How to Make an Evolutionary Tree

While studying evolution, you'll often compare one group of organisms to another. To make the comparisons easier, you can use visual representations of the relationships between these groups of organisms. One visual aid is an evolutionary tree, also called a phylogenetic tree. These diagrams start with a common ancestor of a group of organisms and then show how each organism branches off from that ancestor.

Difficulty: Moderate

Instructions

1. Study the group of organisms you're putting on the tree. Gather as much data as you have available, including physical characteristics, genetic information and evolutionary data. Use the information to infer the relationships between the organisms.

2. Understand the structure of the evolutionary tree. Extant, or living, species are the end "branches." The extinct species that gave rise to the living species are the common ancestral species. The further back the ancestral species occurred and the more extant species it gave rise to, the further down the tree it should be. The "trunk" is the one common ancestor of all the species in the tree diagram.

3. Decide whether you wish to start with the end "branches" or the "trunk." Study your data and see if you have more information about a group of species evolving from one species, in which case, you'd work from the bottom up. If you have more data about multiple ancestral species of single extant species, work from the top down.

4. Pay attention to vertical relationships. Any place that the branches come together is called a "node" and represents an ancestral species. Place the nodes for each ancestral species correctly in reference to the other ancestral species. For example, if you have four closely related species, two evolved from ancestor A and two that evolved from ancestor B, discover which ancestor evolved first. If ancestor A is an older species, put its node lower on the diagram than the node for ancestor B.

5. Work back and forth to fill in the blanks once you've made your initial determinations; use other known information to connect any species that are disconnected from the tree. For example, if you worked from the top down, see if you have any information about the descendants of one of the known ancestors. It may take several passes before you establish all of the relationships.