1. Discuss the purpose of classification.
To gauge students' prior knowledge of biological classification, ask: Why classify Earth’s living things? Accept a variety of ideas, and guide students to consider that classification helps us sort out ancestor/descendent relationships and allows us to map the evolutionary history of living things. For example, the presence or absence of characteristics can help you know how a population of organisms is related or can guide in investigating the evolutionary history of them.

2. Relate the Linnaean system to evolutionary relationships.
Students will likely be familiar with the Linnaean system, named for the Swedish scientist Carl Linnaeus, who developed the system in the 18th century. They should be familiar with its hierarchy: kingdom, phylum, class, order, family, genus, species. Ask: How do you think organisms are classified into specific categories in the Linnaean system? Explain that the classification of living things is based on the number of shared characteristics. Those organisms with the greatest number of shared characteristics are most closely related. Two organisms’ relatedness reflects how recently they diverged from a common ancestor. All living things are related evolutionarily if you go back in time far enough. Ask students to think about characteristics that humans, apes, and chipmunks have in common, and list them on the board. Ask: Which do you think is more closely related to humans, the great ape or chipmunk? (Humans and great apes are more closely related.) How do you think this explains their relationship to ancestors? (Both diverged relatively recently from a common primate ancestor; humans and chipmunks diverged in the more distant past from a common mammal ancestor.) Explain that humans currently reside in the family Hominidae with the great apes, and both share space in the class Mammalia with the chipmunks. The first mammal, in turn, separated from modern-day reptiles still earlier from an ancient reptile that is the ancestor of both. Reptiles and mammals belong—along with fish, birds, and amphibians—in the phylum Chordata. All the chordates, in turn, belong to the kingdom Animalia, along with every insect, mollusk, and worm.

3. Prepare for the classification activity.
Explain that, in this activity, students will group animals based on shared characteristics, and by doing this they will attempt to determine evolutionary relationships. Organize students in groups of three, and give each group a poster-sized sheet of butcher paper, removable tape or glue, scissors, and colored markers. Provide each group with the handout A Grouping Challenge. Have them cut each of the 19 boxes, descriptions of key anatomical and behavioral characteristics for 19 vertebrates.

4. Have students create five to nine vertebrate groups.
Have each group read the descriptions and consider how they might group these in as few as five or as many as nine groups. Ask students to mark with colored dots or highlight the shared characteristics they think are important in classification. For example, students might mark with a color each instance of “warm-blooded” or “feathers” to help with arranging visually. Explain that the goal is not to get a right answer, but instead to develop their own explanation for how they would divide these into groups with shared characteristics that seem most closely related. Discourage the groups from applying names to the groupings, such as mammals or reptiles; it is best if they try to classify them strictly according to their characteristics rather than relying on prior knowledge of how the classification is supposed to be. Have students tape or glue their groupings to the butcher paper and write a short explanation of why they grouped in the way they did.

5. Evaluate the different classification schemes.
Have students present their posters or do a poster walk to view the various grouping explanations. Ask: How similar or different are the classification schemes? Which do you think is closest to how scientists currently classify these animals? Why? Next, list on the board these categories of features that scientists have determined are most indicative of common ancestry: skin covering, warm/cold-bloodedness, skeletal features and anatomy, and methods of reproduction. Have students look at the vertebrate descriptions again, and have them circle any characteristics that fall into these four categories. Groups can reconsider their original groupings now that they have this new information. Poll the class on how many groups they now have.

6. Share the Linnaean classification and more recent approaches.
Traditional Linnaean classification placed these animals in five groups: fish, amphibians, reptiles, birds, and mammals. Share the numbers and names below:

• Fish: 2—bluefish, 3—blue shark, 8—lungfish
• Amphibians: 5—blue-spotted salamander, 7—mudpuppy, 16—caecilian, 19—green frog
• Reptiles: 4—ichthyosaur, 6—blue racer, a snake, 11—pterosaurs, 12—Tyrannosaurus rex, 17—leatherback turtle, 18—blue-tailed skink, a lizard
• Birds: 9—blue penguin, 10—bluebird, 15—blue duck
• Mammals: 1—blue whale, 13—gray bat, 14—duck-billed platypus

Tell students not to worry if they had six or more groups, as today these vertebrates are grouped with more complexity and scientists continue to debate classification issues. Scientists can further analyze evolutionary relationships with technology that enables analysis of DNA. Fish are now usually separated into 3 separate classes: Chondrichthyes or cartilaginous fishes (the blue shark), Actinopterygii or ray-finned bony fishes (bluefish), and Dipnoi (lungfishes). Reptiles traditionally have included snakes, turtles, lizards, and crocodilians, as well as pterosaurs and dinosaurs. Today, turtles, crocodilians, and snakes and lizards are often put into three separate classes. Prehistoric “reptiles” are more confusing still. Tyrannosaurus rex may be closer to birds than reptiles, considering its skeleton and the strong possibility that it might have been warm-blooded and even had feathers in one stage of its development. Recently, scientists have even been able to extract proteins from Tyrannosaurus fossils that further support the idea that these gigantic beasts might actually have been more bird than modern amphibian. Birds, in fact, might better be considered as a subgroup of reptiles. Pterosaurs might have been warm-blooded with a fur-like skin covering unlike any present-day reptile.

7. Discuss the challenges of classification.
Have students compare their approaches to the classification schemes unveiled in step 6. Ask: Did you originally consider characteristics that were not one of these four: skin covering, warm/cold-bloodedness, skeletal features and anatomy, and methods of reproduction? Explain. Discuss the past and present challenges that taxonomists have faced in classifying the vast array of living things.