Cladograms are constructed by comparing traits or aligned sequences, then choosing trees that best explain shared similarities and differences parsimoniously.
Coverage 2015–2025 · Updated 15 Jul 2026
A sequence-based cladogram is a hypothesis that groups taxa according to shared differences in aligned DNA, RNA or amino-acid sequences.
Homologous sequences are aligned so each position can be compared. Candidate trees place the observed changes on branches, and parsimony selects the tree that explains the data with the smallest total number of sequence changes.
Procedure: align homologous sequences; record variable positions; use an outgroup where provided to infer ancestral states; propose alternative branching patterns; count the minimum changes required by each; select the most parsimonious supported tree.
If taxa B and C share two sequence states absent from A and the outgroup, a tree pairing B with C may require fewer independent changes than trees pairing either taxon with A.
Parsimony identifies the simplest explanation for the supplied data, not a guaranteed true history. Different genes or criteria can support a different hypothesis.
This objective is assessed through essay response, structured response, commonly using Explain / State / Describe.
Explain / State / Describe / Discuss / Outline
Build the answer around this relationship: Cladograms can be built from traits, DNA sequences, RNA sequences, or protein sequences.
Choosing a cladogram from visual layout rather than shared trait or sequence evidence.
Representative question
Discuss the use of amino acid sequences of proteins as the basis for constructing cladograms.
A3.2 exam answers are strongest when they sound like evidence arguments. Classification organizes diversity, but fixed ranks and morphology can mislead. Evolutionary classification should match phylogeny using monophyletic clades supported by synapomorphies. Molecular clocks estimate divergence time from calibrated sequence differences. Cladograms are built from aligned sequence data and interpreted by nodes, not tip positions. Cladistics can reclassify old taxa, and rRNA evidence supports the three-domain system.
