David Attenborough recently declared in a program about Darwin that “you’d have to be extraordinarily blinkered if you didn’t stub your toe against the theory of evolution very early on in your life”, particularly if as a child you collected fossils (Charles Darwin and the tree of life 2009). The data to substantiate many of the major transitions in evolution have been amassed. Lines of evidence include cladograms from independent datasets (i.e., fossils, morphology, and molecules), stratigraphy and radiometric dating to establish events in time, and paleoecology to interpret ancient environments. Embryology, behavior, histology, geochemistry, functional morphology, physiology, and biochemistry all add to the array of independent lines of evidence used to test evolutionary hypotheses.
Padian (2008) suggests depicting these various lines of evidence, particularly the ones that are important in major evolutionary transitions, in a single diagram, called an “evogram” (Fig. 3), to help students to understand that evolutionary research is conducted in an integrative way and in a way that relies on testing independent lines of evidence against each other to demonstrate concordance. Evograms have a cladogram as the backbone of the diagram, generated using an entirely separate base of information. Because a cladogram is basic to understanding evolutionary relationships, other types of noncladogram evolutionary diagrams will only confuse students (Catley and Novick 2008). In Fig. 3, fossil evidence of limb structure is depicted in one row. Structures of the fossils, showing corresponding bones in corresponding colors, are interpreted in the next row. Important clade names and important synapomorphies can also be added to the nodes of the cladogram (Padian 2008). Evograms integrate the various independent lines of evidence, providing a more united picture of evolution.
The challenge is that teachers are generally neither trained nor equipped to teach this way. Indeed, tree-thinking has only recently trickled down to high school biology (Goldsmith 2003; Catley 2006; Baum and Offner 2008). Most high school and many college introductory biology texts are woefully lacking in their presentation of comprehensive figures integrating phylogenetic analyses with hypotheses about evolutionary process. In fact, it appears that many of the diagrams may actually be misleading (Catley and Novick 2008). In a recent analysis of evolutionary diagrams in biology textbooks, Catley and Novick (2008) found that even though 72% of the diagrams in their survey of 31 textbooks were cladograms, there were striking differences across grade levels. Middle school texts had the fewest cladograms, with almost twice as many noncladograms. High school texts included more cladograms, but the ratio of cladograms to noncladograms was approximately equal. Such high proportions of noncladograms are disturbing if these support misconceptions about evolutionary processes (Catley and Novick 2008).
I urge teachers to embrace cladistics and tree-thinking as a way of examining the natural world and encourage students to make such thinking a habit. For a first step, the following websites offer basic exercises on cladistics and tree-thinking: Evolution and the Nature of Science Institutes (http://www.indiana.edu/~ensiweb); Understanding Evolution (http://evolution.berkeley.edu); What did T. rex taste like? (http://www.ucmp.berkeley.edu/education/explorations/tours/Trex). Also recommended are ways of teaching about evolution consistent with modern practice in systematics: using “ancestral” and “derived” instead of “primitive” and “advanced” when referring to organisms and characters, which promotes thinking about evolution as a tree rather than a ladder (and also follows, the good example of Darwin, who admonished himself, “Never use the words higher or lower”); including exercises that define clades by synapomorphies and map such characters onto phylogenies (see Staub et al. 2006 and ENSI: Making Cladograms); and clarifying that a cladogram or phylogenetic tree is a hypothesis, given the data, and that a node on a cladogram is a hypothetical ancestor, not an actual ancestor for which we should expect to find a fossil representative. To attain scientific literacy, students need to learn that evolution is a branching process, that paleontologists are not searching for missing links but for transitional features, and that evolutionary research is conducted in a way that enables scientists to use independent lines of evidence to converge on robust conclusions about the history of life. Teachers need to learn these lessons in order to be able to impart them!