IB Biology Topic A3.2.7: Analysing Cladograms Notes

Reading a Cladogram From Root to Tips

By analysing a cladogram, biologists can deduce common ancestry, relatedness, and evolutionary pathways.

The Key Features of a Cladogram

Root

1. The root is the base of the cladogram.
2. It represents the hypothetical common ancestor of all organisms shown.
3. This ancestor is inferred from molecular or morphological data, not always seen in fossils.

Nodes

1. A node is a point where branches split.
2. Each node represents a hypothetical common ancestor of the clades that diverge from it.
3. Usually two clades split at a node, but sometimes three or more emerge (a polytomy).

Terminal Branches (Tips)

1. Terminal branches are the ends of the branches.
2. They represent species or groups alive today, or recently extinct.
3. Each marks the end of an evolutionary lineage.

How to Analyse a Cladogram

1. Locate the root: the starting point, representing the common ancestor of all species shown.
2. Identify nodes and branches:
   1. Nodes show where evolutionary splits occurred.
   2. Branches connect nodes and species, tracing the evolutionary path.
3. Determine relationships:
   1. Species branching from the same node share a more recent common ancestor and are more closely related.
   2. Species whose common ancestor sits closer to the root are less closely related.
4. Identify clades:
   1. A clade includes all species branching from a single node.
   2. Clades can be small, like a genus, or large, like a kingdom.
5. Consider divergence times:
   1. In a scaled cladogram, branch lengths can reflect the time since divergence.
   2. Longer branches suggest more time has passed; shorter ones suggest a recent split.

Assumptions and Caution

1. Cladograms are built on the principle of parsimony: the pathway with the fewest changes is taken as most likely.
2. This makes them probabilistic models, not absolute proof.
3. Sometimes the true pathway is more complex than the simplest model.