Abstract diagrams are important not only in mathematics but in science as well (Novick, 2006). In biology, hierarchical diagrams are especially common. For the past several years, I have been working with Kefyn Catley, a biologist and science educator, to investigate college students' understanding of cladograms, the most important tool that contemporary scientists use to reason about evolutionary relationships. A cladogram is a type of hierarchical diagram that depicts the distribution of characters (i.e., physical, molecular, and behavioral characteristics) among a set of taxa. More specifically, cladograms are hypotheses about nested sets of taxa that are supported by shared evolutionary novelties 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, derived 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 constitute a clade. Because of the nesting inherent in hierarchical diagrams, birds, alligators, and lizards also constitute 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, derived 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. By invoking monophyly to systematize the 3.5 billion year history of life on Earth, they (a) enable evidence-based inference and prediction and (b) provide a conceptual framework for basic and applied biology.

      In the past several years, a number of researchers have called for the inclusion of tree thinking in biology curricula from middle school through the undergraduate years. Tree thinking has at its core cladograms (phylogenetic trees) -- testable hypotheses of relationships among taxa based on observable evidence in the form of synapomorphies. 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 of Earth (i.e., the evolutionary relationships among taxa). Thus, inferences based on this classification scheme are likely to be more informative and 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 (survival!) than basing the choice of antivenin on the king brown snake's similar coloration to the western brown snake. Implementing tree thinking in the biology curriculum is consistent with the National Science Education Standards, which state that students should learn to use the methodologies and tools from professional practice (American Association for the Advancement of Science, 2001; National Research Council, 1996).

Summary of our Research

      Our research on students' understanding of and ability to reason from evolutionary diagrams is multifaceted. In most studies, we compare the performance of students with stronger versus weaker backgrounds in biology in order to determine the learning that results from instruction in (primarily organismal) biology, as well as the gaps in that learning. Weaker background students have taken high school biology and may or may not have taken a college-level course designed for students who are not majoring in a scientific discipline. The performance of these students will inform curriculum development at the high school level. Stronger background students have taken at least one semester of college-level biology for biology majors; in most studies, they have taken an average of just over three semesters of potentially-relevant coursework at the college level (with a range of 1-7 semester-long courses). The performance of these students will inform curriculum development for biology majors. In addition to investigating effects of domain expertise and prior knowledge, we are also examining the effects of (a) cognitive and perceptual principles and (b) diagram format. We have used tasks that require diagram comprehension, translation from one diagram format to another, and inference. Our measures of performance include accuracy, types of errors made, and written justifications (evidence cited) in support of one's responses. This research is currently being supported by a grant from the U.S. Department of Education, Institute of Education Sciences.

      Cladograms are typically depicted in one of two formats, which we have referred to as trees (left diagram in the figure below) and ladders (right diagram in the figure below). In an analysis of the cladograms printed in a professional journal, Novick and Catley (2007) found that trees are by far the preferred format among evolutionary biologists: 83% vs. 17%. In high school and biology textbooks, however, the ladder format appears slightly more often than the tree format: 59% vs. 41% for high school biology texts and 54% vs. 46% for college texts (Catley & Novick, 2008).

      One project is investigating students' ability to translate between the tree format for drawing cladograms and the ladder format. In two studies, we found that students particularly had trouble understanding the structure of ladder diagrams, and this difficulty was especially pronounced for students with weaker backgrounds in biology. We hypothesized that this difficulty stems from the Gestalt principle of good continuation, which makes it difficult to extract the critical information about hierarchical levels from a ladder diagram. Examination of the errors students made in drawing ladder diagrams and in translating from a ladder diagram to a tree diagram supported this hypothesis. A preliminary report of this research may be found in Novick and Catley (2006); the full report is in Novick and Catley (2007). Follow-up research in progress is examining additional factors that may contribute to students' difficulty in extracting the hierarchical structure from the ladder format. This new research will use both paper-and-pencil and eye-movement methodologies.

      A second project is investigating students' ability to comprehend and reason from cladograms in both the tree and ladder formats, such as those depicted in the figure immediately above. In this study (Novick & Catley, 2008), students answered questions such as: (a) "What character was possessed by the most recent common ancestor of lizards and mammals?", (b) "Which taxa did not evolve from an ancestor that had lungs?", and (c) "Are lungfish more closely related to mammals or to flounder? What evidence supports your answer?" Analyses in progress are examining both accuracy and students' justifications to gain insight into their understanding of and ability to reason from cladograms. Briefly, we found that students with a stronger background in biology did better than those with a weaker biology background on all of our core categories of tree-thinking questions. For the simplest questions -- identify characters (a) and identify taxa (b), for which students just had to read information off the cladograms, the difference between the two groups was larger for the ladder format than for the tree format. For the more difficult relationship (c) and inference questions, the biology background difference was consistent across all other manipulations. In general, the stronger background students did better in terms of both accuracy and quality of supporting evidence cited. Replicating the results of Novick and Catley (2007), this study also found poorer performance when the evolutionary relationships were depicted in the ladder format than in the tree format. Except for the easiest question types, even stronger background students found the ladder format more difficult to understand than the tree format. Several cognitive and perceptual factors were found to affect students' performance, including, for example, the Gestalt principle of good continuation and whether the question was worded affirmatively or negatively. Finally, a composite tree-thinking score that quantified the extent to which students cited evidence concerning most recent common ancestry to support their answers indicated that even stronger background students did poorly: Mean of 0.31 (0-1 scale) for all stronger background students reasoning about information depicted in the tree format and mean of 0.45 for the subset of those students who had taken 4-7 potentially relevant courses.

      A third project is examining students' understanding of evolutionary diagrams that are not cladograms (Catley, Novick, & Shade, 2008). Such diagrams, although they are commonly found in high school and college textbooks (Catley & Novick, 2008), appear to be poorly conceived and may reinforce evolutionary misconceptions. We wanted to determine whether the information extracted from such diagrams is in accordance with contemporary understanding of evolutionary theory. Our analyses documented persistent misconceptions that fall broadly into the following categories: (a) Evolution as an anagenic rather than a cladogenic process, (b) evolution as a teleological (purpose-driven) process, and (c) confusion between individuals (organisms) and groups of individuals (taxa). Cladogenesis, the accepted means of speciation, is a process whereby one species splits into two when the population of a parent species is fragmented (as a result of some vicariant event), and selection continues separately in each group driven by the pressures of each local environment. Anagenesis, in contrast, is a process whereby one species evolves directly into (i.e., becomes) another species over time without any branching events. There is no evidence to support anagenesis as a mechanism of speciation. In an on-going follow-up study conducted by Courtney Shade for her honors project, we are comparing students' responses to evolutionary relationship questions when they accompany cladograms (in tree and ladder formats) versus other evolutionary diagrams.

      Some ability to comprehend deep time is a prerequisite for understanding macroevolution. In a fourth project (Catley & Novick, in press), we examined college students' knowledge of deep time in the context of seven major historical and evolutionary events (e.g., the age of the Earth, the emergence of life, the appearance of a pre-modern human -- Homo habilis). Subjects provided startlingly large time ranges for all questions, ranging over several orders of magnitude (e.g., from 1000 to 600 billion years ago for when most dinosaurs became extinct), coupled with a strong tendency to underestimate how long ago the events occurred. Converting absolute time estimates to relative time estimates allowed students' knowledge of the spacing of the events to be examined and also provided a clearer picture of their patterns of over and underestimation. The results of this study suggest that many students are without an effective conceptual framework to make sense of very large time frames. Although there were no consistent differences in the accuracy of students' responses as a function of their biology background, the weaker background students showed greater variability, providing more time estimates at both the low and high extremes.

      A fifth project is examining college students' knowledge of (a) the evolutionary relationships among frogs, lizards, and mammals and (b) convergent evolution (Morabito, Catley, & Novick, 2008). Although lizards are often grouped together with frogs in the category of reptiles and amphibians, they 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 figure at the top of the page). Convergent evolution is an important concept because it is an alternative explanation to a shared MRCA for two taxa sharing the same character (e.g., warm-bloodedness for birds and mammals). A final project, conducted in collaboration with Dan Funk, an evolutionary biologist, is examining changes in performance on our cladogram reasoning task due to instruction in phylogenetics.

Publications and Recent Manuscripts

     Morabito, N., Catley, K. M., & Novick, L. R. (2008). Lizards, Eggs, and Homoplasy: The Effect of Prior Knowledge on Students' Understanding of Macroevolution. Manuscript in preparation.

     Novick, L. R., & Catley, K. M. (2008). Understanding the Tree of Life: The Effects of Biology Background, Cladogram Format, and Cognitive/Perceptual Factors on Tree Thinking. Manuscript under review.

     Catley, K. M., Novick, L. R., & Shade, C.K. (2008). Students' Interpretations of Common Types of Evolutionary Diagrams. Manuscript under review.

     Catley, K. M., & Novick, L. R. (in press). Seeing the wood for the trees: An analysis of evolutionary diagrams in biology textbooks. BioScience.

     Catley, K. M., & Novick, L. R. (in press). Digging deep: Exploring college students' knowledge of macroevolutionary time. Journal of Research in Science Teaching.

     Novick, L. R., & Catley, K. M. (2007). Understanding phylogenies in biology: The influence of a Gestalt perceptual principle. Journal of Experimental Psychology: Applied, 13, 197-223.

     Novick, L. R. (2006). The importance of both diagrammatic conventions and domain-specific knowledge for diagram literacy in science: The hierarchy as an illustrative case. In D. Barker-Plummer, R. Cox, & N. Swoboda (Eds.), Diagrams 2006, LNAI 4045 (pp. 1-11). Berlin: Springer-Verlag.

     Novick, L. R., & Catley, K. M. (2006). Interpreting hierarchical structure: Evidence from cladograms in biology. In D. Barker-Plummer, R. Cox, & N. Swoboda (Eds.), Diagrams 2006, LNAI 4045 (pp. 176-180). Berlin: Springer-Verlag.

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