7.4: Part 2- Cladistics
A newer system of phylogenetic classification, called cladistics , is currently in practice today. A cladogram is a hypothesis about the evolutionary relationships between the organisms depicted on the tree. In this way, a cladogram illustrates the lines of descent for these organisms. A cladogram proposes an answer to the question “Which groups of organisms share a common ancestry?”
Take a look at these two identical, generic cladograms. The capital letters indicate the terminal organisms represented in the tree. The numbers indicate characters present in organisms beyond that point. In addition, the nodes (indicated by the lowercase letters and the dots) represent the common ancestors of the terminal organisms. Even though they look different, if you examine them closely, these two cladograms are depicting the same relationships between critters A, B and C. As a mental exercise, try replacing A, B, and C with 3 animals and see if you place them correctly based on relatedness. Check with your instructor if you are unsure of how they are related.
In this example, you see that A is more closely related to B than it is to C based on the shared derived characteristic 1. Note that at each branch a derived characteristic is indicated that separates the left branch from the right branch of the evolutionary tree.
Now examine the cladogram below illustrating the evolutionary relationships between a hagfish, shark, bony fish, frog, rat, bird, and lizard.
Procedure
- Name each organism on the cladogram.
- Place a dot at every point that represents a common ancestor.
- Indicate one shared derived characteristic that distinguishes each branch.
- Who is more closely related: the shark and bony fish, or the bony fish and frog?