Tree Thinking

An Introduction to Tree Thinking

Abstract diagrams are critically important in most, if not all, science disciplines (Novick, 2006). In biology, hierarchical diagrams are especially common. Since 2004, I have been investigating college and high school students’ understanding of cladograms, the most important tool that contemporary scientists use to reason about evolutionary relationships. Most of this research has been conducted in collaboration with Kefyn Catley, an evolutionary biologist and science educator at Western Carolina University.

A cladogram is a type of hierarchical diagram that depicts hypotheses about nested sets of taxa that are supported by shared, evolutionarily novel characters called synapomorphies. For example, the cladogram shown at the top of the page indicates that one synapomorphy for birds and alligators is that they both possess a gizzard. That is, birds and alligators share a most recent common ancestor (MRCA) that evolved the novel character of possessing a gizzard. A group of taxa consisting of the MRCA and all descendants of that ancestor is called a clade or monophyletic group. Thus, birds and alligators comprise a clade (in the cladogram shown above). Because of the nesting inherent in hierarchical diagrams, birds, alligators, and lizards also comprise a clade. And those three taxa plus mammals (represented by manatees and elephants in the cladogram above) constitute another clade, etc. The synapomorphy supporting the bird/alligator clade distinguishes the MRCA of birds and alligators from the earlier ancestor common to birds, alligators, and lizards. And the synapomorphy supporting the bird/alligator/lizard clade (see UV light) distinguishes the MRCA of those three taxa from the earlier ancestor common to birds, alligators, lizards, and mammals. The latter ancestor evolved the novel character of having an amniotic egg, a critical development in the history of life on Earth that enabled vertebrates possessing this character to complete their life cycles on land.

Biologists use the tool of phylogenetics along with its product, the cladogram, to study macroevolution, the subdiscipline of biology that synthesizes events of Earth history and deep time (the well-established theory that Earth is billions of years old) with mechanisms that generate and maintain the biodiversity of our planet. Macroevolutionary processes operate at the level of species and above, resulting in the formation, radiation, and extinction of higher groups of taxa. Macroevolution explains, for example, both the origin and radiation of mammalian taxa. In contrast, microevolution concerns processes that occur at the level of the organism (i.e., genome, individual, and population). Microevolution explains, for example, the appearance of antibiotic-resistant strains of bacteria.

Cladograms are the most important tool used by evolutionary biologists because they document and organize existing knowledge about the properties of species and higher-order taxa. Tree thinking is the ability to understand and reason with evolutionary relationships depicted in cladograms (phylogenetic trees). The power of tree thinking is that the resulting classification scheme­—for example that alligators are more closely related to birds than to lizards because of their shared MRCA—reflects current understanding of the history of life on Earth (i.e., the evolutionary relationships among taxa). Thus, inferences based on this classification scheme are likely to be more informative and to have greater practical value than inferences based on other criteria. For example, inferring which antivenin to use to counteract the bite of a venomous king brown snake based on its close evolutionary relationship to the red-bellied black snake is more likely to lead to a successful outcome (namely, survival!) than is basing the choice of antivenin on the king brown snake’s similar appearance to the western brown snake.

Summary of My Research

Overview. My research on tree thinking falls into three broad categories: (a) Influences of diagram design on interpretations of evolutionary relationships, (b) assessing and improving students’ tree-thinking skills, and (c) effects of prior knowledge about taxonomic relationships on tree thinking. The studies of diagram design are based primarily in cognitive and perceptual psychology, with strong implications for education. The instructional studies are rooted in science education while being informed by cognitive psychology. The studies of prior knowledge reflect a more even mix of psychological and educational foundations. All studies are informed by expert knowledge of evolutionary biology. This research has used a variety of different kinds of tasks, including those that require diagram comprehension, translation from one diagram format to another, and inference. Measures of performance include accuracy, types of errors made, written explanations (evidence cited) in support of one’s responses, and patterns of eye movements.

Influences of diagram design on interpretations of evolutionary relationships. Consistent with a large cognitive psychological literature on diagram comprehension, we would expect students’ interpretations of Tree-of-Life diagrams to be influenced by how those diagrams are designed. Thus, one major focus of my research program has been to discover how diagram design affects students’ interpretations of a variety of different types of Tree-of-Life representations.

One exciting project compared students’ ability to extract the hierarchical structure from cladograms depicted in different ways. Cladograms are typically drawn in one of two formats: rectangular trees (left diagram in the figure below) and diagonal ladders (right diagram in the figure below). In an analysis of the cladograms printed in a professional journal, Novick and Catley (2007) found that rectangular trees are by far the preferred format among evolutionary biologists: 83% vs. 17%. In high school and biology textbooks, however, the diagonal format was found to occur slightly more often than the rectangular format: 59% vs. 41% for high school biology texts and 54% vs. 46% for college texts (Catley & Novick, 2008).

In several studies (Novick & Catley, 2007, 2013), we found that students had difficulty understanding and reasoning from the diagonal cladogram format and that this difficulty stems from the Gestalt principle of good continuation, which works to conceal the critical information about hierarchical levels in this format. One implication of these results is that if some method can be found to break good continuation at the appropriate points along the continuous lines, students’ ability to correctly extract the hierarchical structure of diagonal cladograms should improve. Consistent with this prediction, we found that adding a synapomorphy to mark each branching point in diagonal cladograms greatly improved students’ ability to translate those cladograms to the rectangular format (Novick, Catley, & Funk, 2010). In a final study in this line of research, we found that biology students preferentially scan diagonal cladograms from left to right, following their highly practiced directional pattern for reading written text, and that they prefer to scan along the main diagonal line at the base of the cladogram (Novick, Stull, & Catley, 2012). This impairs their ability to uncover the correct pattern of nesting in diagonal cladograms as those cladograms are typically drawn in textbooks and the biology literature (see above figure).

I am excited to report that based on our research, many textbooks for introductory biology, evolution, and zoology classes have changed from depicting cladograms in the diagonal to the rectangular format to improve student comprehension and learning. Introductory biology textbooks alone reach approximately 800,000 students every year.

My current research is examining the importance of another Gestalt grouping principle in influencing students’ interpretations of the evolutionary relationships depicted in cladograms. I have recently come to believe that the fundamental difficulty students need to overcome to acquire expertise in tree thinking is to understand that any specific evolutionary tree is a subset of the complete, unimaginably large Tree of Life. My prior research with Kefyn Catley suggests that students instead reify the particular groupings they see and fail to appreciate that these groupings are largely an artifact of the specific taxa that happen to be included in the particular tree under consideration. This reification of particular groupings occurs, I believe, because of the Gestalt principles of grouping, which are part of the foundation of human perception. I am pursuing this new line of research in collaboration with Linda Fuselier, an evolutionary biologist at the University of Louisville. We are examining the role of the Gestalt principle of connectedness in determining students’ interpretations of the relationships depicted in rectangular format cladograms. By testing students enrolled in biology classes at different levels (e.g., introductory biology for majors and nonmajors vs. more advanced classes), we will be able to discern the extent to which reliance on Gestalt grouping versus most recent common ancestry changes as a function of biological expertise.

Assessing and improving students’ tree-thinking skills. As documented in three recent publications (Novick & Catley, 2016, 2017; Novick, Catley, & Schreiber, 2014), using the knowledge we gained from our extensive research on tree thinking, Kefyn Catley and I set out to create, implement, and test a research-based tree-thinking curriculum and assessment instrument. Our efforts were very successful with students from a wide variety of biology backgrounds, ranging from little or no biology coursework in college to extensive biology coursework consistent with being a senior biology major. Over three connected and iterative studies, we were able to show that direct instruction produced skills that transferred to regular classroom practices and lab settings and appeared to enhance student understanding of macroevolutionary patterns and processes. Some of the instructional materials we developed are available for download here and from the lessons and resources for teachers section of the Understanding Evolution web site maintained by the University of California Museum of Paleontology.

Effects of prior knowledge about taxonomic relationships on tree thinking. A third focus of my research program concerns students’ folkbiological knowledge about taxonomic relationships among living things and the impact of such knowledge on their ability to engage in tree thinking. Students’ folkbiological knowledge often conflicts with well-established scientific taxonomy. For example, although students (even after an introductory biology course for majors) group lizards together with frogs in the folkbiological category of reptiles and amphibians, lizards are in fact more closely related to mammals because those taxa share a MRCA that evolved the novel character of possessing an amniotic egg (see the cladogram at the top of this page).

In one project (Novick & Catley, 2014), I examined how college and high school students responded when their prior knowledge conflicted with the evolutionary information provided in rectangular format cladograms. In two studies, college and high school students received matched pairs of cladograms that depicted an identical pattern of relationships among either familiar or unfamiliar taxa. When the taxa were familiar, the cladograms showed (correct) relationships that conflicted with students’ prior knowledge. For example, one such cladogram showed that mushrooms are more closely related to animals than to plants, contradicting folkbiological taxonomy that mushrooms are plants. Students answered evolutionary relationship questions about both cladograms in each matched pair. For both student groups, accuracy was higher when the cladograms depicted relationships among unfamiliar rather than familiar taxa (i.e., when folkbiological knowledge was not available to contradict the scientific information presented).

An additional study reported in Novick and Catley (2014) examined college students’ willingness to include birds in the reptile category, where they belong, as a function of the strength of the supporting evidence. Even with salient visual evidence in the cladogram supporting this grouping, approximately half the students resisted this classification. On the positive side, students did at least choose a coherent definition of reptiles. For example, when they excluded birds from the category, they also excluded crocodiles, to which birds are most closely related. Evidently, the strength of many students’ prior belief that birds are not reptiles is greater than their prior belief that crocodiles are reptiles.

The difficulty of persuading students of the inaccuracy of their prior knowledge may relate in part to the length of time over which their misconceptions have been reinforced. Brenda Phillips, a former postdoctoral fellow in my laboratory, collected some preliminary data on pre-K through 6th grade children’s and college students’ knowledge about the relationships among sets of three familiar taxa (e.g., camels, elephants, and zebras; beavers, snakes, and frogs). In several respects, the responses of K-1st grade, 4th-6th grade, and college students were remarkably similar. For example, given the set of beavers, snakes, and frogs, most students in all age groups responded, incorrectly, that snakes and frogs are most closely related. See if you can figure out the age group of the student providing each of the following three explanations for this response: (a) “Both live near/in water and are reptile family members”; (b) “They are both not mammals”; (c) “They’re both amphibians and can go underwater and stay underwater, and can both go on land. They both like bugs.” [**Answers are at the bottom of this page.]

Research Support

Much of the research described here was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant R305A080621 to Vanderbilt University (Laura R. Novick, PI; Kefyn M. Catley, Co-I). The opinions expressed are those of the authors and do not represent views of the Institute or the U.S. Department of Education. My current research is being supported by a small grant from Peabody College of Vanderbilt University.

Instructional Materials Available for Download

As part of the above-mentioned IES grant, Kefyn Catley and I developed a variety of instructional materials for teaching tree thinking to undergraduates. Some of these materials are available for download here, as well as from the lessons and resources for teachers section of the Understanding Evolution web site maintained by the University of California Museum of Paleontology.

 

** (a) Vanderbilt student, (b) kindergarten or first grade student, (c) 4th-6th grade student.