What a Cladogram Is (and Isn’t)
The cladogram represents a hypothetical relationship between groups of organisms, including their common ancestors. It is derived from the Greek words clados, which means “branch,” and gramma, which means “character.” It resembles the branches of a tree that extend outward from the trunk. A cladogram, however, does not necessarily have a vertical shape. Diagrams can be branched from the sides, tops, bottoms, or centers. It is possible for cladograms to be very simple, comparing only a few groups of organisms, or very complex, potentially classifying all life forms. As a result, cladograms are more commonly used to classify animals than other forms of life.
A cladogram is constructed by comparing synapomorphies between groups. Synapomorphies are shared heritable characteristics, such as fur, shelled eggs, or warm blood. Modern cladograms rely on DNA and RNA sequencing data and proteins to determine synapomorphies.
In cladistics, hypotheses are made about relationships between organisms and cladograms are constructed. A phylogeny represents the hypothetical relationships between organisms. Phylogenetics is the study of evolutionary history and relationships between organisms.
Key Takeaways: What Is a Cladogram?
- The cladogram shows hypothetical relationships between groups of organisms.
- Branches branch out from a main trunk in a cladogram.
- The root, clades, and nodes are the key components of a cladogram. In all branches branching off from the root, there is a common ancestor. Clades are the branches that indicate related groups and their common ancestors. Hypothetical ancestors are represented by nodes.
- The original cladograms were based on morphological characteristics, but modern cladograms are based on genetic and molecular data.
Parts of a Cladogram
A cladogram’s root indicates the ancestor common to all groups branching from it. In a cladogram, branching lines form clades, which are groups of organisms that share a hypothetical ancestor. Nodes are the points where lines intersect and represent the common ancestors.
There are two identical cladograms here. Creative Commons Attribution-Share Alike 3.0 Alexei Kouprianov
Cladogram vs. Phylogram
In phylogenetics, cladograms are one type of tree diagram. Phylograms and dendrograms are other types of diagrams. The terms are sometimes used interchangeably, but biologists recognize distinct differences between the two.
Cladograms show common ancestry but not evolutionary time between an ancestor and descendant group. Different cladogram lines may have different lengths, but these lengths have no meaning. A phylogram, on the other hand, has branch lengths proportional to evolutionary time. Thus, a long branch indicates a longer period of time than a short branch.
Cladograms and dendrograms may appear similar, but they differ as well. As opposed to cladograms, dedrograms represent both taxonomic and evolutionary relationships between groups of organisms.
How to Construct a Cladogram
Using cladograms, organisms are compared based on similarities and differences. Therefore, a cladogram could be constructed to describe relationships between different types of animals, but not between individuals. To construct a cladogram, follow these simple steps:
- Groups should be separated. The groups could include cats, dogs, birds, reptiles, and fish, for example.
- Create a table or list of characteristics. List only inherited characteristics, not those influenced by the environment. Among the examples are vertebrae, hair/fur, feathers, egg shells, and four limbs. List traits until you have one trait common to all groups and enough differences between them to make a diagram.
- Organizing organisms before drawing a cladogram is helpful. Venn diagrams are useful for showing sets, but you can also list groups. Cats and dogs, for example, both have fur, four limbs, and amniotic eggs. Reptiles and birds are vertebrates with four limbs and shelled eggs. Unlike vertebrates with four limbs, fish have eggs.
- Create a cladogram. The root of the problem is the shared trait. The animals in the example are all vertebrates. The first node leads to the group of organisms that have the least in common with the others (fish). Another node off the trunk leads to reptiles and birds. Cats and dogs are the final branches off the trunk. How do you decide whether the second node leads to reptiles/birds or cats/dogs? Reptiles and birds follow fish because they lay eggs. Based on the cladogram, the transition from shelled eggs to amniotic eggs occurred during evolution. It is possible for a hypothesis to be incorrect, which is why modern cladograms rely on genetics rather than morphology.