AeroLab Reading room

Biology / IGCSE

Characteristics and classification

Classification groups organisms by shared features and evolutionary relationships.

Learning objectives

The big idea

Classification groups organisms by shared features and evolutionary relationships. Species are named using a genus and a specific name; identification keys use observable differences to narrow choices.

Animals, plants, fungi, prokaryotes and protoctists have characteristic cellular features, but members vary. Viruses have genetic material and a protein coat yet depend on host cells for replication; they are not simply tiny bacterial cells.

Worked example

A key first divides six organisms into groups of two and four. Your specimen matches the group of two. How many candidates remain?

Follow the solution

A matching decision excludes the other four candidates, leaving the two in that branch.

Investigate and explain

Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.

No exact PhET model is claimed for this topic. Use diagrams and supplied-data reasoning instead.

Open the interactive investigation

Check your understanding

1. Which part of a binomial name is written first?

  1. Habitat
  2. Genus
  3. Individual number
Check the answer and reasoning

Genus

The genus precedes the specific name, for example Homo sapiens.

2. A key first divides six organisms into groups of two and four. Your specimen matches the group of two. How many candidates remain?

  1. 4
  2. 6
  3. 2
Check the answer and reasoning

2

A matching decision excludes the other four candidates, leaving the two in that branch.

3. Can an identification key alone prove evolutionary ancestry?

  1. No
  2. Yes
  3. Only with many questions
Check the answer and reasoning

No

A key identifies by chosen features; evolutionary relationships need broader evidence such as molecular comparisons.