Teaching Tree-Thinking to Undergraduate Biology Students

Abstract

Evolution is the unifying principle of all biology, and understanding how evolutionary relationships are represented is critical for a complete understanding of evolution. Phylogenetic trees are the most conventional tool for displaying evolutionary relationships, and "tree-thinking" has been coined as a term to describe the ability to conceptualize evolutionary relationships. Students often lack tree-thinking skills, and developing those skills should be a priority of biology curricula. Many common student misconceptions have been described, and a successful instructor needs a suite of tools for correcting those misconceptions. I review the literature on teaching tree-thinking to undergraduate students and suggest how this material can be presented within an inquiry-based framework.

Fig. 1

Phylogenetic trees show the relationships of four species (A, B, C, and D). Each tree represents the same relationships of the four species: species C and D are the closest relatives, species B is equally related to both C and D, and species A is the outgroup. a A large arrow indicates the direction of time from the past to the present. The most recent common ancestor (MRCA) of all four species is indicated by a circle and small arrow. An arrow indicates the root of the tree. The tree on the right is a diagonal representation of the evolutionary relationships. b Two clades are indicated on each tree: clade 1 consists of species C and D, and it is nested within clade 2, consisting of species B, C, and D. c A fifth species (Z) has been added to the phylogeny, but the relationships of species A, B, C, and D have not changed. d A circles-within-circles diagram can represent the nested hierarchical relationships of species A, B, C, and D. e The branches of the tree have been rotated around various nodes, but they still depict the same evolutionary relationships

Fig. 2

Characters have been mapped onto a tree (the branches are not drawn to scale). Black bars show upon which branch a character evolved. Given this tree, students should be able to infer the characters in each of the extant species

Fig. 3

An example of the Great Clade Race (Goldsmith 2003) is shown, with one possible solution. The rectangles represent index cards with symbols drawn on them

Fig. 4

Two different examples of the universal tree of life are shown. a In the simplified version, linear descent from a common ancestor is assumed. b In the more complex version, horizontal gene transfer is common amongst prokaryotic organisms, violating the assumption of linear descent. Additionally, the complex version contains a representation of the endosymbiosis event that gave rise to the eukaryotes