Cambridge O Level Biology · Syllabus 5090 · Classification
Dichotomous Key
What is Dichotomous Key?
A dichotomous key is an identification tool made of numbered pairs of contrasting statements. At each numbered stage the user chooses whichever one of the two statements matches the specimen, and is sent either to another numbered pair or to the name of the organism, so repeated choices narrow an unknown specimen down to a single identification.
This definition is part of the Classification chapter in Cambridge O Level Biology.
Dichotomous Key in context
Classification is the placing of living organisms into groups on the basis of the features they share, so that organisms with many features in common sit in the same group. Cambridge O Level Biology (5090) Chapter 2 covers four things: the definition of a species and the binomial naming system, dichotomous keys, the five kingdoms (animal, plant, fungus, prokaryote and protoctist), and the observable features that separate the vertebrate groups, the arthropod groups, ferns from flowering plants and monocotyledons from dicotyledons. Viruses sit apart from all five kingdoms because they have no cellular structure.
Questions students ask about Dichotomous Key
How does a dichotomous key work?
A dichotomous key is a series of numbered pairs of contrasting statements. At each stage you choose whichever one of the two statements matches your specimen, and it sends you either to another numbered pair or to the name of the organism, so repeated choices narrow an unknown organism down to a single identification. The statements must be observable and mutually exclusive: "wings present / wings absent", not "has wings / is small", which one organism could satisfy at the same time.