Nearly every biology student has heard of cladistics and cladograms, however, cladograms can be difficult to construct. This instructable will show you step by step how to make one, and how to avoid confusing derived traits with ancestral ones. Having the skills to construct a cladogram will help your understanding of evolutionary relationships.
List of Terms:
- derived characteristics
- ancestral characteristics
- Something to write with
Step 1: Pick Organisms for Your Cladogram
Pick four to six organisms to be in your cladogram. Make sure that they are within the same order or family. The exception to this could be the organism in the outgroup, as the outgroup is meant to be used as a point of comparison against the other organisms.
Step 2: Pick One Ancestral and One Derived Characteristic to Designate the Outgroup
What do all of the organisms you chose have in common? The purpose of the ancestral characteristic is to link the animal in the outgroup to the other animals. It also serves as a point of comparison between the them. For example, if you chose a dog, a cat, a bear, and a rabbit, your ancestral trait, the trait they all share, could possibly be hair, or warm-bloodedness (endothermy).
What makes your organisms different from each other? Think of traits that are physical or physiological in nature that are unique to that order or family of organisms. In our example, we could use consumption of meat to separate our bear, dog, and cat from our rabbit. Doing this makes the rabbit our outgroup.
Step 3: Pick Derived Characteristics for the Ingroup (Part 1)
What makes the organisms in your ingroup different from each other? Now that we have included the bear, dog, and cat in our ingroup, we can begin choosing derived characters to separate them. Let's separate the bear and dog from the cat by using enhanced sense of smell as a derived characteristic.
Step 4: Pick Derived Characteristics for the Ingroup (Part 2)
Now that we have separated the dog and bear from the cat, it is time to separate them from each other. What characteristic of bears is unique to them and is not found in the dog, cat, or rabbit? In our example, hibernation would be a derived characteristic unique to bears.
Step 5: Pick Derived Characteristics for the Ingroup (Summary)
By this point, you should have derived characteristics for each animal in your ingroup such that you could make a sort of list of them, in order from ancestral to most derived. In this list, each derived characteristic chosen should separate the organisms that share the character from those that do not. In our example, the ancestral characteristic of either fur or endothermy groups our rabbit, bear, cat, and dog together; but the derived character of meat consumption separates our cat, dog, and bear from the rabbit, the derived character of reliance on sense of smell separates the cat from the dog and bear, and the derived character of hibernation separates our bear from our dog. This makes the rabbit the most "ancestral" and the bear the most "derived".
Step 6: Constructing Your Cladogram: Place and Draw the Outgroup
Cladograms typically have the format shown above, a tree-like pattern.
Place the organism in the outgroup at the first perpendicular line at the left as shown.
Step 7: Constructing Your Cladogram: Place and Draw the Ingroup (Part 1)
Place the least derived organism in the ingroup (the second organism in the list you made in step five) at the line next to the outgroup as shown.
Step 8: Constructing Your Cladogram: Place and Draw the Ingroup (Part 2)
Place the second derived organism in the ingroup (the third organism in the list you made in step five) at the line next to the outgroup as shown.
Step 9: Constructing Your Cladogram: Place and Draw the Ingroup (Part 3)
Finally, place the most derived organism at the end of the diagonal line as above.
Step 10: Check Your Cladogram for Errors
Review the previous steps and the following video to check your cladogram for errors.
Congratulations! You have constructed your first cladogram.