By Shana McAlexander
Product Developer

Phylogenetic trees are used to model the evolutionary relationship between groups of organisms. The 2-dimensionality of a phylogenetic tree confuses some students as to the extent of relatedness of the different organisms depicted. To help eliminate this confusion, have students create 3-dimensional trees in the form of mobiles so that the branching nodes pivot. Students can manipulate the mobile to see the relationships more clearly.

National Science Education Standards

Grades 9–12

Life Science

• Molecular basis of heredity
• Biological evolution

Materials needed per group to make 1 mobile

• 3 Craft Sticks or Tongue Depressors
• 6' String
• Piece of Card Stock, 5 × 8"
• Pen or Marker
• Scissors
• Clear Adhesive Tape
• Ruler

Teacher preparation

1. Set up a materials station where students can pick up the supplies they need.
2. Either print for distribution or prepare to display the student procedure.

Teacher procedure

1. Organize students into pairs or groups. Each group will make a mobile.
2. Briefly introduce your expectations for the product and supply or display the student procedure. For the organism tags, students may take different approaches. Consider having them write the name of the organism, draw a picture, or attach a photo.
3. Have students select which of the 3 phylogenetic trees provided that they want to model.
4. Assist students as they construct their mobiles. They may need extra help figuring out how to balance the weight. Prior knowledge of levers and fulcrums will be useful.
5. Once the construction is complete, demonstrate how the branches can rotate around each node.

Discussion

To demonstrate how a 2-dimensional tree can sometimes be misleading, have students draw the various configurations of the tree or demonstrate them using the mobile against a flat surface. (There are 8 variations of alignment of the 4 organisms depicted, but each alignment shows the same information regarding evolutionary relationship.) Below are 2 diagrams of the same phylogenetic tree. Each node represents a divergence from a common ancestor. Letters (a–d) represent extant organisms. Organisms a and b are more closely related to each other than they are to organism c. Organism d is the outgroup and is more distantly related to the others than organisms a, b, and c are to one another. These relationships are indicated by the branching from the nodes rather than by the left–right proximity of the organisms.

For any student who has assumed that relative distance between organisms (from left to right) in the tree indicates their relatedness, the mobile demonstrates that the points of divergence (the nodes) supply that information.

Extension

• If your students are ready for something more complex, have them construct mobiles with more nodes or with scaled branch lengths that depict relative lengths of time from divergence.
• Students may use the hominid evolution example provided below to examine a phylogenetic tree that shows both extant and extinct species.

Related products

• Studying Classification with Cladograms Kit
• Genetic Kinship: Following the Globin Genes Through Time
• Download the Student Procedure Guide