Reading: Limitations of Phylogenetic Trees

It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings or after the evolution of a major new adaptation, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure 1 shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs appear more similar than lizards and rabbits.

The ladder-like phylogenetic tree starts with a trunk at the left. A question next to the trunk asks whether a vertebral column is present. If the answer is no, a branch leads downward to lancelet. If the answer is yes, a branch leads upward to another question: is a hinged jaw present? If the answer is no, a branch leads downward to lamprey. If the answer is yes, a branch leads upward to another question: are legs present? If the answer is no, a branch leads downward to fish. If the answer is yes, a branch leads upward to another question: does the egg have amnion? If the answer is no, the branch leads downward to frog. If the answer is yes, the branch leads upward to another question: is hair present? If the answer is no, the branch leads downward to lizard. If the answer is yes, the branch leads upward to rabbit.

Figure 1. This ladder-like phylogenetic tree of vertebrates is rooted by an organism that lacked a vertebral column. At each branch point, organisms with different characters are placed in different groups based on the characteristics they share.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, the length of a branch does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 1, the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure 1, the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops.

So, for the organisms in Figure 1, just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.

Head to this website to see interactive exercises that allow you to explore the evolutionary relationships among species.