DragonflyIssuesInEvolution13 Wiki

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 Vocabulary[]

  • 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: http://stackoverflow.com/questions/265809/what-is-an-internal-node-in-a-binary-search-tree):



       /   \

      o     o INTERNAL NODES

      |    / \

      o   o   o EXTERNAL NODES ( or leaves)

Weblinks and Sources:[]

Weblinks- For more information check out these two sites:

  1. Understanding Phylogenies.  Available at: http://evolution.berkeley.edu/evolibrary/article/evo_05
  2. How to Read a Phylogenetic Tree.  Available at: http://epidemic.bio.ed.ac.uk/how_to_read_a_phylogeny
  3. Taxonomy and the Tree of Life. Available at: http://www.khanacademy.org/science/biology/tree-of-life/v/taxonomy-and-the-tree-of-life


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.