This is an excellent introduction to phylogenetic trees and their several versions. The suggested presentations would be perfect for high school biology, filling a gap in biology textbooks and providing a deeper understanding of biology. Evolutionary trees provide an excellent visual aid to understanding evolution and its relationship to classification, but there are several conventions in their use that should be explained in order to prevent or remove misconceptions.

Seeing tree diagrams as the "forest" overview of what they represent at the close-up "trees" level of resolution is most impressive. Figures show how we can "zoom out" from the mating lines between various individuals in a sample population to how this would look for many generations over time from a great distance: a fuzzy branching tree.

Variations of trees showing the same relationships are explained, as are the degrees of relationships reflected in the number of nodes between species on the branch tips. Cladograms encourage (require?) groups to be arranged in such a way that members demonstrated to be in a single clade (and their common ancestor) must be placed together on their own branch. Some of the more common misconceptions about phylogenetic trees are also explained.

An interesting and useful application for phylogenies is to show your class where a particular physiological or anatomical trait being studied first arose (or ask them to figure out where it arose, requiring them to recognize the groups where the structure or process occurs). This shows how evolution works as a central unifying theme.

Another value is to show how phylogenies are predictive, leading to new discoveries, as paleontologist Neil Shubin describes in his excellent book Your Inner Fish (Pantheon 2008). In a remote Canadian area, he was looking for better evidence of early tetrapod features in fish fossils expected in the 380 million year old rocks found in that region. And he found Tiktaalik, a fish whose forelimbs have typical tetrapod bones while the hind limbs were just fins. This find was reported in April, 2006 in the journal Nature.

One use for an evolutionary tree that was not presented, but should have, was to show how the taxonomic hierarchy relates to the recency of branching in the tree. Species, genera, families, orders, etc. are simply our recognition of different levels in the nested groups of ever-larger clades. To find a version useful for classroom instruction, go to http://www.indiana.edu/~ensiweb/lessons/unt.ev.f.html Scroll down to "What Evolution IS" and click on the Macroevolution Diagram for a PDF copy with directions for use.

The authors encourage the frequent teaching of tree-thinking and a useful resource for such ideas at http://www.tree-thinking.org/resources.html I strongly recommend this article.

The illustrations used in this article are excellent, and we will try to make large sharp copies available to teachers. If you are interested, contact the webmaster. They could be used on an overhead or PowerPoint presentation.

There are several lessons on the ENSI site that involve the use of phylogenies and the proper construction of cladograms. Go to the Evolution Index at http://www.indiana.edu/~ensiweb/evol.fs.html and click on "Synopses." Then scroll down to the "Classification, Hierarchy & Relationships" section for descriptions of these excellent lessons. Trees will be found in many of the other lessons, as well. Take a look at the Chronology Lab, Classroom Cladogram and Primate Cladogram lessons (in the Human Evolution Patterns section), the new Patterns in Time lesson in the Geo-Paleo Patterns list, or the Becoming Whales lesson (Procedure #10, with a Thewissen's "Family Tree of Whales," or the newer chronological look at whale relationships in the "Provisional Phylogeny of Whales and Ungulates").

Larry Flammer
ENSI webmaster