Explain how molecular sequence data can be used to determine whether two species belong to the same clade.

IB DP Biology Higher Level (2023 syllabus) — A3.2 Classification and cladistics · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

DNA or protein sequences of the two species are aligned and compared with those of an outgroup.
Shared derived sequence changes (synapomorphies) that are absent in the outgroup show a common ancestor.
The larger the proportion of shared differences relative to the outgroup, the closer the two species are.
A cladogram is then drawn, placing the two species together in a clade because they share a more recent common ancestor with each other than with any other group or the outgroup.

Examiner tips

Use the word ‘shared derived characters’ or ‘synapomorphies’ to show understanding of cladistic terminology.
Explain the role of an outgroup in determining which changes are derived.
Show that a higher proportion of shared differences indicates a closer relationship.
Mention that the cladogram will place the two species together if they share a more recent common ancestor with each other than with the outgroup.

Common mistakes

Confusing shared ancestral traits (symplesiomorphies) with synapomorphies.
Failing to mention the outgroup or its purpose.
Not linking the proportion of shared differences to the closeness of relationship.

Mark scheme (4 marks)

DNA (or amino acid / protein) sequences are compared between the two species and an outgroup
Shared derived characters (synapomorphies) / shared sequence similarities that are not present in the outgroup indicate common ancestry
The greater the proportion of shared sequence differences (relative to an outgroup), the more recently the two species diverged / the more closely related they are
A cladogram is constructed placing the two species in the same clade if they share a more recent common ancestor with each other than with any other group / the outgroup

Key terms in this question

clade · molecular sequence data