Introduction to Classification of Organisms
Senior 1 · Biology
Introduction to Classification of Organisms
Lesson 4 of 5
25 min
A school trip drives through Akagera National Park. Ishimwe photographs six large mammals, but he has never seen most of them before and the guide is busy. How can he find their names without guessing?
Biologists use a tool called an identification key. It asks you to look at one feature at a time and choose between two descriptions. Each choice sends you to the next step, until only one name is left.
Definition
Identification key: a set of steps, based on observable features, that leads the user to the name of an organism.
Dichotomous key: a key in which every step offers exactly two contrasting choices. Dichotomous means "cut in two".
Couplet: one numbered step of a dichotomous key: a pair of statements about the same feature, such as "1a. Coat has stripes" and "1b. Coat has no stripes". Each statement ends either with a name or with "go to" another couplet.
| No. | Statement | Go to or name |
|---|---|---|
| 1a | Body covered all over in a bold pattern of stripes or patches | go to 2 |
| 1b | No pattern covering the body: coat one colour, or darker above and paler below | go to 3 |
| 2a | Pattern of black and white stripes | Plains zebra, Equus quagga |
| 2b | Pattern of brown patches separated by pale lines | Giraffe, Giraffa camelopardalis |
| 3a | Legs very short: the barrel-shaped belly almost touches the ground | Hippopotamus, Hippopotamus amphibius |
| 3b | Legs long: the belly is held well off the ground | go to 4 |
| 4a | Feet with soft pads and claws | Lion, Panthera leo |
| 4b | Feet with hooves | go to 5 |
| 5a | Dark grey to black; heavy body | African buffalo, Syncerus caffer |
| 5b | Reddish-brown back, paler sides and white belly; slender body | Impala, Aepyceros melampus |
Figure 1: The same key drawn as a branching diagram. Start at couplet 1 and follow the branch that matches your animal until you reach a name.
Worked example 1: identifying animal X
Ishimwe's first photo shows a large grey animal with no stripes or patches. Its belly nearly touches the mud, and it is standing at the edge of a lake.
Answer: hippopotamus, Hippopotamus amphibius. Notice that "standing at a lake" was not needed: a key uses only the features in its statements.
Worked example 2: identifying animal Y
The second photo shows a graceful animal with a reddish-brown back, paler sides, a white belly, long thin legs and hooves. It has no horns.
Answer: impala, Aepyceros melampus. It has no horns because it is a female: only male impala grow them. That is exactly why this key does not use horns as a feature.
Warning
Mistakes that give the wrong name. (1) Reading only the first statement of a couplet: always read both before you choose. (2) Using a key outside its range: this key is for six adult mammals, so a warthog or a young lion cub (which has faint spots) will be misidentified. (3) Jumping ahead to the couplet you "expect" instead of following the "go to" instruction.
Park rangers use keys to record which birds and mammals they see; farmers and agronomists use them to tell a harmless caterpillar from a crop pest; nurses and laboratory technicians use them to identify the mosquito or parasite behind a disease. The botanists who describe the plants of Nyungwe and the Rwandan hills use keys hundreds of couplets long.
Dichotomous Keys: Identification Achievement Unlocked (Amoeba Sisters). Watch how each step splits the group in two, and pause to try the example before they solve it.
Tip
Memory hook: a dichotomous key is like a path through a forest where every fork has only two ways. Read both signs, take one, and never skip a fork.
Practice: using a dichotomous key
Key points
Sources and credits
Written by Ganzaa: the lesson text, the key to Akagera mammals, Figure 1, the worked examples and the practice questions.
