What Is Shown by Cladograms?

Evolution

• A cladogram is always read from left to right or bottom-up in order to communicate the idea behind evolutionary science that organisms are constantly mutating and changing over time, but never reversing the process. The order and arrangement of different groups in a cladogram is very important as it displays a shared evolutionary history. Although cladograms are used to display phylogenetic relationships, sometimes biologists use cladograms to show hypothetical evolutionary relationships displaying potential common ancestors, extinctions and more, while reserving phylogenetic trees to portray accepted evolutionary history.

Clades

• A group of organisms in a cladogram is called a clade, and the spacing and length of the lines branching off of each clade are particularly significant. Closely related species will be found on adjacent branches with little space in between, indicating a recent shared ancestor and other similar characteristics. Meanwhile, distant relatives that diverged from a common ancestor thousands of years ago will be located at the end of a long, unbranched line.

Common Ancestors

• Every time a line branches in a cladogram it signifies two species or organisms that evolved later on from a single species in the past. This shared heritage is called a common ancestor, and often times these ancestors may be extinct by the time their later descendants appear on the cladogram, according to professor of biology at DeAnza College Brian McCauley. Some cladograms seek to demonstrate a single ancestral organism for dozens or even hundreds of more recent species.

Shared Characteristics

• Sometimes a cladogram will group organisms, not by species, but by a shared characteristic found across multiple species. In this type of cladogram, clades branch off each time a group evolves significant or noticeable genetic changes, and may eventually lose the original characteristic altogether. More detailed cladograms may note the transmission of inherited characteristics by labeling each branch with descriptions.