Algae are a fascinating and diverse organismal group, with global ecological importance, a storied evolutionary history and deep connections to both contemporary and historical human societies. Yet non-experts who teach algal diversity face a lack of examples in many general biology textbooks and the difficulty of generalizing a group that includes many distantly-related lineages that don't share a single common ancestor. This lesson embraces the complexity of algae using a sorting game and tree-building activity. Students work in groups to decide which organisms from a provided set are eukaryotic algae. The class creates consensus statements about what exactly defines organisms as "algae" and self-discover that exceptions exist for every seemingly definitive algal trait. Students then build simple phylogenetic trees and map their organisms across the phylogenetic Tree of Eukaryotes in order to explore the complex evolutionary relationships between the major eukaryotic algal lineages. Student written responses recorded before and after the sorting game indicate students become more nuanced and expert-like in their descriptions of algae. This lesson is an engaging way to introduce students to algae and can be modified for a variety of courses including high school, non-majors biology courses and introductory biology courses.

Primary image: A photo of the phylogenetic trees made by students during the tree-building activity. Photo taken by the author, B. Clarkston.