Dichotomous key
Dichotomous key
Construction and Use of Dichotomous Keys
• Biological keys are sets of statements that act as clues leading to the identification of an organism.
• By following the keys we can be able to place an organism in its group.
• The most common key is the dichotomous key.
• This is a biological tool for identification of unknown organisms.
• The word dichotomous means branching into two.
• A single characteristic is considered at a time.
• Two contrasting statements are put forward to describe the characteristics in such a way as to separate the organisms.
• This continues until all the organisms have been identified.
Rules Used to Construct a Dichotomous Key
• Use morphological characteristics as far as possible e.g. type of leaf - simple or compound.
• Select a single characteristic at a time and identify it by number. 1. Type of leaf. .
• Use identical forms of words for two contrasting statements e.g.:
a) Flowers scented.
b) Flowers not scented.
• Start with a major characteristic that divide the organisms into two large groups then proceed to lesser variations that would separate the organisms further into smaller groups.
• Use positive statements especially the first one.
• Avoid generalizations e.g. short plants. Be specific in your description e.g.:
a) Plants above 1m tall.
b) Plants below 1m tall.
Some Common Features Used for Identification
In Plants
Leaves
1. Type of leaf Leaf
Compound leaves.
• Simple leaf
• Trifoliate
• Pinnate
• Type ofleaf margin.
Type of venation.
• Type ofleaf arrangement on stem.
• The colour of leaf.
• The texture ofleaf; whether hairy or smooth.
• Shape of the leaf e.g. palmate.
Stem
• Type of stem - woody or herbaceous.
• Shape of stem - cylindrical or rectangular.
• Texture of stem smooth or spiny.
Infloresence
• Are flowers terminal or lateral
• For each flower:
• Is the flower regular or irregular?
• Number of floral parts for each whorl.
• Are floral parts free or fused?
Roots
• Type of root system- Taproot or fibrous?
• Function of the root.
In Animals
Features used to identify animals:
• Type of mouthparts.
• Type of skeleton.
• Presence or absence of antennae.
• Body segmentation.
• Body covering: scales, fur, hair or feathers.
• Number of body parts.
• Locomotory structures: legs, wings and fins.
• Presence or absence of vertebral column.
• Presence and type of eves.
• Biological keys are sets of statements that act as clues leading to the identification of an organism.
• By following the keys we can be able to place an organism in its group.
• The most common key is the dichotomous key.
• This is a biological tool for identification of unknown organisms.
• The word dichotomous means branching into two.
• A single characteristic is considered at a time.
• Two contrasting statements are put forward to describe the characteristics in such a way as to separate the organisms.
• This continues until all the organisms have been identified.
Rules Used to Construct a Dichotomous Key
• Use morphological characteristics as far as possible e.g. type of leaf - simple or compound.
• Select a single characteristic at a time and identify it by number. 1. Type of leaf. .
• Use identical forms of words for two contrasting statements e.g.:
a) Flowers scented.
b) Flowers not scented.
• Start with a major characteristic that divide the organisms into two large groups then proceed to lesser variations that would separate the organisms further into smaller groups.
• Use positive statements especially the first one.
• Avoid generalizations e.g. short plants. Be specific in your description e.g.:
a) Plants above 1m tall.
b) Plants below 1m tall.
Some Common Features Used for Identification
In Plants
Leaves
1. Type of leaf Leaf
Compound leaves.
• Simple leaf
• Trifoliate
• Pinnate
• Type ofleaf margin.
Type of venation.
• Type ofleaf arrangement on stem.
• The colour of leaf.
• The texture ofleaf; whether hairy or smooth.
• Shape of the leaf e.g. palmate.
Stem
• Type of stem - woody or herbaceous.
• Shape of stem - cylindrical or rectangular.
• Texture of stem smooth or spiny.
Infloresence
• Are flowers terminal or lateral
• For each flower:
• Is the flower regular or irregular?
• Number of floral parts for each whorl.
• Are floral parts free or fused?
Roots
• Type of root system- Taproot or fibrous?
• Function of the root.
In Animals
Features used to identify animals:
• Type of mouthparts.
• Type of skeleton.
• Presence or absence of antennae.
• Body segmentation.
• Body covering: scales, fur, hair or feathers.
• Number of body parts.
• Locomotory structures: legs, wings and fins.
• Presence or absence of vertebral column.
• Presence and type of eves.
Examples of key construction
What is a dichotomous key?
• A biological device (tool) which enables one to identify an organism by progressively opting between two alternative observable characteristics
Necessity of using a dichotomous key
• used to identify organisms quickly and accurately
• by following the statements in the key we are able to identify each organism on the basis of a characteristic which is not to be found in other specimens
Rules followed in constructing a dichotomous key
• use observable characteristics only
• start with major characteristics, placing organisms into two groups at each stage
• use a single characteristics at a time
• use contrasting characteristics at each stage e.g 1(a) short, 1(b) tall
• avoid repeating the same characteristics
Procedure of using a dichotomous key and a list of major features of the characteristics to be identified
• look at the features of similarities
• look at the features of differences between the organisms
• we can then be able to identify the organisms by distinguishing one from another
• the key uses a method of elimination by following statements that are correct only for the organism
Example
i)You are provided with a specimen leaves(Araicaria, Cynodon, Grevellea, Kalanchoa, Brassica, Tradescantia, Commelina). Use the dichotomous key below to identify the taxonomic group to which the specimen belongs. Show the steps (number and letter) in the key that you followed to arrive at the identify of the specimen
1 a) leaf broad ...................................................................................................................................go to 2
b) leaf narrow ..................................................................................................................................Araicaria
2 a) leaf parallel veined .................................................................................................................Cynodon
b) leaf net-veined ............................................................................................................................go to 3
3 a) leaf with one lobe (simple) ...................................................................................................go to 4
b) leaf with many lobes (compound) .....................................................................................Grevellea
4 a) leaf fleshy ........................................................................................................................................Kalanchoa
b) leaf not fleshy ...............................................................................................................................go to 5
5 a) leaf petiole modified to form sheath................................................................................ go to 6
b) leaf petiole not modified to form sheath ......................................................................Brassica
6 a) leaf purple .....................................................................................................................................Tradescantia
b) leaf green ....................................................................................................................................Commelina
Steps – 1a, 2b, 3a, 4b, 5b Identify – Brassica
ii) You have been provided with four animals labeled K (mature adult housefly), L (mature adult grasshopper, M(maize flour beetle) and N(worker termite) use the dichotomous key below to identify the specimens. Write down in the correct order, the steps (number and letter) in the key that you followed to arrive at your answer.
Dichotomous key
1 a) animal with wings.............................................................................................................................go to 2
b) animal without wings.................................................................................................................... go to 7
2 a) with two pairs of wings ............................................................................................................... go to 3
b) with one pair of wings ................................................................................................................. Diptera
3 a) with membranous wings ............................................................................................................go to 4
b) hind pair of membranous wings ...........................................................................................go to 6
4 a) with long abdomen ..................................................................................................................Odontata
b) medium sized abdomen ............................................................................................................go to 5
5 a) wings with coloured scales............................................................................................. Lepidoptera
b) wings without scales...................................................................................................... hymenoptera
6 a) forewings hard and shell-like ...................................................................................... coleopteran
b) forewings hard but not shell-like................................................................................ Orthoptera
7 a) body horizontally flattened ................................................................................................... Isoptera
b) body laterally flattened...........................................................................................Symphonoptera
Identify the orders o the various specimens as per the table below
• A biological device (tool) which enables one to identify an organism by progressively opting between two alternative observable characteristics
Necessity of using a dichotomous key
• used to identify organisms quickly and accurately
• by following the statements in the key we are able to identify each organism on the basis of a characteristic which is not to be found in other specimens
Rules followed in constructing a dichotomous key
• use observable characteristics only
• start with major characteristics, placing organisms into two groups at each stage
• use a single characteristics at a time
• use contrasting characteristics at each stage e.g 1(a) short, 1(b) tall
• avoid repeating the same characteristics
Procedure of using a dichotomous key and a list of major features of the characteristics to be identified
• look at the features of similarities
• look at the features of differences between the organisms
• we can then be able to identify the organisms by distinguishing one from another
• the key uses a method of elimination by following statements that are correct only for the organism
Example
i)You are provided with a specimen leaves(Araicaria, Cynodon, Grevellea, Kalanchoa, Brassica, Tradescantia, Commelina). Use the dichotomous key below to identify the taxonomic group to which the specimen belongs. Show the steps (number and letter) in the key that you followed to arrive at the identify of the specimen
1 a) leaf broad ...................................................................................................................................go to 2
b) leaf narrow ..................................................................................................................................Araicaria
2 a) leaf parallel veined .................................................................................................................Cynodon
b) leaf net-veined ............................................................................................................................go to 3
3 a) leaf with one lobe (simple) ...................................................................................................go to 4
b) leaf with many lobes (compound) .....................................................................................Grevellea
4 a) leaf fleshy ........................................................................................................................................Kalanchoa
b) leaf not fleshy ...............................................................................................................................go to 5
5 a) leaf petiole modified to form sheath................................................................................ go to 6
b) leaf petiole not modified to form sheath ......................................................................Brassica
6 a) leaf purple .....................................................................................................................................Tradescantia
b) leaf green ....................................................................................................................................Commelina
Steps – 1a, 2b, 3a, 4b, 5b Identify – Brassica
ii) You have been provided with four animals labeled K (mature adult housefly), L (mature adult grasshopper, M(maize flour beetle) and N(worker termite) use the dichotomous key below to identify the specimens. Write down in the correct order, the steps (number and letter) in the key that you followed to arrive at your answer.
Dichotomous key
1 a) animal with wings.............................................................................................................................go to 2
b) animal without wings.................................................................................................................... go to 7
2 a) with two pairs of wings ............................................................................................................... go to 3
b) with one pair of wings ................................................................................................................. Diptera
3 a) with membranous wings ............................................................................................................go to 4
b) hind pair of membranous wings ...........................................................................................go to 6
4 a) with long abdomen ..................................................................................................................Odontata
b) medium sized abdomen ............................................................................................................go to 5
5 a) wings with coloured scales............................................................................................. Lepidoptera
b) wings without scales...................................................................................................... hymenoptera
6 a) forewings hard and shell-like ...................................................................................... coleopteran
b) forewings hard but not shell-like................................................................................ Orthoptera
7 a) body horizontally flattened ................................................................................................... Isoptera
b) body laterally flattened...........................................................................................Symphonoptera
Identify the orders o the various specimens as per the table below
| Specimen | order | steps followed | identity |
| K | Diptera | 1a,2b | Housefly |
| L | Orthoptera | 1a, 2a, 3b, 6b | Grasshopper |
| M | Coleoptera | 1a, 2a, 3b, 6a | Beetle |
| N | Isoptera | 1b, 7a | Termite |