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Dichotomous keys: Making and
interpreting them
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.
- 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")
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Choose a second definable feature and further
subdivide the two groups. This feature might be different depending
on whether or not it is appropriate. For example, it may not be
appropriate to divide up boys further by observing whether or not
they're wearing a dress or slacks! |
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children |
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boys |
girls |
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blond hair |
non-blond hair |
wearing dress |
wearing
slacks | |
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wearing
jumper |
no jumper |
wearing
jumper |
no jumper |
ribbon in
hair |
no ribbon |
ribbon
in hair |
no ribbon | |
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James |
Left-
handed |
right-
handed |
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This key could be continued to separate out all the individual
students. 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.
color, 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.
| 1a. |
Boys |
2 |
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| 1b. |
Girls |
3 |
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| 2a. |
Blond hair |
4 |
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| 2b. |
Non- blond |
5 |
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| 3a. |
Wearing dress |
6 |
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| 3b. |
Wearing slacks |
7 |
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| 4a. |
Wearing jumper |
8 |
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| 4b. |
No jumper |
9 |
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| 5a. |
Black hair |
18 |
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| 5b. |
Hair neither blond or black |
19 |
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| 6a. |
Ribbon in hair |
12 |
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| 6b. |
No ribbon in hair |
13 |
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| 7a. |
Ribbon in hair |
14 |
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| 7b. |
No ribbon in hair |
15 |
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| 8a. |
Ear-ring |
16 |
James |
| 8b. |
No ear-ring |
17 |
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9, 10, 11, 12, 13, 14, 15, 16, choose other appropriate
features, each requiring a choice between two alternatives (ie. dichotomous)
| 17a |
Left- handed |
Scott |
| 17b |
Right- handed |
Matthew | | |
