A cladogram is a specific type of diagram used in the biological field of cladistics to hypothesize and visualize evolutionary relationships among different species or groups of organisms, known as taxa. This branching illustration provides a structured way to understand patterns of common ancestry and divergence. The diagram focuses on the order in which distinct lineages separate from a shared ancestral line. It serves as a visual hypothesis for the historical connections between groups.
Anatomy of a Cladogram
The structure of a cladogram is built on a few fundamental components. The lines extending through the diagram are called branches, representing a lineage evolving over time. These branches terminate at the tips, which are labeled with the names of the organisms or groups being compared, referred to as taxa or terminal taxa.
Where two or more branches diverge is known as a node. Every node represents a hypothetical common ancestor that existed when a single lineage split into two or more distinct descendant lineages. The base of the entire cladogram is the root, which signifies the oldest common ancestor shared by all the organisms shown.
The diagram often includes an outgroup, a taxon that branched off before the common ancestor of all the other groups being studied. This distantly related group acts as a point of reference to help determine which characteristics are unique to the main groups under investigation. The arrangement of these components illustrates the sequence of speciation events.
Reading Relatedness and Common Ancestry
The primary purpose of a cladogram is to identify clades, which are groups of organisms that include a common ancestor and all of its descendants. To determine relatedness, trace the branches back until they meet at a shared node. The closer that shared node is to the tips, the more recently the two organisms shared a common ancestor, and the more closely related they are.
Organisms that share a common node are called sister taxa, representing the closest relatives on the tree. The relationships are defined strictly by the branching pattern, not by the physical distance between the names at the branch tips. The traits used to group organisms are called synapomorphies, which are shared derived characteristics that appear in a common ancestor and are passed on to its descendants.
For example, the appearance of hair is a synapomorphy that defines the clade of mammals, as it is shared by all mammals but not by their earlier ancestors. When reading the diagram, a characteristic listed on a branch is present in all the lineages that branch off after that point. Tracing these shared traits back to the nodes provides a hypothesis for the sequence in which different characteristics evolved.
How Cladograms Differ from Phylogenetic Trees
While the terms cladogram and phylogenetic tree are sometimes used interchangeably, they have a technical distinction concerning the information conveyed by their branch lengths. A cladogram shows only the pattern of branching, or the relative order of common ancestry. In a cladogram, the length of the branches is arbitrary and conveys no information about the amount of evolutionary change or the passage of time.
A phylogenetic tree is a broader category of diagrams that often incorporates additional data. In many phylogenetic trees, the length of the branches is meaningful, scaled to represent either the actual amount of evolutionary time elapsed or the degree of genetic distance (the number of genetic changes) between groups. Therefore, a cladogram illustrates only the topological relationship, whereas a scaled phylogenetic tree provides an inference about the rate or timing of evolutionary change.