Characters Are Key: The Effect of Synapomorphies on Cladogram Comprehension

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.