By analysing cladograms, biologists can deduce common ancestry, relatedness, and evolutionary pathways.
Key Features of a Cladogram
1. Root
Definition
Root
The base of the cladogram, representing the most recent common ancestor of all species in the diagram.
- The base of the cladogram.
- Represents the hypothetical common ancestor of all organisms shown.
- This ancestor may not be directly observable in fossil records but is inferred from molecular or morphological data.
2. Nodes
Definition
Node
A branching point where two or more lineages split, representing a hypothetical common ancestor shared by the diverging groups.
- Points where branches split.
- Each node represents a hypothetical common ancestor of the clades that diverge from it.
- Usually two clades split at a node (binary branching), but sometimes three or more emerge (polytomy).
3. Terminal Branches (Tips)
Definition
Terminal branch
The endpoints of the branches, representing the species or groups being studied.
- The ends of branches.
- Represent individual species or groups of species alive today (or recently extinct).
- Each terminal branch marks the end of an evolutionary lineage.
- Confusing terminal branches with ancestors.
- Terminal branches represent descendants, not ancestral forms.
- Nodes do not represent specific species we can observe today.
- Instead, they are inferred based on evidence such as DNA similarities and shared traits.
How to Analyse a Cladogram
- Locate the Root: The root is the starting point of the cladogram. It represents the common ancestor of all species in the diagram.
- Identify Nodes and Branches
- Nodes show where evolutionary splits occurred.
- Branches connect nodes and species, tracing the evolutionary path.
- Determine Relationships
- Species that branch from the same node share a more recent common ancestor and are more closely related.
- Species with a more distant common ancestor (closer to the root) are less closely related.
- Identify Clades
- A clade includes all the species branching from a single node.
- Clades can be small (e.g., a genus) or large (e.g., a kingdom).
- Consider Divergence Times
- If the cladogram is scaled, branch lengths can reflect time since divergence.
- Longer branches indicate more time has passed, while shorter branches suggest a recent split.
To identify a clade, start at any node and include all the branches and terminal species that stem from it.
Assumptions and Caution in Cladogram Analysis
- Cladograms are constructed under the principle of parsimony → the evolutionary pathway requiring the fewest sequence changes is considered the most likely.
- This makes cladograms probabilistic models, not absolute proof.
- Sometimes the true evolutionary pathway is more complex than the simplest model.
- Don’t assume that branch length always represents time.
- In many cladograms, branch lengths are arbitrary unless explicitly stated.
- Define root, node, and terminal branch in the context of a cladogram.
- How can you tell which species are more closely related in a cladogram?
- What does a node represent, and why is it considered hypothetical?
