is that they are ordered in such a way
that a series of choices is made that leads the user to the correct identity of the item they are looking at. If the user makes the correct choice every time, the name of the organism will be revealed at the end.
• "Dichotomous" means, "divided into two parts." Therefore, dichotomous
keys always offer two choices for each step, each of which describes key characteristics of a particular organism or group of organisms.
• There are two kinds of descriptions that might be presented to the user of
a dichotomous key: qualitative and quantitative descriptions.
1. 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."
2. 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.
• You simply compare the characteristics of an unknown organism against
an appropriate dichotomous key. These keys will begin with general characteristics and lead to couplets indicating progressively specific characteristics.
• If the organism falls into one category, you go to the next indicated couplet. By following the key and making the correct choices, you should be able to identify your specimen to the indicated taxonomic level (depends on what level the key is designed).
• Couplets can be organized in several forms. The couplets can be
presented using numbers (numeric) or using letters (alphabetical). The couplets can be presented together or grouped by relationships. There is no apparent uniformity in presentation for dichotomous keys.
When you follow a dichotomous key, your task becomes simpler if you adhere to a few simple rules of thumb: A. Read both choices in a couplet carefully. Although the first description may seem to fit your sample, the second may apply even better. B. Keep notes telling what sequence of identification steps you took. This will allow you to double-check your work later and indicate sources of mistakes, if they have been made. C. If you are unsure of which choice to make in a couplet, follow both forks (one at a time). After working through a couple of more couplets, it may become apparent that one fork does not fit your sample at all. D. Work with more than one sample if at all possible. This will allow you to tell whether the one you are looking at is typical or atypical. This is especially true when working with plants – examine more than one leaf, branch, cone, seed, flower,…etc. E. When you have keyed out an organism, do not take your effort as the final result. Double check your identification scheme, using your notes. Find a type specimen (if available) and compare your unknown to the type specimen. If a type specimen is unavailable, find a good description of the indicated taxonomic group and see if your unknown reflects this description. F. When reading a couplet, make sure you understand all of the terms used. The best keys will have a glossary of technical terms used in the key. If a glossary is unavailable, find a good reference work for the field (textbook, biological dictionary,…etc.) to help you understand the term.
G. When a measurement is indicated, make sure that you take the measurement using a calibrated scale. Do not “eyeball” it or take a guess. Take home exercise on using dichotomous keys: go to the following web site: http://dendro.cnre.vt.edu/dendrology/idit.htm
1. Try & key out the leaf types that you collected in lab. Although this is a computerized version of a dichotomous key, the basic principle you employed with the paper version is the same. This Virginia Tech key (or VTREE app) is pretty good, and it works for native trees etc. in the region, and outside.
2. Tell me what you found out next week. You won’t be graded, but it
was probably fun anyway!
3. See if you can find a pretty good computerized key for animal taxa, mammal skulls would be good since you tend to find them while walking in the woods, by the roadside etc. since bleached bone stands out. If it’s a good one, we can use it in lab for the future.
Additional Resources: • This website is helpful for understanding dichotomous keys (a couple of
online exercises) and also about Biological Classification & Phylogeny reconstruction. It’s worth a look!