What Is a Cladogram? Definition and Examples

A cladogram is a diagram that represents a hypothetical relationship between groups of organisms, including their common ancestors. The term "cladogram" comes from the Greek words clados, which means "branch," and gramma, which means "character." The diagram resembles the branches of a tree that extend outward from a trunk. However, the shape of the cladogram isn't necessarily vertical. The diagram can branch from the side, top, bottom, or center. Cladograms can be very simple, comparing only a few groups of organisms, or highly complex, potentially classifying all forms of life. However, cladograms are more often used to classify animals than other forms of life.

Scientists use synapomorphies to compare groups to construct a cladogram. Synapomorphies are shared common heritable characteristics, such as having fur, producing shelled eggs, or being warm-blooded. Originally, synapomorphies were observable morphological traits, but modern cladograms use DNA and RNA sequencing data and proteins.

The method of hypothesizing relationships between organisms and constructing cladograms is called cladistics. The hypothetical relationships between organisms is called a phylogeny. The study of the evolutionary history and relationships between organisms or groups is called phylogenetics.

Parts of a Cladogram

The root is the central trunk of a cladogram that indicates the ancestor common to all groups branching from it. A cladogram uses branching lines that end in a clade, which is a group of organisms sharing a common hypothetical ancestor. The points where the lines intersect are the common ancestors and are called nodes.

Cladogram vs. Phylogram

A cladogram is one of several types of tree diagrams used in phylogenetics. Other diagrams include phylograms and dendrograms. Some people use the names interchangeably, but biologists recognize distinct difference between the tree diagrams.

Cladograms indicate common ancestry, but they do not indicate the amount of evolutionary time between an ancestor and a descendant group. While the lines of a cladogram may be different lengths, these lengths have no meaning. In contrast, the branch lengths of a phylogram are proportional with respect to evolutionary time. So, a long branch indicates a longer time than a shorter branch.

While they may appear similar, cladograms also differ from dendrograms. Cladograms represent hypothetical evolutionary differences between groups of organisms, while dedrograms represent both taxonomic and evolutionary relationships.

How to Construct a Cladogram

Cladograms are based on comparing similarities and differences between groups of organisms. So, a cladogram could be constructed to describe relationships between different types of animals, but not between individuals. Follow these simple steps to construct a cladogram:

1. Identify separate groups. For example, the groups could be cats, dogs, birds, reptiles, and fish.
2. Make a list or table of characteristics. Only list characteristics that can be inherited and not those that are influenced by environmental or other factors. Examples include vertebrae, hair/fur, feathers, egg shells, four limbs. Continue listing traits until you have one trait common to all groups and enough differences between other groups to make a diagram.
3. It's helpful to group organisms before drawing the cladogram. A Venn diagram is useful because it shows sets, but you can simply list groups. For example; Cats and dogs both are vertebrates with fur, four limbs, and amniotic eggs. Birds and reptiles are vertebrates that lay shelled eggs and have four limbs. Fish are vertebrates that have eggs, but lack four limbs.
4. Draw the cladogram. The shared common trait is the root. All of the animals in the example are vertebrates. The first node leads to the branch of organisms with the least in common with the other groups (fish). The next node off the trunk leads to another node that branches off to reptiles and birds. The final node off the trunk branches to cats and dogs. You may be wondering how to decide whether the second node leads to reptiles/birds or to cats/dogs. The reason reptiles/birds follow fish is that they lay eggs. The cladogram hypothesizes the transition from shelled eggs to amniotic eggs occurred during evolution. Sometimes a hypothesis may be incorrect, which is why modern cladograms are based on genetics rather than morphology.

Sources

• Dayrat, Benoît (2005). "Ancestor-Descendant Relationships and the Reconstruction of the Tree of Life". Paleobiology. 31 (3): 347–53. doi:10.1666/0094-8373(2005)031[0347:aratro]2.0.co;2
• Foote, Mike (Spring 1996). "On the Probability of Ancestors in the Fossil Record". Paleobiology. 22 (2): 141–51. doi:10.1017/S0094837300016146
• Mayr, Ernst (2009). "Cladistic analysis or cladistic classification?". Journal of Zoological Systematics and Evolutionary Research. 12: 94–128. doi:10.1111/j.1439-0469.1974.tb00160.x
• Podani, János (2013). "Tree thinking, time and topology: Comments on the interpretation of tree diagrams in evolutionary/phylogenetic systematics" . Cladistics. 29 (3): 315–327. doi:10.1111/j.1096-0031.2012.00423.x
• Schuh, Randall T. (2000). Biological Systematics: Principles and Applications. ISBN 978-0-8014-3675-8.