Phylogenies

Phylogenies Overview

1. What is a Phylogeny?

  • Definition: The evolutionary history of a group of organisms.

  • Phylogenetic Tree: A diagram illustrating patterns of ancestry between ancestors and their descendants, highlighting evolutionary relationships.

2. Reading a Phylogenetic Tree

  • Basic Structure:

    • Branch: Represents a population over time.

    • Node: A point where an ancestral group splits into two or more descendant groups.

    • Tip: Represents a group (species or larger taxon) without descendants.

    • Outgroup: A taxon closely related to a particular monophyletic group.

    • Sister Taxa: Closely related taxa on adjacent branches from the same node.

    • Root of Tree: The bottom, most ancient node of the tree.

    • Polytomy: A node that branches into more than two groups.

3. Types of Groups in Phylogenetics

  • Monophyletic Group:

    • Definition: An evolutionary group containing an ancestral population and all of its descendants (e.g., all descendants of ancestral reptiles).

  • Paraphyletic Group:

    • Definition: Includes an ancestral population and some of its descendants (e.g., Class Reptilia excluding birds).

  • Polyphyletic Group:

    • Definition: A group whose members' last common ancestor is not part of the group (e.g., warm-blooded animals).

4. Synapomorphies & Homoplasies

  • Synapomorphies:

    • Shared, derived traits that help to define monophyletic groups.

  • Homoplasies:

    • Similar traits due to reasons other than shared ancestry (example: wings in birds vs. bats).

5. Applying the Principle of Parsimony

  • Definition: The simplest explanation or model, requiring the fewest evolutionary changes, is preferred.

  • Importance in tree construction: The phylogenetic tree with the minimum number of changes is considered the most likely to be correct.

6. Phylogenetic Species Concept

  • Definition: Identifies species based on the smallest monophyletic group on a phylogenetic tree, emphasizing evolutionary history among populations.

7. Biologists' Use of Phylogenetic Trees

  • Applications:

    • Discovering evolutionary relationships among organisms.

    • Addressing applied biological questions (e.g., sources of new diseases, conservation issues).

8. Practical Exercise: Fictitious Running Race

  • Scenario:

    • Runners start at the same location but take different paths (akin to evolutionary pathways).

    • Each runner's path is recorded by stamping cards that reflect their unique stamps along the way, paralleling how traits are recorded in taxa.

  • Illustrative Purpose:

    • Represents how phylogenetic trees are constructed by analyzing branches, nodes, and species relationships based on collected traits.

9. Summary of Key Terms

  • Homology: Traits similar due to shared ancestry (e.g., bones in birds and bats).

  • Homoplasy: Traits similar not due to common ancestry but due to convergent evolution or other factors.