The big idea: A cladogram (an evolutionary tree) used to be drawn from what organisms look like: their shape and structure. Today, trees are built mainly from molecular evidence.
What we compare between species
- The base sequence of a gene, such as the gene for rRNA.
- The amino-acid sequence of a protein, such as haemoglobin.
The key rule: The more differences there are between two species' sequences, the longer ago they split from a common ancestor. Fewer differences means they are more closely related.
- Molecular evidence
- Data from the sequences of molecules — the bases in DNA/RNA or the amino acids in proteins — used to work out how closely species are related.
- Base sequence
- The order of the bases (A, T, C, G) along a gene; comparing the same gene in different species shows how much their DNA has changed.
- Amino-acid sequence
- The order of amino acids in a protein; because the protein is built from the gene, this also reflects differences in the DNA.
- Mutation
- A change in a base (or amino acid). Mutations build up gradually over time, so they act as a record of how long two lines have been separate.
| Type of molecule | What is compared | Why it works |
|---|---|---|
| A gene's base sequence (e.g. the gene for rRNA) | The order of bases (A, T, C, G) in the same gene in different species | rRNA is found in every cell and changes very slowly, so it can be lined up even between very distantly related groups (bacteria vs animals) |
| A protein's amino-acid sequence (e.g. haemoglobin, cytochrome c) | The order of amino acids in the same protein in different species | The protein is the gene's product, so differences in amino acids reflect differences in the underlying DNA |
Why molecules beat appearances: Looks can mislead — unrelated species can evolve to look alike (a dolphin and a shark), and close relatives can look very different.
A sequence is just a string of letters, so it can be counted and compared objectively — even between organisms that share no obvious features at all.
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The big idea: Several cladograms can be drawn for the same species. Parsimony picks the one that needs the fewest sequence changes: the most probable tree.
The points to remember
- Align the sequences: the same gene or protein, position by position.
- Count the differences between every pair of species.
- Species with the fewest differences are joined at the most recent node.
- For each possible tree, count the changes needed to explain every difference.
- The tree with the fewest changes is the most parsimonious: the most likely one.
Remember it as: Fewest changes, best tree.
Example: sample data: In the player, tree 1 explains all eight positions with 6 changes; tree 2 needs 9. Both fit the data, but tree 1 is the simpler explanation.
A cladogram is a hypothesis:
Different criteria, different trees: Another gene, or another way of choosing, can give a different tree. A cladogram is the best hypothesis for the data, not a proven fact.
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How it is tested: Paper 1B gives sequence data and asks how they are used to build a cladogram; Paper 2 asks you to discuss amino acid sequences or to name another type of evidence.
IB-style questionExplain[4 marks]
The base sequence of the same gene was found in four species of whale. Explain how these sequences can be used to construct a cladogram.
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Another type of evidence: If a cladogram was built from DNA, other evidence could be amino acid sequences of proteins, or homologous morphological traits.