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By the end of this section, you will be able to:
  • Discuss the need for a comprehensive classification system
  • List the different levels of the taxonomic classification system
  • Describe how systematics and taxonomy relate to phylogeny
  • Discuss the components and purpose of a phylogenetic tree

In scientific terms, the evolutionary history and relationship of an organism or group of organisms is called its phylogeny    . A phylogeny describes the relationships of an organism, such as from which organisms it is thought to have evolved, to which species it is most closely related, and so forth. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.

Phylogenetic trees

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree    is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back 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 ( [link] ).

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 miniscule these groups are compared with other organisms. Unrooted trees don’t show a common ancestor but do show relationships among species.

The phylogenetic tree in part a is rooted and resembles a living tree, with a common ancestor indicated as the base of the trunk. Two branches form from the trunk. The left branch leads to the domain Bacteria. The right branch branches again, giving rise to Archaea and Eukarya. Smaller branches within each domain indicate the groups present in that domain. The phylogenetic tree in part B is unrooted. It does not resemble a living tree; rather, groups of organisms within the Archaea, Eukarya, and Bacteria domains are arranged in a circle. Lines connect the groups within each domain. The groups within Archaea and Eukarya are then connected together. A line from the Archaea/ Eukarya domains, and another from the Bacteria meet in the center of the circle. There is no root, and therefore no indication of which domain arose first.
Both of these phylogenetic trees shows the relationship of the three domains of life—Bacteria, Archaea, and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor while the (b) unrooted tree does not. (credit a: modification of work by Eric Gaba)

In a rooted tree, the branching indicates evolutionary relationships ( [link] ). The point where a split occurs, called a branch point    , represents where a single lineage evolved into a distinct new one. A lineage that evolved early from the root and remains unbranched is called basal taxon    . When two lineages stem from the same branch point, they are called sister taxa    . A branch with more than two lineages is called a polytomy    and serves to illustrate where scientists have not definitively determined all of the relationships. It is important to note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split apart at a specific branch point, but neither taxa gave rise to the other.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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