(DIV) Dichotomous Keys Lesson
Dichotomous Keys Lesson
Taking a walk on a nature trail or a park is a good way to observe the different types of organisms in your environment. Let's say you're hungry and you want to eat something right from nature, but you want to be sure it's edible and safe. Or you touched that plant over there and you think it may have been poison ivy! Wouldn't it be great if you could identify different species of organisms easily to avoid trouble on your hike? That is why a dichotomous key is useful.
A dichotomous key uses pairs of contrasting descriptions (called couplets) to identify unknown plants, animals, other living things, or even objects. A dichotomous key progresses through a series of choices (general to specific), much like a "choose your own adventure story." Dichotomous means "divided in two parts".
At each step of the process of using the key, the user starts at the first couplet and is given two choices; each alternative leads to another question and to another (and so on) until the item is identified. The dichotomous key allows for the scientist to ask a series of questions with yes or no answers. Each question should be phrased so that the answer will either be yes or no .
Here is an example of a diagnostic dichotomous key for some eastern United States oaks based on leaf characteristics:
1a. Leaves usually without teeth or lobes: 2
1b. Leaves usually with teeth or lobes: 5
2a. Leaves evergreen: 3
2b. Leaves not evergreen: 4
3a. Mature plant a large tree - Southern live oak Quercus virginiana
3b. Mature plant a small shrub - Dwarf live oak Quercus minima
4a. Leaf narrow, about 4-6 times as long as broad - Willow oak Quercus phellos
4b. Leaf broad, about 2-3 times as long as broad - Shingle Oak Quercus imbricaria
5a. Lobes or teeth bristle-tipped: 6
5b. Lobes or teeth rounded or blunt-pointed, no bristles: 7
6a. Leaves mostly with 3 lobes - Blackjack oak Quercus marilandica
6b. Leaves mostly with 7-9 lobes - Northern red oak Quercus rubra
7a. Leaves with 5-9 deep lobes - White oak Quercus alba
7b. Leaves with 21-27 shallow lobes - Swamp chestnut oak Quercus prinus
This key differentiates between oaks with entire leaves with normally smooth margins (live oaks, Willow oak, Shingle oak), and other oaks with lobed or toothed leaves. The steps created smaller and smaller groups (e.g. red oak, white oak), until the species has been keyed out.
Rules for Using Dichotomous Keys
- Read both parts of each couplet carefully.
- If you do not understand a term used, look it up!
- If you are asked to measure something, do not guess at the measurement, use the proper tool!
- If you are unsure which choice to make, follow both paths and see where each takes you; inevitably one will not fit.
- Write down the order of steps you took (couplet 1b, to 4b, to 6a)
- Double check your work!
For Practice: Smiley Face Dichotomous Key
Instructions: Use the key below to identify the species name of each of the smileys. To check your work, drag each smiley to it's correct name below.
1a. Teeth visible .....go to 2
1b. Teeth not visible .....go to 4
2a. Has a wide, toothy smile .....Smilus toothyus
2b. Is not smiling .....go to 3
3a. Visibly crying .....Smilus dramaticus
3b. Frowning .....Smilus upsettus
4a. Eyes are symmetrical .....go to 5
4b. Eyes not symmetrical .....go to 8
5a. Eyes shaped like hearts .....Smilus valentinus
5b. Eyes are shaped as ovals .....go to 6
6a. Smiling, happy face .....Smilus traditionalis
6b. Not happy, frowning or other .....go to 7
7a. Mouth curved down, frowning .....Smilus saddus
7b. Mouth is a small circle .....Smilus suprisus
8a. Has a pirate eye patch ......Smilus piratus
8b. Does not have eye patch .....go to 9
9a. One eye is much larger than the other eye.....Smilus mutatus
9b. One eye is winking .....Smilus winkus
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