Phylogenics & Tree-Thinking

Abstract

Phylogenetic trees, once peculiarities of systematics, now permeate almost all branches of biology and are appearing in increasing numbers in biology textbooks. While few state standards explicitly require knowledge of phylogenetics, most require some knowledge of evolutionary biology and many scientists and educators would hold that it is impossible to really understand evolution without an ability to accurately interpret phylogenetic trees (O'Hara, 1988, 1997). Evolution, at its core, is a claim that living species are related by descent from common ancestry, and as such it is a theory of evolutionary trees. Additionally, trees help integrate evolutionary concepts throughout the curriculum (e.g., Offner, 2001) and provide students with an organizational framework for structuring knowledge of biological diversity. Therefore, biological literacy requires some exposure to tree-thinking--the ability to conceptualize evolution in terms of phylogenetic trees. As noted by O'Hara (1997): ... just as beginning students in geography need to be taught how to read maps, so beginning students in biology should be taught how to read trees and to understand what trees communicate. Such research as exists suggests that students hold significant misconceptions about trees and that these views may be deeply held and persistent (Baum et al., 2005). Therefore, the challenge faced by teachers, most of whom have had little exposure to phylogenetics, is significant. In this article we will provide a brief overview of some important principles of tree-thinking and a list of specific skills in which high school and college students should become proficient. We will also briefly discuss strategies for bringing trees into the broader biology curriculum. What a Tree Represents A phylogenetic tree is a depiction of the inferred evolutionary relationships among a set of species (or other taxa). When introducing trees to students it can be helpful to make clear the connection between reproduction within populations over short time frames and the evolution along the branches of a tree over a longer period of time. A useful strategy is to zoom from a single population at a single point in time to a phylogeny representing much longer periods of time. One of us (DB) uses Figures 1-3 for this purpose, both in a lecture format and in an assigned reading. [FIGURE 1 OMITTED] Students are asked to imagine one generation of plants of a particular species, for example, shepherd's purse, Capsella bursa-pastoris, growing side by side in a meadow and produc ing offspring by exchanging pollen. Five individual plants in a parental generation (GI) and an offspring generation (G2) could have a pedigree like that shown in Figure IA. You can expand the frame to encompass all the plants in this population and several generations (Figure IB). To encourage students to examine this figure closely you can give students a version without the time axis included. They can usually figure out the direction of time from the fact that each individual has two parents, but gives rise to a variable number of offspring. [FIGURE 2 OMITTED] [FIGURE 3 OMITTED] The next step is to imagine taking the preceding figure and getting rid of the organisms, keep ing only the descent relationships, since it is these that glue together the members of a sexual population. The resulting image might look like Figure IC. One can then expand the field of view to include many more individuals and generations. For example, Figure 2B is like Figure 2A except it includes about 250 individuals and 80 generations. As you can see, if one were to try to represent a typical population of several thousand individuals that persists for hundreds or thousands of generations, all one would see would be a fuzzy line. Individual populations may be fairly isolated for some period of time. However, on an evolutionary timescale, seeds and pollen occasionally move between the discrete populations that comprise a typical species. …