THE TREE ROOM
Primer on trees
Field guide to evolutionary trees
How to build a tree
For museums and zoos
The Tree Room : Tree misinterpretations
Misinterpretations and intuitive ideas about evolutionary 'progress' or 'advancement'
Research suggests that students, teachers, and the general public tend to misinterpret evolutionary 'progress' or 'advancement in the following ways:
INTUITIVE INTERPRETATION: Taxa (different groups of living things) are organized into a Great Chain of Being, in which some taxa (e.g., humans) are higher or more advanced than others. 1, 2, 3, 4
SCIENTIFIC INTERPRETATION: The relationships among taxa are best represented by a branching tree-like structure (a phylogeny), in which taxa appear at the tips of the phylogeny, visually reinforcing the idea that no taxon has a higher or lower status than others.
EXPLANATION: The idea of "higher" and "lower" organisms is intuitively appealing and has many antecedents in the history of science; however, this idea reflects a human-centered, biased perspective on the biological world in which other organisms are measured by their similarity to humans. Taking an unbiased view, it is clear that there is no universal yardstick against which we can measure species. For example, we could focus on photosynthetic ability (which would make plants the "higher" beings), sheer number of individuals (which would pick out bacteria and microorganisms as special), or any number of other traits. Each trait would suggest a very different group of "higher" organisms. Diagrams that represent relationships using a central trunk with side branches reinforce the incorrect idea that evolution is directional and progressive. Phylogenetic trees are preferred because they convey information about evolutionary relationships without reinforcing intuitive ideas about evolutionary progress by placing some taxa above or below others. A similar intuitive idea is that some living species are more evolved than others; this idea is explored in the section 'everyday interpretations about time.'
INTUITIVE INTERPRETATION: On a phylogeny, there is a main lineage representing the progress of evolutionary history, and other lineages on the phylogeny represent side tracks to this main line. 1, 5, 6
SCIENTIFIC INTERPRETATION: There is no "main line" of evolutionary history. It is equally valid to focus on any lineage of descent represented by a phylogeny.
The blue lineage on the leftmost phylogeny does not represent the "main" line of evolution. The equivalent phylogenies shown here emphasize that this same lineage could be shown in different ways depending on how the branches of the phylogeny are rotated.
EXPLANATION: With trees that are particularly unbalanced (e.g., the leftmost tree above), it is easy to assume that one lineage (e.g., the one shown in blue above) represents the main trajectory of evolution. However, from a scientific perspective, evolution is neither progressive nor goal-directed and so cannot have a "main trajectory." For example, we could look at this phylogeny of vertebrates and trace the evolution of mammals as shown; however, this is not the "main trajectory" of evolution, it is just one of many histories we could explore. We could just as easily look at that same phylogeny and focus on the amphibians or ray-finned fishes. No particular pathway of evolution is the "main" one. If an unbalanced phylogeny encourages this intuitive conception, it may be helpful to note that rotating branches around nodes (as shown in the trees on the right above) yields equivalent phylogenies that visually emphasize different lineages of descent.
INTUITIVE INTERPRETATION: Taxa that appear near the top of a side-oriented tree or on the right-hand side of an upright phylogeny are more advanced than other organisms on the tree.
SCIENTIFIC INTERPRETATION: A taxon's position on a phylogeny is a function of its relationship to other taxa and the way that the phylogeny's branches are rotated. The position or placement of a terminal taxon is not an indication of how adaptive, specialized, or extreme its traits are.
Pines appear at the right-hand side of this phylogeny, but are not necessarily advanced, specialized, or extreme. Furthermore, rotating branches shows that many equivalent phylogenies exist in which pines are not in the rightmost position.7
EXPLANATION: When exploring such intuitive reasoning, it's important to note first that the idea of evolutionary "advancement" is not a particularly scientific idea. It is tempting to view organisms that are more similar to humans as more "advanced"; however, this is a biased and invalid perspective. There is no universal scale for "advancement" that favors human-like traits over spider-like, whale-like, or fir-like traits. We can use phylogenetics to study the evolution of eyes, photosynthetic ability, or any other trait, but such traits are not the equivalent of evolutionary advancement. Second, note that taxa with extreme versions of traits (e.g., complex eyes or a complicated photosynthetic pathway) may occur on any terminal branch irrespective of branch location. Tree designers sometimes place such taxa near the top or right-hand side of a phylogeny, but by rotating branches around nodes, we can generate many equivalent phylogenies in which taxa with extreme traits appear in different positions on the tree.
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