Explain how the use of a dichotomous key allows an organism to be identified and assigned to a known species.

IB DP Biology Standard Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A dichotomous key presents a series of paired statements (couplets) with contrasting observable characteristics. At each couplet the user selects the statement that matches the observable characteristics of the organism, directing them to the next couplet. By following through successive couplets, the number of possible species is progressively narrowed down. The process ends when a species name (binomial) is reached, thereby assigning the organism to a known species.

Examiner tips

Use the word ‘dichotomous key’ and ‘couplets’ to show understanding of the tool.
Explain the step‑by‑step selection process and the narrowing down of possibilities.
Mention that the final step gives the binomial species name.

Common mistakes

Confusing a dichotomous key with a multi‑choice test; not describing the pairwise nature of the statements.
Failing to state that the final outcome is a binomial species name.
Using vague terms like ‘list of characteristics’ instead of ‘paired statements’.

Mark scheme (4 marks)

A dichotomous key presents a series of paired statements (couplets) with contrasting observable characteristics.
At each couplet the user selects the statement that matches the observable characteristics of the organism, directing them to the next couplet.
By following through successive couplets, the number of possible species is progressively narrowed down.
The process ends when a species name (binomial) is reached, thereby assigning the organism to a known species.

Key terms in this question

dichotomous key · species