A Name by Any Other Tree

Evo Edu Outreach (2009) 2:303–309 DOI 10.1007/s12052-009-0122-7 VIEWS FROM UNDERSTANDING EVOLUTION A Name by Any Other Tree Anastasia Thanukos Published online: 18 April 2009 # The Author(s) 2009. This article is published with open access at Springerlink.com Keywords Taxonomy . Linnaean classification . Phylogenetics . Teaching If you read other articles in this issue carefully, you might begin to wonder if animal life is going through a hushed up identity crisis: Hummingbirds and canaries have finally come out as dinosaurs (Angielczyck 2009; Chiappe 2009). Despite lacking the four feet of tetrapods (tetra = four, pod = foot), snakes and whales have been embraced by this group (Clack 2009). Elephants are simply confused; they may or may not be ungulates (Prothero 2009). This taxonomic soul searching and the resulting strange bedfellows (e.g., lumping budgies with brachiosaurus) do not, of course, stem from long therapy sessions, but from a quiet revolution gripping biology: phylogenetics. Phylogenetics is the area of biology that deals with evolutionary relationships among organisms. In the past few decades, technological advances have produced a flood of genetic data, computing power has exploded, and scientists have developed new mathematical algorithms for building phylogenies—or evolutionary trees. As a result, biologists have been increasingly able to reconstruct the evolutionary histories of groups of organisms. We can now draw the family trees of close-knit groups, like Darwin’s finches (Petren et al. 1999), and sketch the deepest lines of descent that connect animals, plants, fungi, and bacteria (Ciccarelli et al. 2006; Figs. 1 and 2). The widespread availability of evolutionary trees is reshaping how biologists ask and answer biological questions. For a review of how to read and understand evolutionary trees, see Gregory (2009). A. Thanukos (*) University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780, USA e-mail: URL: http://evolution.berkeley.edu Phylogenies have even invaded what is, perhaps, the most foundational area of biology: taxonomy—how we classify and name life forms. This is the reason that elephants are left on the fence about their identity, while birds have landed in the dinosaur camp, and snakes can boast the paradoxical title of tetrapod. Biologists have changed the fundamental ways that they think about classification. Ever Since Linnaeus The Linnaean system of classification is how most of us learned about biodiversity, and it remains deeply embedded in biology textbooks today. Memorizing the Linnaean ranks of kingdom, phylum, class, order, family, genus, and species, was—and in many classrooms, still is—a rite of passage en route to more advanced material. But should it be? This system was created in the 1700s, long before scientists understood that life evolves. Today, biologists have moved away from the aspect of traditional Linnaean classification that groups organisms according to similarity of specific characteristics or overall similarity. Instead, biologists are adopting a system of classification based on phylogenetics, which reflects organisms’ evolutionary history. In another article in this issue, Angielczyck (2009) provides an introduction to phylogenetic classification. Here, we explore how the new system works and why biologists are bothering to make the switch. Naming Clades As opposed to naming phyla and families, phylogenetic classification only gives names to clades—groupings that include an ancestor and all the organisms (whether living or extinct) descended from that ancestor. A clade may include hundreds of thousands of species or just one. Using a phylogeny, it is easy to tell if a group of lineages forms a 304 Evo Edu Outreach (2009) 2:303–309 ground finches cactus finches sharp-beaked finch tree finches which it belongs. So, for example, if we work our way backward from the evolutionary twig belonging to the Common Ostrich (Struthio camelus), we see that it first joins the shoot belonging to all ostriches (Struthionidae) and that this shoot springs from the Ratite bird branch (Struthioniformes), which attaches to the bough belonging to all birds (Aves), which is itself just one offshoot of the dinosaur limb (Dinosauria)...and so on, all the way back to the root of the tree of life (Fig. 6). Because the Common Ostrich is in each of these clades, it is given all of these names. It is an ostrich, a ratite, a bird, and a dinosaur. mangrove finch woodpecker finch Practically Speaking vegetarian finch warbler finch Cocos finch warbler finch Fig. 1 Biologists are now able to reconstruct the phylogenies of many groups of organisms, like this one showing relationships among the Galapagos finches. Illustration adapted with permission from the Understanding Evolution website and based on phylogenies presented in Grant and Grant (2008) clade. Imagine clipping a single branch off the tree. All of the organisms on that branch make up a clade (Fig. 3). If any organisms that spring from that branch are excluded from the grouping, it does not form a clade. A quick glance at the phylogeny of tetrapods (Fig. 4) makes it clear why phylogenetic classification leads us to view birds as dinosaurs. Since birds evolved from dinosaurs, there is just no way to clip a single branch from this tree that includes Triceratops and Tyrannosaurus rex, but excludes birds. In the same way, snakes and whales are tetrapods because they evolved from four-legged tetrapod ancestors. On the other hand, the status of elephants as ungulates is currently uncertain because scientists need to gather more evidence in order to be confident about where the pachyderm lineage connects to the rest of the tree of life. Figure 4 also highlights why the definition of Reptilia that you might have learned in school—a group of cold-blooded, scaly, terrestrial vertebrates made up of turtles, lizards, snakes, and crocodiles—is not a valid grouping according to phylogenetic classification. You cannot snip a branch that includes the traditional reptiles but excludes dinosaurs and birds. So either we cannot use the term reptile as a scientific name or we need to start thinking of birds as reptiles. Biologists have opted for the latter approach. Birds are considered members of the group Reptilia. Clades form a nested hierarchy—that is, they are nested within one another (Fig. 5). For classification purposes, an organism accumulates all the names of all the clades to Phylogenetic classification means a shift in the way we think about classification, but it does not mean a radical change in what organisms are actually called. Drosophila, Escherichia coli, T. rex, and most of the other scientific names you have gone to the bother of learning are still the same. At issue is really what we can take away from a name. If two lineages are both in the group Drosophila, we know that biologists have evidence that they (...truncated)