“Making Cladograms” Online Activity

One way that scientists classify organisms is based on evolutionary evidence that includes homologous structures, as well as DNA comparisons. This approach, called cladistics, attempts to answer questions such as: “Are sharks more closely related to lizards or bony fishes?” In this activity, you will answer this question by analyzing and constructing cladograms, phylogenetic trees with branch points defined by derived characters of organisms in a particular branch (clade).

Clades are nested within one another — they form a nested hierarchy. A clade may include many thousands of species or just a few. Some examples of clades at different levels are marked on these phylogenies. Notice how clades are nested within larger clades.

Evolution predicts that living things will be related to one another in what scientists refer to as “nested hierarchies.” According to evolutionary theory, groups of related organisms should share certain homologous traits, and the number of these shared homologous traits should increase with the degree of relatedness. This is indeed what we observe in the living world and in the fossil record.

Understanding a phylogeny is a lot like reading a family tree. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendents of that ancestor. As you move from the root to the tips, you are moving forward in time.

Part 1: Interpret Cladograms

Study the cladogram below and answer the questions that follow.

Study the cladogram below and answer the questions that follow.

Part 2: Construct a Cladogram

Follow the directions in the light blue box to find out more about how cladograms are constructed. Afterwards, answer the questions.