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THE TREE ROOM

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The Tree Room :

Intuitive ideas and misconceptions about evolutionary trees
by the Understanding Evolution team

Research shows that many students, teachers, and members of the general public misinterpret trees in predictable ways. Such misinterpretations are commonly known as 'misconceptions;' however, many of these "wrong" ideas are actually perfectly sound ways of reasoning that learners simply apply in the wrong situation or in the wrong way. In other words, common explanations (such as change resulting from intentional choices) work perfectly well for many everyday situations, but these explanations are not helpful for thinking about evolutionary change. While the term 'misconception' is a familiar way to describe such ideas, they are more accurately thought of as "intuitive" or "everyday" interpretations.

Reviewing the following intuitive conceptions, gleaned from the research literature, can help you anticipate ways that learners are likely to misinterpret evolutionary trees. Though it might be tempting to try to root out such conceptions and "replace" them with correct ones, this approach is unlikely to be effective and fails to recognize the useful roles that these ways of reasoning may play in other contexts. Instead, here are a few general tips for designing instructional experiences to help students build on their intuitive conceptions:

  • Encourage learners to recognize phylogenies as a specialized representational form that requires setting aside everyday ways of reasoning
  • Encourage learners to reflect on their explanations (i.e., engage in metacognition) and be explicit about their intuitive approaches to phylogenies
  • Help learners recognize why they use these intuitive ways of reasoning (because they are useful in many other situations!) and compare such intuitive approaches with scientifically acceptable ones
  • Engage learners with instruction that acknowledges students' intuitive conceptions about phylogenies, directly refutes them, and introduces scientific interpretations as viable alternatives
  • Help learners see the similarities between trees that they are able to interpret correctly and trees for which their intuitive conceptions lead them astray

For suggestions on how to design phylogenies that discourage misinterpretations, visit Tips for phylogeny design.


Intuitive interpretations of evolutionary 'progress' or 'advancement'

Intuitive interpretations of relatedness

Intuitive interpretations of time

Intuitive interpretations of evolutionary change



Intuitive interpretations of evolutionary 'progress' or 'advancement'

  • 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.

    a misleading and an accurate portrayal of primate relationships
    5

    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.'

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  • 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, 6, 7

    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.

    several equivalent portrayals of one tree
    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.

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  • 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.

    several equivalent portrayals of one tree
    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.8

    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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Intuitive interpretations of relatedness

  • INTUITIVE INTERPRETATION: Superficial, overall similarity is an indicator of evolutionary relatedness; taxa that are more closely related to one another, as shown on a phylogeny, are more similar to one another than they are to more distantly related taxa. 1, 7, 9

    SCIENTIFIC INTERPRETATION: Common ancestry is the currency of evolutionary relatedness; lineages that share a more recent common ancestor are more closely related. Furthermore, closely related lineages are not necessarily very similar to each other in terms of morphology or general appearance. By the same token, species that are similar to each other in terms of general appearance are not necessarily very closely related.

    EXPLANATION: Although morphology can inform us about the relatedness of taxa, biologists focus on specific shared characters that indicate common ancestry, rather than on overall similarity in appearance. For example, a garter snake might look like an earthworm because of its long body and lack of legs, but its basic body structures (e.g., a bony skeleton and external body scales) provide evidence of the snake's close relationship to lizards. Because rates of evolutionary change can vary in different lineages and because of convergent evolution, taxa that are very closely related may be relatively dissimilar from one another, and taxa that are distantly related may appear to be quite similar to one another superficially (e.g., dolphins and sharks look similar to one another, but dolphins are mammals, whereas sharks are cartilaginous, or non-bony, fishes). Hence, the branching patterns seen in tree diagrams may be inconsistent with our intuitive sense of similarity based on general appearance.

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  • INTUITIVE INTERPRETATION: The proximity of tips on a phylogeny is an indicator of relatedness: taxa that are closer together are more closely related. 1, 7, 9, 10, 11

    SCIENTIFIC INTERPRETATION: The branching pattern of a phylogeny is the indicator of taxon relatedness; taxa that share a more recent common ancestor are more closely related.

    several equivalent portrayals of one tree
    The circle and triangle taxa are adjacent on this phylogeny, and the triangle and oval taxa are further apart; however, the triangle and oval taxa are more closely related because they share a more recent ancestor with one another than do the circle and triangle taxa.

    EXPLANATION: It's tempting to think that taxa that are nearer one another on a phylogeny must be more closely related than taxa positioned further from one another. After all, we often group similar things together in a chart or diagram. However, rotating branches around nodes demonstrates that this cannot be a valid guideline. Taxa that are adjacent on one phylogeny may be separated by many taxa on an equivalent phylogeny. Instead, the true indicator of evolutionary relatedness is the age of a common ancestor. Taxa that share a more recent common ancestor are more closely related than taxa whose most recent common ancestor is older. For example, on the phylogeny above, the circle and triangle taxa are adjacent to one another, while the triangle and oval taxa are further apart. However, this does not mean that the circle and triangle are more closely related. On the contrary, the most recent common ancestor of the triangle and oval taxa is younger than the most recent common ancestor of the circle and triangle taxa; therefore, the triangle and oval taxa are more closely related to one another than either is to the circle taxon.

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  • INTUITIVE INTERPRETATION: The degree of relatedness between two taxa on a phylogeny is indicated by the number of nodes separating them (e.g., a pair of taxa with three nodes between them is more distantly related to each other than is a pair of taxa with two nodes between them). 1, 7, 10, 11

    SCIENTIFIC INTERPRETATION: Taxa that share more recent common ancestors are more closely related; thus, the branching pattern of a phylogeny is the true indicator of taxon relatedness.

    The number of nodes separating two taxa does not indicate their degree of relatedness. As shown here, this number is affected by which taxa the tree includes
    The number of nodes separating two taxa does not indicate their degree of relatedness. As shown here, this number is affected by which taxa the tree includes.

    EXPLANATION: Nodes represent common ancestors, but the specific number of nodes between any two taxa shown on a tree will vary depending on what other taxa are included. For example, in the tree above at left, three nodes separate the amphibians and the echinoderms (i.e., starfish relatives). If you were to include sharks on the tree (as shown on the rightmost phylogeny), four nodes would separate the amphibians and the echinoderms. However, the degree of relatedness between amphibians and echinoderms has not changed. The appropriate indicator of relatedness is the age of two organisms' most recent common ancestor. Organisms that share a more recent common ancestor are more closely related than organisms with a most recent common ancestor that is older.

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  • INTUITIVE INTERPRETATION: Some taxa at the tips of a phylogeny (usually those depicted near the left-hand side of an upright phylogeny or near the bottom of a side-oriented tree) are the ancestors of taxa on other tips. 1, 9, 11

    SCIENTIFIC INTERPRETATION: Terminal taxa are each others' evolutionary cousins. They do not have an ancestor/descendant relationship.

    EXPLANATION: The ancestors of taxa at the tips of a phylogeny are represented by the lineages and nodes leading to those tips, not by other terminal taxa. Each terminal taxon depicted in a tree has a separate evolutionary lineage that can be traced back to ancestors shared with the other taxa in the tree at some point the past — making them the equivalent of evolutionary cousins. For example, if a tree includes humans, chimpanzees, and gorillas, and the chimpanzees are placed on the left hand side of humans in the diagram (as shown below left), it does not mean that chimps are the ancestors of humans. Instead, chimps and humans are closely related evolutionary cousins. Just as your cousin is not your ancestor, chimps are not the ancestors of the human species. Keep in mind that rotating branches around nodes yields an equivalent phylogeny and changes the order of the taxa across the tips of the tree, as shown in the diagram below right. If your tree includes terminal taxa that are often misunderstood as being ancestors of other taxa on the tree (such as chimpanzees and humans), you might want to rotate branches to avoid placing the presumed "ancestor" taxon on the left or bottom. You might also contrast a tree that elicits ancestor-descendant ideas to a tree with the same layout but different taxa that are less likely to elicit the same intuitive reasoning patterns.

    Terminal taxa are each others' evolutionary cousins. They do not have an ancestor/descendant relationship.
    Humans appear to the right of chimpanzees in the lefthand tree, but this does not mean that they are the descendents of chimpanzees. As shown at right, rotating branches changes the order of taxa at the tips of a tree without changing the underlying relationships.

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Intuitive interpretations of time

  • INTUITIVE INTERPRETATION: Terminal taxa on the left-hand side of an upright phylogeny or near the bottom of a side-oriented tree evolved earlier or are older than other terminal taxa on the tree (i.e., on phylogenies, the arrow of time flies across the tips). 1, 11, 12

    SCIENTIFIC INTERPRETATION: On phylogenies, time is read from root to tip, not across the tips.

    Time flows from the root of a phylogeny to its tips.
    Time flows from the root of a phylogeny to its tips.

    EXPLANATION: The branching pattern from the root to the tip of a tree represents the evolutionary relationships among the taxa through time; therefore, time runs from root to tip. This means that in an upright tree the direction of time runs from bottom to top. Similarly, in a phylogeny that is oriented to the side, time flows from left (root) to right (tip). The particular order of the taxa at the tips has no significance or meaning in terms of geological age or time (e.g., appearance in the fossil record, etc.).

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  • INTUITIVE INTERPRETATION: Some living (i.e., extant) species have longer evolutionary histories than others (i.e., have been evolving for a longer time), and so some species are more or less "evolved" than other extant species. 1

    SCIENTIFIC INTERPRETATION: Since all extant species are alive today and share a common ancestor (one that lived more than 3.5 billion years ago!), all extant species have been evolving the same amount of time.

    Though their clades first arose at different times, modern sharks and mammals trace their ancestry back to the same lineage and so have had the same amount of time to evolve.
    Though their clades first arose at different times, modern sharks and mammals trace their ancestry back to the same lineage and so have had the same amount of time to evolve.

    EXPLANATION: Some living organisms such as mosses and sharks represent clades that appear early in the geological record. Others (such as grasses and birds) represent clades that appear more recently. It is tempting to think of living members of a clade that appeared 160 million years ago (such as the mammals) as having a shorter history than members of a clade that appeared 440 million years ago (such as the cartilaginous fishes, sharks and rays). However, this intuition does not apply because of all living clades trace their evolutionary history back to shared ancestors among the earliest forms of life. For example, the fact that the clade that includes sharks appears early in the fossil record does not mean that modern sharks have had a longer evolutionary history than any other modern species.

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Intuitive interpretations of evolutionary change

  • INTUITIVE INTERPRETATION: A long, unbroken branch on a phylogeny indicates that little evolutionary change has occurred in that lineage and that this lineage is likely to retain ancestral characteristics — or may even be the ancestor of other organisms on the phylogeny. 1, 6, 11

    SCIENTIFIC INTERPRETATION: In most phylogenies, branch length does not indicate anything about the amount of evolutionary change. When branch length is used to depict evolutionary change, longer branches indicate more evolutionary change.

    EXPLANATION: Long, unbroken branches appear on a phylogeny when one small clade is the sister group to a large clade—either because the small clade is comprised of only a few lineages or because the tree designer opted to show only a few members of the clade on the phylogeny. The long, unbroken branch is caused by the relative size of the clades and has nothing at all to do with the amount of evolution a lineage has undergone. For example, in the phylogeny at left below, the flowering plants are the sister group to a larger clade and so have a long branch. However, in the phylogeny at right, more examples of angiosperms have been included on the phylogeny and the clade's taxa have shorter branches. Outgroups often appear as long, unbroken branches because their full diversity is rarely represented, not because they necessarily retain many ancestral characteristics. Even if a particular group actually is comprised of just a few lineages, it doesn't indicate anything about the amount of evolutionary change that has occurred within that lineage.

    Long branches do not indicate taxa that have undergone little evolutionary change. As shown within the angiosperms, branch length can be affected simply by including more taxa in the phylogeny.
    Long branches do not indicate taxa that have undergone little evolutionary change. As shown within the angiosperms, branch length can be affected simply by including more taxa in the phylogeny.8

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  • INTUITIVE INTERPRETATION: Evolutionary change occurs only at times that correspond to the nodes of a phylogeny. 1, 9

    SCIENTIFIC INTERPRETATION: The nodes on a phylogeny correspond to the relative time of lineage splitting. Evolutionary change may occur at any point along the branches of a phylogeny.

    Many evolutionary changes occurred in the lineage that led to modern birds, but they did not all occur at the moment that this lineage split from that of Archaeopteryx.
    Many evolutionary changes occurred in the lineage that led to modern birds, but they did not all occur at the moment that this lineage split from that of Archaeopteryx.13

    EXPLANATION: An evolutionary tree depicts the relatedness of taxa through a series of branches that diverge at nodes. The branches leading up to a node represent the common ancestry of descendent lineages. However, a divergence between populations at a particular point in the past does not necessarily mark the precise moment of evolutionary change in physical, behavioral, or molecular characters that make descendent lineages distinct. Such changes may have occurred at this point and/or may have occurred later in these lineages' evolutionary history.


1 Gregory, T.R. 2008. Understanding evolutionary trees. Evolution and Education Outreach 1:121-137.

2 Nee, S. 2005. The great chain of being. Nature 435:429.

3 O'Hara, R.J. 1992. Telling the tree: Narrative representation and the study of evolutionary theory. Biology and Philosophy 7:135-160.

4 O'Hara, R.J. 1997. Population thinking and tree thinking in systematics. Zoologica Scripta 4:323-239.

5 Perelman, P., W.E. Johnson, C. Roos, H.N. Seuanez, J.E. Horvarth, M.A.M. Moreira, … and J. Pecon-Slattery. 2011. A molecular phylogeny of living primates. PLoS Genetics 7:e1001342.

6 Crisp, M.D., and L.G. Cook. 2005. Do early branching lineages signify ancestral traits? Trends in Ecology and Evolution 20:122-128.

7 Halverson, K.L., C.J. Pires, and S.K. Abell. 2011. Exploring the complexity of tree thinking expertise in an undergraduate systematics course. Science Education 95:794-823.

8 Soltis, P.S., and D.E. Soltis. 2013. Angiosperm phylogeny: A framework for studies of genome evolution. In I.J. Leitch et al. (eds.), Plant Genome Diversity Volume 2. Springer-Verlat, Wien.

9 Baum, D.A., S.D. Smith, and S. Donovan. 2005. The tree-thinking challenge. Science 310:979-980.

10 Novick, L.R., and K.M. Catley. 2013. Reasoning about evolution's grand patterns: College students' understanding of the tree of life. American Educational Research Journal 50:138-177.

11 Meir, E., J. Perry, J.C. Herron, and J. Kingsolver. 2007. College students' misconceptions about evolutionary trees. American Biology Teacher 69:71-76.

12 Dodick, J. 2010. Phylogeny exhibits and understanding geological time. Paper presented at the Understanding the Tree of Life, Carnegie Museum of Natural History, Pittsburgh, PA.

13 Zimmer, C. 2010. The Tangled Bank: An Introduction to Evolution. Roberts and Co., Greenwood Village, CO.

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Design phylogenies that discourage misinterpretations — visit Tips for phylogeny design.