Phylogenetic trees

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back in time to confirm the proposed relationships. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms.

Figure 1: Both of these phylogenetic trees show the relationship between the three domains of life (Bacteria, Archaea, and Eukarya), and the rooted tree on the left attempts to identify when various species diverged from a common ancestor while the unrooted tree on the right does not. (credit: modification of work by Eric Gaba)

Unlike a taxonomic classification diagram, a phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains (Bacteria, Archaea, and Eukarya) diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and minuscule these groups are when compared with other organisms. Unrooted trees don’t show a common ancestor but do show relationships among species.

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 dissimilar than other groups that are not as closely related, but look similar because of convergent evolution.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for the length of time but only the evolutionary order. In other words, a long branch does not typically mean more time has passed, nor does a short branch mean less time has passed, unless otherwise specified on the diagram.