Constructing a Phylogenetic Tree (Cladogram) K.L. Wennstrom, Shoreline Community College

Biologists use phylogenetic trees to express the evolutionary relationships among groups of organisms. Such trees are constructed by comparing the anatomical structures, embryology, and genetic sequences of different species. Species that are more similar to one another are interpreted as being more closely related to one another.

Before you continue, you should carefully read BioSkills 2, “Reading a Phylogenetic Tree” in your textbook. The BioSkills units can be found at the back of the book. BioSkills 2 begins on page B-3.

Steps in creating a phylogenetic tree 1. Obtain a list of characters for the species you are interested in comparing. 2. Construct a character table or Venn diagram that illustrates which characters the groups have in common. a. In a character table, the columns represent characters, beginning with the most common and ending with the least common. The rows represent organisms, beginning with the organism with the fewest derived characters and ending with the organism with the most derived characters. Place an X in the boxes in the table to represent which characters are present in each organism. b. In a Venn diagram, the circles represent the characters, and the contents of each circle represent the organisms that have those characters.

Organism Characters Rose Leaves, flowers, thorns Grass Leaves Daisy Leaves, flowers

Character Table Venn Diagram

leaves

thorns flowers

Grass X Daisy X X Rose X X X

3. Using the information in your character table or Venn diagram, construct a cladogram that represents the relationship of the organisms through evolutionary time. The organisms will be the tips of the branches. Nodes represent the most recent common ancestors and are often represented by circles. The evolution of specific characters is marked by symbols on the branches of the tree (usually boxes or lines). See the example below: