Naming with a key — answer key

1. KQ1 · Practice

Model answer. a. Pair 1: one blade, go to 3. Pair 3: no spines, go to 4. Pair 4: lobes along both sides of the midrib: oak.

A full answer includes

• a: start at pair 1 and follow the numbers: one blade (go to 3), no spines (go to 4), lobes along both sides of the midrib: oak.

Also accept

• a

Watch for

• “You can jump straight to the pair that looks right.” — Always start at pair 1 and follow the numbers: each pair only makes sense after the ones before it.

2. KQ2 · Practice

Options as numbered on the pupil sheet: 1 — Write the feature as a pair of opposite statements; 2 — Run each organism through the key to check it reaches its own name; 3 — List features that some of the organisms have and others do not; 4 — Do the same for each smaller group until each organism is on its own; 5 — Choose a feature that splits the whole group into two.

Model answer. First look at all the organisms and list features that some have and others do not. Then choose a feature that splits the group into two. Write it as a pair of opposite statements. Repeat for each smaller group until every organism is on its own. Last, test the key by running each organism through it.

A full answer includes

• List features → choose one that splits the group in two → write it as two opposite statements → repeat for each smaller group → test the key.

Also accept

• list features, choose a splitting feature, write two opposite statements, repeat for each smaller group, test

Watch for

• “You only need to test a key if it looks wrong.” — You can check each pair as you write it, but always finish by running every organism through the whole key: that is the only way to show each one reaches its own name.

3. KQ3 · Practice

Model answer. b. Every animal either has legs or has none, and the question splits the four two and two. "Big or small" (a) with no measurement is judged differently by different people, a key never asks for the name (c), and colour (d) varies between animals of one kind and does not split these four clearly.

A full answer includes

• b: a good step asks about a feature every animal clearly has or has not (legs), and splits the group, here two and two.

Also accept

• b

Watch for

• “"Is it big?" makes a good step in a key.” — Big compared with what? A spider and a beetle can be the same size. Use a feature that is either there or not, such as legs.
• “A key can ask "Is it a spider?"” — The name is what the key is for; a step must ask about a feature.

4. KQ4 · Practice

Model answer. c. Pair 1: it has legs, go to 2. Pair 2: more than six legs, go to 3. Pair 3: grey plates and seven pairs of legs (fourteen legs): woodlouse.

A full answer includes

• c: follow the pairs in order, reading both statements each time: legs (go to 2), more than six legs (go to 3), grey plates and seven pairs of legs: woodlouse.

Also accept

• c

Watch for

• “Any animal with lots of legs is a centipede.” — Read both statements at pair 3: grey plates and seven pairs of legs describe a woodlouse.

5. KQ5 · Practice

Model answer. a, c and e. Leaflets, spines and lobes are either there or not, so two people looking at the same leaf agree. "Large" (b), with no measurement, is judged differently by different people, and a young leaf is smaller than an old one. "Pretty" (d) is an opinion. "Green" (f) does not split these leaves: all six are green in summer, and leaf colour changes in autumn.

A full answer includes

• a, c and e: leaflets, spines and lobes are either there or not, so two people agree; "large", "pretty" and "green" do not split these leaves clearly.

Also accept

• a, c, e

Watch for

• “"Is it big?" makes a good step in a key.” — "Big" means different things to different people. A measured size, such as "longer than 10 cm", can work, but only if the leaves really differ that much.
• “Colour is a good feature for leaves.” — All six of these leaves are green in summer, and many turn brown or yellow in autumn, so "green" does not split them.

6. KQ6 · Practice

Model answer. All six leaves are green, so all six go to the same side: the pair does not split the group at all. Many leaves also change colour in autumn, so the key would not work for leaves collected then. A better first pair uses a feature some leaves have and others do not, written as opposites, such as "Leaf divided into leaflets / Leaf is one blade".

A full answer includes

• All six leaves are green, so they all go the same way: the pair does not split the group in two.
• The colour of many leaves also changes in autumn, so the key would not work for leaves collected later.
• Better: an identifiable feature some have and others do not, e.g. "divided into leaflets / one blade".

Also accept

• all go the same way, so no split; colour changes in autumn; better: a feature some have and others do not, e.g. leaflets / one blade

Watch for

• “Any feature you can see makes a good step.” — A step must split the group into two, so it needs a feature that some organisms have and others do not.

7. KQ7 · Practice

Model answer. One key that works: 1. Has legs, go to 2 / Has no legs, go to 3. 2. Smooth, moist skin: frog / Dry, scaly skin: lizard. 3. Has fins: goldfish / Has no fins: snake. Four animals need three pairs. Other keys work too (for example starting with fins / no fins), as long as each pair is two opposite statements about a feature every animal clearly has or has not, and each animal ends on its own.

A full answer includes

• Three pairs of statements for four animals.
• Each pair is two opposite statements about an identifiable feature (legs, skin, fins).
• Each animal ends on its own with its name; for example 1 has legs → 2 / no legs → 3; 2 smooth moist skin → frog / dry scaly skin → lizard; 3 has fins → goldfish / no fins → snake.

Also accept

• three pairs of opposite statements about identifiable features; each animal reaches its own name; any valid order

Watch for

• “A key can have three choices at a step.” — A dichotomous key has exactly two statements at each step.
• “One pair can name two animals at once.” — Keep splitting until every animal is on its own.

8. KQ8 · Practice

Model answer. Each pair of statements splits one group into two, adding one more group. Starting from one group of 12, it takes 11 splits to end with 12 groups of one beetle each. So the key needs 11 pairs of statements.

Checked automatically

• Number of pairs of statements: 11 (exact value)
  • Common wrong answer 12 : One pair fewer than the number of species: each pair splits one group into two.
  • Common wrong answer 6 : Halving 12 is one step; you must keep splitting every group until each beetle is on its own.
  • Common wrong answer 22 : That is the number of statements (two in each of 11 pairs); the question asks for the number of pairs.
  • Common wrong answer 24 : That is two statements for each species. Each pair splits one group into two, so 12 species need 11 pairs.

A full answer includes

• 11 (one pair fewer than the number of species).

Also accept

• 11

Watch for

• “A key needs one pair for each organism.” — Each pair splits one group in two, so a key for n organisms has n − 1 pairs.

9. KT1 · Apply

Model answer. (a) Pair 1 uses size. A magpie and a blackbird are larger than a robin, but "large" means different things to different people, so two people might choose differently. Pair 3 asks "Is it a blue tit?": a key must ask about a feature, because the name is what you are trying to find. (b) Pair 1: "Feathers only black, or black and white / Has some orange, blue or yellow feathers". Pair 3: "Orange-red breast: robin / Yellow breast and a blue cap: blue tit". A measured size would also work at pair 1 ("longer than 20 cm / shorter than 20 cm"), because these four birds really differ that much. Each statement is now a feature two people would agree on.

A full answer includes

• (a) Pair 1 uses size ("large / small") with no measurement: people judge it differently, so it is not a clear feature. Pair 3 asks for the name ("Is it a blue tit?"), not a feature.
• (b) For example: pair 1 "Feathers only black, or black and white / Has some orange, blue or yellow feathers"; pair 3 "Orange-red breast / Yellow breast and a blue cap" (or a measured size, such as "longer than 20 cm / shorter than 20 cm").

Also accept

• (a) pair 1 size without a measurement is not clear; pair 3 asks the name; (b) replace with clear opposite features, e.g. black or black-and-white feathers / coloured feathers (or longer / shorter than 20 cm); orange-red breast / yellow breast and blue cap

Watch for

• “"Large / small" is fine because a magpie is clearly bigger than a robin.” — It works for these two, but "large" for a robin and a blue tit side by side? Use a feature or a measurement, so that everyone draws the line in the same place.
• “A key can ask "Is it a blue tit?"” — A pair must ask about a feature, not the name.

10. KT2 · Apply

Model answer. (a) The key was built for six trees only. Another maple can also be one blade with no spines and with smooth lobes spreading from one point, so it passes every pair correctly and reaches "field maple", the only name at the end of that route, although it is a different species. (b) The visitor should stop at pair 4. If neither statement fits, either the leaf is not one of the trees the key was made for, or it is not a typical leaf: a holly leaf from high up on an old tree can have no spines, and it would end up here. Choosing the statement that fits "better" would force the leaf to a wrong name. The visitor should look at typical leaves from the same tree; if they still fit neither statement, the tree is not in the key. (c) A key only works for the organisms it was built for. Check the name it gives against a full description or picture, and when a pair does not fit, find a key made for all the organisms that live in that place.

A full answer includes

• (a) The key was built for six trees only; another species can have every feature on the route to field maple, so it reaches that name although it is a different species.
• (b) Stop at pair 4: neither statement fits, so either the leaf is not one of the six trees or it is not a typical leaf (an old holly leaf can have no spines); do not choose the statement that fits better, which would give a wrong name; look at typical leaves of the same tree.
• (c) A key only works for the organisms it was made for: check the name against a description or picture, and use a key that includes all the organisms of the place.

Also accept

• (a) another species can share every feature on a route and reach a name in the key; (b) stop: neither statement fits, so the leaf is not in the key or not typical; do not choose the nearer statement; check typical leaves; (c) a key only works for the organisms it was made for; check the name, use a wider key

Watch for

• “A key always gives the right name.” — A key can only reach the names that are in it, and a species it was not built for can share every feature on a route.
• “If neither statement fits, choose the one that is closest.” — Stop instead: the leaf is not in the key, or it is not a typical leaf, and forcing it on would give a wrong name.