Retiring Rankings

When it comes to teaching the tree of life, a new study finds that descent with modification is the way to go.

During evolution units, the phrase, “King Phillip Came Over For Good Soup,” echoes from introductory biology classrooms. Instructors use this mnemonic device to drive home the levels of taxonomic classification — kingdom, phylum, class, order, family, genus, species. Despite the popularity of this outdated method, overwhelming evidence shows the more inclusive, clade-based approach, which mirrors Darwin’s “descent with modification,” should replace the rank-based hierarchy.

While working as a post-doctoral researcher at Cornell University, Cissy Ballen teamed up with Harry Greene to examine how evolution and biodiversity are taught to introductory biology students. For Ballen, now a post-doctoral associate in Sehoya Cotner’s lab in the College of Biological Sciences: “The collaboration (with Greene) came easily as a result of our shared interest in teaching natural history and the tree of life.” Ballen and Greene share teaching strategies and examine the implementation of the updated method in textbooks with a recently published article in PLoS Biology, “Walking and Talking the Tree of Life: Why and How to Teach About Biodiversity.”

The evolutionary tree of life (TOL) captures the biological diversity of more than 1.5 million species through time; it is as central to biology as gravity is to physics and as the periodic table is to chemistry.

The clade-based approach eliminates rote memorization, as it does not sort and rank species into levels of taxonomic classification. Instead, the TOL organizes species based on shared common ancestors, ultimately tracing back all life to the last universal common ancestor. By following the TOL through time, students learn the branches of the tree diverge when new species arise. Key moments in evolutionary history occur when features that typify a clade — or a group of species who share a common ancestor — appear or disappear.

At Cornell, Ballen and Greene co-taught an evolutionary biology and biodiversity course. They found that teaching descent with modification requires students to utilize critical thinking skills as they discern the traits shared by groups of species. In addition, an active learning curriculum allows students to explore the TOL at varied paces while gaining a deeper understanding of biodiversity.

Although evolutionary biologists at large embrace this TOL approach, classroom teaching still lags behind research findings. After interviewing biology teachers and researching introductory textbooks, Ballen and Greene concluded that the rank-based approach still dominates pedagogical practices. Although some high schools and universities are beginning to explore flipped classrooms, traditional passive lecture reigns supreme.

However, the days of King Phillip mnemonics are likely numbered. Several new online resources allow students to visualize the TOL and move between nodes of phylogenetic trees.

“Not only is clade-based classification more accurate,” Ballen says, “but it also provides students with a richer understanding of relationships among organisms that appear superficially unrelated.” – Claire Wilson