Phylogenetic Trees - Biology

1. Fundamental Concepts

Definition: Phylogenetic trees are diagrams that represent the evolutionary relationships among various biological species based upon similarities and differences in their physical and/or genetic characteristics.
Branch Points (Nodes): Represent common ancestors of descendant groups.
Branch Lengths: Can represent time or the number of genetic changes.

2. Key Concepts

Rooted vs Unrooted Trees: $A$ rooted tree has a designated root node, indicating the direction of evolution, while an unrooted tree does not specify the direction.

Clades: A clade is a group of organisms consisting of a common ancestor and all its descendants.

Monophyletic Groups: These include all descendants of a common ancestor within a single branch.

3. Examples

Example 1 (Basic)

Problem: Identify the common ancestor for species A and B in the following phylogenetic tree.

Step-by-Step Solution:

Trace back from species A and B to find the first shared node.
The first shared node represents the common ancestor.

Validation: The common ancestor is correctly identified at the first shared node.

Example 2 (Intermediate)

Problem: Given the phylogenetic tree below, determine if the grouping of species C, D, and E forms a monophyletic group.

Step-by-Step Solution:

Identify the common ancestor of species C, D, and E.
Check if all descendants of this ancestor are included in the group.

        /\       /  \      /    \     C      /\            /  \           D    E

Validation: Species C, D, and E form a monophyletic group because they share a common ancestor and include all its descendants.

4. Problem-Solving Techniques

Visual Strategy: Use different colors to highlight different branches and nodes.
Error-Proofing: Double-check the connections between nodes to ensure no missing links.
Concept Reinforcement: Practice with real-world examples to understand the application of phylogenetic trees.