Misinterpretations and intuitive ideas about relatedness

Research suggests that students, teachers, and the general public tend to misinterpret the representation of evolutionary relatedness on trees in the following ways:

• 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.
• The proximity of tips on a phylogeny is an indicator of relatedness: taxa that are closer together are more closely related.
• 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).
• 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.

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, 2, 3}

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.

INTUITIVE INTERPRETATION: The proximity of tips on a phylogeny is an indicator of relatedness: taxa that are closer together are more closely related. ^{1, 2, 3, 4, 5}

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.

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.

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, 2, 4, 5}

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.

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 and 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.

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, 3, 5}

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.