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! |
children
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boys
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girls
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blond hair
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non-blond hair
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wearing dress
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wearing slacks
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wearing
jumper
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no jumper
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wearing
jumper
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no jumper
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ribbon in
hair
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no ribbon
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ribbon
in hair
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no ribbon
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James
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Left-
handed
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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.
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.
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
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James
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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
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17b |
Right- handed |
Matthew
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