Which species is it?: Taxonomists use a variety of tools to identify an organism: to find out which species it belongs to.
The points to remember
- Identification means finding out which species one organism belongs to.
- Dichotomous keys: pairs of statements about visible features.
- Reference collections: compare the specimen with ones already named by experts.
- DNA surveys: compare its DNA with known species.
- Apps and databases: match a photo or a sound with named records.
Remember it as: Key, collection, DNA, app: four ways to put a name on it.
A dichotomous key works from what you can see. A reference collection and DNA are used by experts when looks are not enough.
Real example: London's Natural History Museum holds about 80 million named specimens, which scientists use to check new finds.
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A dichotomous key offers two choices at each step. 'Dichotomous' means cut in two.
How to use a key
- Start at couplet 1 and read both statements.
- Choose the one that fits the specimen.
- Follow 'go to' to the next couplet...
- ...until you reach a species name.
- Give one name per specimen: only the first name counts.
Worked example: Specimen C is red, so 1a sends you to 2. It has seven black spots, so 2b: it is a seven-spot ladybird.
Try the other three yourself, then check the tree.
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To build a key, you choose features that split the species into two groups, again and again.
How to build a key
- Choose visible, fixed features: spots, fins, number of tails, leaf edges.
- Each question splits the group into two.
- Keep splitting until each species is alone at an end point.
- Write it as numbered pairs with 'go to', or yes/no questions.
- Do not use size, behaviour, diet or where it lives.
Here, a fin on the back splits one dolphin from two; fin shape splits the last two.
Features that do not count: Size changes with age, and behaviour, diet and location cannot be seen on the specimen. 'The colour is different' is not enough: say which colour, such as 'red wing cases'.
Real example: these three dolphins all live off South America. A key that asked 'Is it large?' would fail: a young bottlenose dolphin is smaller than an adult tucuxi.
Each tool has strengths and limits, so taxonomists often use more than one.
Strengths and limits of the tools
- Keys: cheap and quick in the field, and no expert needed...
- ...but damaged or young specimens and look-alike species cause errors.
- Reference collections: accurate, named by experts, but kept in museums.
- DNA: separates species that look the same, but needs a laboratory and money.
- Apps: fast, from a photo or a sound, but can be wrong and need a signal.
In the field
- Dichotomous keys.
- Apps: photo or sound.
In the laboratory or museum
- Reference collections.
- DNA surveys.
Real example: two British bats, the common and the soprano pipistrelle, look almost the same. Their calls and their DNA showed in the 1990s that they are two species.
Apps: iNaturalist lets anyone upload a photo; experts and the app suggest the species.
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How this comes up: Every few sessions: use a key to name species [2], build a key [3], or state features for a key [1-2].
Figure 1 shows four animals caught by students in a stream in the Lake District, England.
State three structural features that could be used to construct a dichotomous key for the animals in Figure 1.
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