Group organisms by shared features
Use specified similarities to organise a collection without relying on appearance alone.
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Read the stated observations before choosing a group.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should a grouping criterion do?
What you will learn
- Choose relevant shared features.
- Explain why a classification criterion needs consistency.
Make the grouping rule explicit
A classification places organisms into groups using features they share. A useful rule can be applied consistently to each specimen. A backbone, feathers, jointed legs or the organisation of cells can be more informative than a general impression such as looks similar.
A classification task may provide observations in a table. Use those observations, rather than guessing from the name, habitat or colour. More than one trait can support a grouping, and further evidence can change an initial proposal.
A shared location is not the same as shared structure
A dolphin and a fish both live in water, but the dolphin has mammalian features. Grouping everything aquatic together can be useful for an ecological question while failing to represent the animal groups used in this syllabus. State the purpose of your rule.
A dichotomous key identifies a specimen from choices; a classification organises groups. They are connected tools, but reaching the same branch in an identification key does not by itself prove the closest evolutionary relationship.
Pause and explain
Which evidence best supports a structural group?
Worked example
Three specimens have a backbone; a fourth has an external skeleton and jointed legs. Propose a structural grouping.
Show the worked solution
- Use the same criterion for all specimens.
- Place the three with a backbone together.
- Keep the jointed-leg specimen in a different structural group.
Answer Group by backbone presence, using the reported structure.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Specimens A and B have cellulose walls and chloroplasts. C has neither. Give a grouping rule for these observations.
Give me a hint
Use the two reported cell features.
Compare my reasoning
- Identify features shared by A and B.
- Group them by cellulose walls and chloroplasts.
- Keep C separate on the supplied evidence.
A and B together by the two shared cell features; C separately.
Look for these in your work
- State the grouping features.
- Use the observations provided.
A dolphin and a trout both live in water. Explain why habitat is insufficient to place them in one vertebrate group.
Give me a hint
Look for structural and reproductive evidence.
Compare my reasoning
- Different vertebrate groups can occupy water.
- Dolphins have mammalian features such as milk production.
- Trout have fish features such as gills and fins.
Shared habitat does not override different diagnostic features.
Look for these in your work
- Avoid grouping only by habitat.
- Give contrasting features.
An unknown specimen lacks a clearly visible feature needed for your proposed group. What should you report?
Give me a hint
Separate missing observation from confirmed absence.
Compare my reasoning
- A hidden feature has not been demonstrated absent.
- Seek a clearer observation or an additional diagnostic feature.
- Keep the grouping provisional.
The grouping is provisional until the required feature is observed or supported by other evidence.
Look for these in your work
- Do not guess absence.
- Identify useful additional evidence.
Common mistakes
- Habitat alone is often insufficient.
- An identification key is not automatically an evolutionary tree.
What can you explain now?
Why is having feathers more useful than living near a lake for identifying a bird?
Compare with the explanation
Feathers are a defining structural feature of birds.
Many unrelated animal groups share the same habitat.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain a consistent grouping rule and one reason to keep a grouping provisional.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.