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BIOLOGY · Stage 7 · Chapter 1: Classification and Life

Biology 1.8 Making your own identification keys

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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 5Planning the key

  • Know the organisms
    • it is important to know the names of the organisms before making the key.
  • See the organisms
    • it helps to see them so that their size can be judged.
  • 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
    • a good key separates the whole group in a logical sequence, with each question dividing the remaining specimens into two groups.

Section 2 of 5Building 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
    • a fully branched key for n organisms needs n − 1 questions.
  • 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 5A scientific key for plants

  • Scientific questions
    • a key is more scientific if it uses the characteristics of the main groups whenever possible.
  • 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).
  • 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.
  • Size clue
    • only seed plants grow really big, so a very big tree is a seed plant even if flowers cannot be seen.
  • Habitat clue
    • conifers survive harsh conditions such as snow and have needle leaves.

Section 4 of 5Testing and improving

  • Test
    • ask a classmate who has not seen the specimens to use the key and check that they reach the right names.
  • Vague words
    • replace any vague word with a measurement or a count.
  • Overlap
    • both statements in a pair must not be true together; "has wings" and "has large wings" overlap.
  • Gaps
    • no specimen may fit neither statement; "red" and "blue" miss a green specimen.
  • 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 5Ways to present a key

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

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