When delivering a new concept to students, it is important to provide them with cues to allow them the opportunity to find meaning or a personal connection with the subject matter. This does not necessarily need to be within the confines of a formal lesson hook. In the “Background Information” section of this topic, I correlated the classification of species to the organization of songs within iTunes. This gives students the opportunity to relate previous knowledge to the new subject material.
As a possible consolidation activity after the first lesson, have students create their own analogies to show they understand how classification according to shared traits can be seen in everyday life.
Students should be able to define the following terms, make the appropriate connections between them, and also operationalize and use them actively in their work: cladistics, clade, cladogram, derived trait, primitive trait, outgroup, apomorphy, plesiomorphy, synapomorphy, parsimony. It is important that students display the ability to use the terms apomorphy, plesiomorphy, derived trait and primitive trait interchangeably when appropriate. The understanding of these terms will be acquired through the review and creation of cladograms.
Students should also understand the methodology involved in the creation of a cladogram as they will be asked to create their own in future lessons. A methodology refers to a guideline system for solving a problem, with specific components such as phases, tasks, methods, techniques and tools. Cladistics methodology is as follows:
- Choose the taxa whose evolutionary relationships interest you. These taxa must be clades if you hope to come up with plausible results.
- Determine the characters (features of the organisms) and examine each taxon (pl. taxa) to determine the character states (decide whether each taxon does or does not have each character). All taxa must be unique.
- Group taxa by synapomorphies (shared derived characteristics) not plesiomorphies (original, or “primitive”, characteristics).
- Work out conflicts that arise by some clearly stated method, usually parsimony (minimizing the number of conflicts).
Reviewing a Cladogram
Cladograms are used to illustrate the evolutionary relationships (or phylogeny) of different groups of special of organisms. Before asking students to create their own cladograms, it would be beneficial for them to be presented with a completed cladogram and some questions for them to answer regarding it.
- Which species is designated as the outgroup?
- Which trait was more recently derived, C or D?
- According to the cladogram, which two species are most closely related?
Creating a Cladogram
- Use the morphological evidence presented in the table below to construct a cladogram.
- Based on the cladogram, describe the phylogeny of the organisms.
- One group must be considered an outgroup. An outgroup is a group that is not closely related to the groups of interest and therefore unlikely to share any recent traits with other groups. In this case, a lizard was chosen as a distantly related vertebrate.
- Consider each characteristic and judge which trait is the more recently derived trait.
- This can usually be done by comparing traits with the outgroup.
Step 1: Consider each characteristic and judge which trait is the more recently derived trait. This can usually be done by comparing the traits with the outgroup.
In this case we can make the following inferences:
- Two digits on each foot is a derived trait (having five digits is the primitive condition)
- Having hair is a derived trait (having scales like reptiles and fish is a primitive condition)
- Having grasping hands is a derived trait (having four non-grasping hands is the primitive condition)
- The lack of a tail is a derived trait (having a tail is the primitive condition)
Step 2: Create a table of synapomorphies (shared derived traits).
Step 3: Draw a “V” with the outgroup at the upper left. The base of the V represents the common ancestor to all animals. (Fig. a)
Step 4: All the animals except the lizard share the feature of having hair. We can indicate the evolution of hair on the right branch leading away from the lizard. (Fig. b)
Step 5: The remaining animals fall into two groups – those with two digits and those with grasping hands. We therefore split the right branch into two and locate the evolution of these traits above the split. We can divide the deer/cow branch in two and place the names at the end of each branch.
Notice that when you split a branch, the choice of left or right branch for positioning the groups is arbitrary. We could have chosen to place the deer/cow lineage on the left rather than on the right.
Step 6: The chimpanzee and the human both lack a tail, so we create a new branch and locate this derived trait above the split.
Conclusion: Based on the completed phylogeny, we can infer that the cow and deer are more closely related to each other than to other groups. Similarly, humans and chimpanzees are more closely related to each other than to other groups. We can also conclude that lemurs are more closely related to chimps than to cows and deer.