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Phylogenetic Supertrees and Narrative in an Age of Mass Extinction

Elizabeth Callaway

Since the invention of the term in 1985, biodiversity has become one of the most crucial and contested concepts within international and extra-national conservation discourses and practices. From its status as the rallying point for conservation, to scientific debates about how to best measure it, biodiversity is mediated and remediated across disciplines and mediums. One of the most prominent and powerful ways of visualizing vast numbers of species (and the diversity between them) is in phylogenetic supertrees.

These trees, which are combinations of other (many times contradictory) evolutionary trees are one of the primary places that many hundreds and thousands of species are organized together at once. They act as conceptual tools to help us think through a variety of life so vast as to be overwhelming, and they do it in almost every domain of life. There are supertrees of all birds, mammals, crawfish, flowering plants, dinosaurs, as well as bacterial trees of life and “comprehensive” trees of life. Yet all of these trees are remarkably similar visually; they are all circular and crowded with time running outward from the center. Set against the backdrop of past linear and surreptitiously hierarchical trees, the new circular design traffics in aesthetics of evenness and prohibits teleological descriptions of evolution’s “direction,” “progression,” or “crowning achievements.” Yet the ways in which these visualizations picture evolutionary history are not without their own problems. Their design affects how we can think about the current global biodiversity crisis.

While supertrees remain an exceptionally powerful tool for visualizing relationships, we need to be aware that formal choice made in their construction means that they are better at portraying certain kinds of stories than others. There are two primary ways I find the design of current phylogenetic supertrees to be discordant with an era characterized by a mass extinction. First, the kinds of stories it is possible for these trees to tell are not the ones so we might need in a time of species loss. Their circular construction and branching structure lend themselves to stories of proliferation, but are nearly incapable of communicating diminishment. A circle simply has more inscription space near its edge than anywhere in the middle, so in any visualization where time runs from the interior of a circle to its edge, there will be more room for organism names along the edge that denotes the present, despite the fact that most species that have existed on this Earth are now extinct. Second, these trees end up acting as analgesics for extinction. Phylogenetic organization is the instrument by which a certain kind of biodiversity conservation gets carried out: that in which phylogenetic diversity or evolutionary uniqueness of a species determines its conservation priority. But this type of accounting for life often figures the loss of a species as a loss of “millions of years of evolutionary history.” This evolutionary history, however, ends up being recoverable in related species, hybrid species, or frozen specimens even after a species has gone extinct. Thus, the equating of a species with its evolutionary history frames extinction as not as a true and final loss of the essence of a species. The focus on evolutionary history takes the sting out of extinction instead of providing space to feel the discomfort of the irrecoverable.

The narratives we tell about what biodiversity is and can be affect conservation priorities and decisions carried out on the ground. In this way, we shape the world to match our conceptualization of biodiversity. We must cast a critical eye toward the strengths and limitations of the media, genres, and narratives we use to think through biodiversity, so that we can be sure we are conserving the types of biodiversity we want available in the future and inviting a variety of voices to conservation decisions.

Last Updated: 2/6/17