Dichotomous keys: Making and interpreting them

Dichotomous keys

Taxonomists are scientists who classify and name plants and animals. Taxonomists attempt to group and classify similar organisms to aid communication and understanding.

In the 1700s, Carolus Linnaeus (a Swedish botanist) used a two name system which is now called "binomial nomenclature" to classify 12,000 organisms grouped according to similar characteristics. This means that, when told that Cygnus atratus is a bird , everyone instantly knows it has feathers, is warm-blooded, and has a four-chambered heart like all birds.

Dichotomous keys
To classify a large collection of different objects, we look for similarities and group them into increasingly smaller groups which share many features in common.

A dichotomous key is a method for identifying and classifying objects, where each feature selected to help identification requires a choice between two alternatives.

As an example, let us make a dichotomous key for a class of 30 children.

1. Choose a definable difference to separate them into two groups. For example, the difference can't be "big or small" (because these choices are not well enough defined, but could be "under 100 cm in height "and "100 cm or over in height")

children boys girls

children
boys	girls
blond hair non-blond hair wearing dress wearing slacks
wearing jumper no jumper wearing jumper no jumper ribbon in hair no ribbon ribbon in hair no ribbon
ear-ring no ear-ring
James Left- handed right- handed
Scott Matthew

Using this key, a visitor to the classroom could quickly work out which child was James. Similarly, the visitor would be able to work out the name of the blond, ear-ringed boy with the jumper.

Note: Some features may not be as useful as others. What happens if James gets hot and takes off his jumper? And what if Scott peroxides his hair?

Using these characteristics in this situation, James, Scott and Michael share many features in common, which may be very different from the others in the group.

The usefulness of the key depends on the features used to sub-divide the group.

colour, size, habitat, and so on are not the best features to use for living things, as these can change within an organism's lifetime.

Biologists mainly use structural characteristics as the basis for their work of classifying, because these usually reflect a close evolutionary relationship. For example, a red parrot has more in common with a blue wren than with a red-backed spider.

An alternative method of recording the dichotomous key for the children is shown below.1a.

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