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1.8Chapter 1: Classification and Life

Making your own identification keys — Alhussan Biology - Stage 7 Revision Notes & Topical Questions

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📚 Alhussan Biology - Stage 7 Hub / Topical Revision
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Lesson summary

Biology 1.8 Making your own identification keys — Alhussan International Academy — Biology (Stages 7–9) notes: Design a dichotomous key for a group of organisms using clear, observable, scientific features.Know the organisms — it is important to know the names of the organisms be…

  • See the organisms
  • Observation
  • Number of questions
  • Animals and shells
  • Scientific questions
  • First question

Section 1 of 7Planning the key

Know the organisms
  • Know the organisms it is important to know the names of the organisms before making the key.
See the organisms
  • See the organisms it helps to see them so that their size can be judged.
Observation
  • Observation study each specimen carefully and make scientific observations, looking for similarities and differences.
  • List features that can be observed and compared, such as leaf shape, edge, size with units and surface.
Logic
  • Logic a good key separates the whole group in a logical sequence, with each question dividing the remaining specimens into two groups.

Section 2 of 7Building the key

  • Questions
    • every question must be answered YES or NO or the key will not work.
1
List all the organisms.
2
Choose a feature that splits them into two sets, ideally of similar size.
3
Write one statement or question for each set, with a direction to the next question or a name.
4
Repeat for each set until every organism has its own name.
Number of questions
  • Number of questions a fully branched key for n organisms needs n − 1 questions.
Animals and shells
  • Animals and shells use features such as legs, wings or antennae for animals and the number of whorls or ridges for shells.

Section 3 of 7A scientific key for plants

1Scientific questions
  • Scientific questions a key is more scientific if it uses the characteristics of the main groups whenever possible.
2First question
  • First question does it make seeds?
  • This divides all plants into seed plants and non-flowering plants.
  • Seed plants split into conifers (seeds in cones, needle-like leaves) and flowering plants (flowers, fruits).
  • Non-flowering plants split into ferns (fronds with spores underneath) and mosses (small, flat, no true roots).
3Evidence limits
  • Evidence limits a plant with no visible flowers may not be non-flowering,
    • because flowers may not be out,
    • so look for cones, seeds, fruit or spores as well.
4Size clue
  • Size clue only seed plants grow really big, so a very big tree is a seed plant even if flowers cannot be seen.
5Habitat clue
  • Habitat clue conifers survive harsh conditions such as snow and have needle leaves.

Section 4 of 7Testing and improving

Test
  • Test ask a classmate who has not seen the specimens to use the key and check that they reach the right names.
Vague words
  • Vague words replace any vague word with a measurement or a count.
Overlap
  • Overlap both statements in a pair must not be true together; "has wings" and "has large wings" overlap.
Gaps
  • Gaps no specimen may fit neither statement; "red" and "blue" miss a green specimen.
Final check
  • Final check run every organism through the key and confirm each ends at its own name and no name is reached twice.

Section 5 of 7Ways to present a key

1Numbered statements
  • Numbered statements each pair of choices gives a name or sends you to another number.
2Flow chart
  • Flow chart boxes and arrows, with a question in each box and two arrows labelled yes and no.
3Branching diagram
  • Branching diagram a tree of choices.
4End points
  • End points every end point is labelled with an organism name.
5Value
  • Value keys let other people identify species accurately without needing an expert.

Section 6 of 7Key terms

TermMeaning
Dichotomous keyA key with two choices at each step.
Observable featureA characteristic that can be seen or measured.
Flow chartA diagram of questions and arrows leading to answers.
Branching diagramA tree-shaped key.
SpecimenA sample organism used for study.
End pointThe place in a key where an organism's name is given.

Section 7 of 7Exam technique

1
Using vague features such as pretty or large — use observable features and measurements.
2
Writing statements that can both be true at once — each specimen must fit only one.
3
Leaving an organism without an end point — every organism needs a name.
4
Using features that vary within a species, such as age — use stable features.

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