Description

Grade Level: Post Secondary Courses

(vocabulary and activities can be modified for use in upper-level high school courses)

Subject: Biological Sciences- Evolution (phylogenetics and tree building)

Duration: Two class periods with homework between classes.

Description: In order for students to develop expertise in phylogenetic systematics and become efficient problem solvers, students must cultivate representational competency associated with tree-thinking. This lesson helps students build the fundamental skills for reading and building phylogenetic representations they are expected to develop upon completing a plant systematics or evolution course. This particular lesson uses hypothetical flowering plants to assess and instruct essential phylogenetic tree-building skills, specifically: appropriate use of evidence, identification of key tree features, understanding of significant patterns, transferability across representations, and ability to verbally describe depicted relationships.

Goals and Objectives: This lesson explicitly addresses and helps overcome the following challenges associated with phylogenetic tree building:

1. Lumping organisms based on single characteristics rather than looking holistically at the organisms.
2. Ignoring critical data & using uninformative evidence to construct phylogenetic trees.
3. Difficulties transferring empirical data into a visual representation illustrating evolutionary relationships.
4. Creating consensus nodes to address discrepancies.
5. Altering the format or orientation of the tree alters the relationships depicted.
6. Generating a comb-like branching structure without incorporating hierarchical

lineages.

7. Using trees to reconstructing ancestral states.
8. Identifying the most likely tree by comparing representations

This two-class period Pseudocots lesson has been used with secondary and post-secondary students. Students engaged in tasks such as: selecting appropriate data sources to group taxa into monophyletic groups (clades), constructing a phylogenetic tree representing these clades, adjusting the representation to incorporate new taxa, comparing their tree to other hypotheses, and forming predictions about historic and future character states.

Objectives:

• Students will be able to identify informative data that can be used to build a phylogeny.
• Students will be able to generate a phylogenetic tree from hypothetical data.
• Students will be able to alter phylogenetic models to accommodate new data.
• Students will be able to compare phylogenetic trees and determine how different models provide support or dispute their phylogeny.

It helps if students are familiar with reading phylogenetic trees prior to this lesson. Thus, I suggest that the Pipe Cleaner Phylogeny lesson (Halverson, K.L. (2010). Using pipe cleaners to bring the tree of life to life. The American Biology Teacher, 72(4), 223-224. doi: http://dx.doi.org/10.1525/abt.2010.72.4.4) or similar tree-reading unit be implemented prior to this lesson. It is also helpful to make sure students are familiar with the following terms:

Apomorphy – a derived character state unique to one lineage

Branch – illustrates the different lineages after speciation events

Character - a specific heritable trait described in terms of its state

Consensus Tree (or node) - a phylogenetic tree derived by combining common features in multiple trees, often leading to undefined relationship hierarchies of sister taxa.

Internode - a shared lineage between speciation events

Lineage - a continuous line of decent

Monophyletic (Clade) - a grouping of a common ancestor and all of the descending lineages

Node - a hypothetical common ancestor or speciation event

Outgroup - a distantly related species that resides outside of the clade of interest

Pseudocot - hypothetical flowering plant clade

Phylogenetic tree - an evolutionary tree (shows relationships among organisms)

This lesson is very dependent upon the timing of when data is introduced to the students. Thus, it is important to be aware of when to hand out the different portions of supplementary data or unintended outcomes may result. To implement this lesson, you will need copies of provided student worksheets, supplementary data, scissors, and it is helpful to have colored pencils or markers.

Cite this work