Understanding Phylogenetic Trees-305:33

Understanding Phylogenetic Trees-3

A "phylogenony" is a term used to describe the evolutionary history of an organism or group of organisms.  To chart evolutionary relationships in a graphical form, scientists often use phylogenetic trees (Freeman & Herron, 1998, p. 364).

A Lesson in Phylogenetic Tree VocabularyEdit

  • Branches- Represent evolutionary lineages changing over time.
    • Longer branches represent a greater period of change.
    • Shorter branches represent a shorter period of change.
  • Nodes
    • There are two types of nodes:
      1. External nodes- sometimes called "tips" or "leaves", these are endpoints which may represent either extant or extinct species.
      2. Internal nodes- are located somewhere between the root of the tree and the tips/leaves. These nodes have branches off of them which lead to other nodes (either internal or external).
  • Roots- The root of the tree represents the ancestral lineage of all the nodes.
    • Phylogenetic trees may or may not be "rooted".
      • Rooted trees- Generally have a single node repressenting a common ancestor for all the species at the tips of the tree.
      • Unrooted trees- May show relatedness of the tip/leaf nodes, but don't necessarily point to a common ancestor.

This diagram illustrates the difference between internal and external nodes (Available from:



       /   \

      o     o INTERNAL NODES

      |    / \

      o   o   o EXTERNAL NODES ( or leaves)

Weblinks and Sources:Edit

Weblinks- For more information check out these two sites:

  1. Understanding Phylogenies.  Available at:
  2. How to Read a Phylogenetic Tree.  Available at:
  3. Taxonomy and the Tree of Life. Available at:


Freeman, S., & Herron, J. C. (1998). Evolutionary analysis. Upper Saddle River (NJ): Pearson Education.

Gregory, T. R. (2008). Understanding evolutionary trees. Evolution: Education and Outreach, 1(2), 121-137.

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