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Unit 4: Maintaining life

Using keys — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 10 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Using keys — HIA INTERNATIONAL notes: Use dichotomous keys of both styles to identify organisms.Reference sources — pictures in a book or online can help but do not always work.Key = a set of questions about the organism you want to identify; each answer leads to another question…

  • Questions → statements
  • Arrows → numbers
  • Observable
  • Avoid opinions
  • Avoid unreliable features
  • Measurements

Section 1 of 8Identifying organisms

Reference sources
  • Reference sources pictures in a book or online can help but do not always work.
Key
Key a set of questions about the organism you want to identify; each answer leads to another question or a name.
Dichotomous key
Dichotomous key a key in which each step branches into two.
Specimen
  • Specimen imagine you have the whole animal, not just a picture.

Section 2 of 8Using a branching key

1
Choose one organism to identify.
2
Start at the top and answer the first yes/no question.
3
Follow the line to the next question.
4
Keep going until you arrive at a name.
Example
  • Does it have legs?
  • 1Yes2more than six legs3crab
Example
  • Example legs, exactly six, four wings → dragonfly; two wings → housefly.
No legs
  • No legs body made of rings → earthworm; not → slug.

Section 3 of 8Using a key with pairs of statements

StepStatementGo to
1aIt has legs2
1bIt does not have legs3
2aExactly six legs4
2bMore than six legscrab
3aBody made up of ringsearthworm
3bBody not made up of ringsslug
4aFour wingsdragonfly
4bTwo wingshousefly
Route
  • Route to identify the dragonfly work through 1a, 2a, 4a.
Earthworm
  • 1b, 3a.

Section 4 of 8Converting between styles

Questions → statements
  • Questions → statements each question becomes a numbered pair 1a/1b, 2a/2b.
Arrows → numbers
  • Arrows → numbers the arrows are replaced by “go to” numbers or names.
Example
  • “Does it have stripes?” becomes “1a The fish has stripes — go to 2;
    • 1b The fish does not have stripes — horn shark”.

Section 5 of 8Good features for a key

Good features for a key
Observable
  • Observable use features you can see or count, with a clear yes/no answer.
Avoid opinions
  • Avoid opinions do not use “Is it pretty?”.
Avoid unreliable features
  • Avoid unreliable features size or colour that varies with age or season.
Measurements
  • Measurements use a measurement if comparing, e.g. “Is the body longer than 2 cm?”.

Section 6 of 8Number of questions

Rule
  • Rule a dichotomous key for n organisms needs exactly n − 1 questions.
Use
  • Use scientists and ecologists use keys to identify organisms quickly in the field.

Section 7 of 8Key terms

TermMeaning
KeyA set of questions used to identify an organism.
Dichotomous keyA key that branches into two at each step.
FeatureA characteristic of an organism that can be observed.
SpecimenA sample or example of an organism.
IdentifyTo find the name of an organism.
SpeciesA kind of organism.

Section 8 of 8Exam technique

1
A key can be used by skipping steps — always start at the first question and follow the path.
2
Dichotomous means many choices — it means two choices at each step.
3
Colour is always a good feature — colour can vary, so use features that are clear.
4
A key with a branching diagram is different information — it is the same key in a different style.

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