Characters Are Key: The Effect of Synapomorphies on Cladogram Comprehension

Abstract

Cladograms, phylogenetic trees that depict evolutionary relationships among a set of taxa, are one of the most powerful predictive tools in modern biology. They are usually depicted in one of two formats-tree or ladder. Previous research (Novick and Catley 2007) has found that college students have much greater difficulty understanding a cladogram's hierarchical structure when it is depicted in the ladder format. Such understanding would seem to be a prerequisite for successful tree thinking. The present research examined the effect of a theoretically guided manipulation-adding a synapomorphy on each branch that supports two or more taxa-on students' understanding of the hierarchical structure of ladder cladograms. Synapomorphies are characters shared by a group of taxa due to inheritance from a common ancestor. Thus, their depiction on a cladogram may facilitate the understanding of evolutionary relationships. Students' comprehension was assessed in terms of success at translating relationships depicted in the ladder format to the tree format. The results indicated that adding synapomorphies provided powerful conceptual scaffolding that improved comprehension for students with both weaker and stronger backgrounds in biology. For stronger background students, the benefit of adding synapomorphies to the ladders was comparable to that of approximately two hours of instruction in phylogenetics that emphasized the ladder format.Keywords Cladograms . Synapomorphies . Evolution education . Phylogenetics . Evolutionary relationships Cladograms are phylogenetic trees that depict evolutionary relationships among a set of taxa in terms of nested levels of common ancestry. Synapomorphies are (morphological, molecular, or behavioral) characters shared by a group of taxa due to their inheritance from a common ancestor. Synapomorphies thus constitute evidence for historical relationships and their associated hierarchical structure. For example, the cladogram in Fig. 1a shows the evolutionary relationships, supported by synapomorphies, among genera of Mollusca. Cladograms are one of the most powerful predictive tools in modern biology. Using monophyly-groupings comprising all descendent taxa and their most recent common ancestor (a.k.a. clades)-to organize and make sense of the 3.5 billion year history of life on Earth, they provide a conceptual framework for basic and applied biology in fields as disparate as conservation, ecology, behavior, molecular biology, epidemiology, and pharmacology (e.g., Futuyma 2004;Hillis 2004;Nickels and Nelson 2005;Yates et al. 2004). Given the importance of cladograms in biology, it is not surprising that a number of researchers and educators have called for the inclusion of tree thinking in biology curricula for both biology majors and nonmajors, as well as at the high school level (e.g., Baum et al.

Assessment of Student Learning Associated with Tree Thinking in an Undergraduate Introductory Organismal Biology Course

“…Figure 2 shows the scoring rubric that was used for part A of the PhAT, which connects to the idea that having synapomorphies displayed on branches allows students to understand trees better (Novick et al ., 2010). In part A, students were asked to map the evolution of large canine teeth, expanded metatarsals, and large incisors onto two different phylogenetic trees (alternative hypotheses) that were chosen out of the 15 possible phylogenetic trees for the relationships of rats, rabbits, dogs, and cats, with opossums as the outgroup (see Smith and Cheruvelil, 2009).…”

Biology Education Research: Lessons and Future Directions

“…A notable case of the latter comes from evolutionary biology, a field in which cognitive scientist Laura Novick and biologist Kefyn Catley have conducted extensive research about how students understand evolutionary relationships when different types of evolutionary tree representations are used (Catley and Novick, 2008; Novick et al ., 2010). Their research shows that the form of representation that is most commonly used in undergraduate biology texts leads to the least understanding of this important evolutionary concept.…”

Depicting the tree of life in museums: guiding principles from psychological research

“…Novick et al (2010) found that including such characters on trees was critical to undergraduate students' ability to understand the structure (i.e., branching pattern) of the depicted phylogenies. Likewise, Ainsworth and Saffer's (2013) study points to the importance of synapomorphies in influencing children's understanding of cladograms.…”