Cladogram & Phylogenetic Tree
Build cladograms and phylogenetic trees to visualize evolutionary relationships between organisms based on shared derived characteristics. Explore how cladistics uses synapomorphies to determine common ancestry, construct branching diagrams, and interpret evolutionary history through the principle of parsimony.
CLADISTICS: MAPPING EVOLUTIONARY HISTORY
A **cladogram** is a diagram used to show relationships among organisms. It is built using **shared derived characters**—traits that are present in the current group but were absent in the common ancestor. Cladograms represent hypotheses about the evolutionary history (**phylogeny**) of a group.
BUILDING A CLADOGRAM
1. **The Outgroup**: An organism that is distantly related to the rest of the group, used as a point of comparison. 2. **Nodes**: Points where a single lineage branches into two, representing a common ancestor. 3. **Clade**: A group of organisms that includes a common ancestor and ALL its descendants.
HOW TO USE THIS VISUALIZATION
1. **Identify Traits**: Look at the character table for traits like 'hair,' 'jaws,' or 'lungs.' 2. **Construct the Tree**: Drag the organisms onto the branches based on how many traits they share. 3. **Map the Changes**: Place the hash marks for each derived trait at the correct points on the lineage. **Try This**: Look at the 'Four Limbs' trait. If both a frog and a human have it, where should that trait be placed on the tree relative to their common ancestor? Does a fish belong inside that clade?
AP EXAM CONNECTION
Unit: Unit 7: Natural Selection (Topic 7.9)
Learning Objective: EVO-3.C
COMMON MISCONCEPTIONS
- Thinking an organism at the top of a cladogram is 'more evolved' (all living organisms are equally evolved).
- Confusing the physical order of the tips with the actual branching relationships.
- Believing cladograms are permanent (they are revised with new DNA evidence).
KEY TAKEAWAYS
- Nodes represent common ancestors.
- Trees are based on shared traits.
- DNA evidence is the gold standard.
- Cladograms are testable hypotheses.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): What does a branch point (node) on a cladogram represent?
Show Answer & Explanation
Answer: A common ancestor.
Explanation: The node is the point in time where two lineages diverged from a single ancestral population.
Q2 (CONCEPTUAL): In a cladogram of vertebrates, which animal would likely serve as the outgroup: Shark, Frog, Crocodile, or Tuna?
Show Answer & Explanation
Answer: Shark (or Tuna).
Explanation: An outgroup should be the most distantly related member, typically lacking the derived traits shared by the rest of the 'ingroup' (like lungs or four limbs).
