10
Learning Goals
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Discuss the need for a comprehensive classification system
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List the different levels of the taxonomic classification system
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Describe how systematics and taxonomy relate to phylogeny
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Discuss the components and purpose of a phylogenetic tree
Introduction
Phylogenetic Trees
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. 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.
The point where a split occurs called a branch point represents where a single lineage evolved into a distinctly new one (Fig. 3). A lineage that evolved early from the root and remains unbranched is called a 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.
Diagrams can serve to show a pathway and aid our understanding of evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the “trunk” of the tree, one can discover that species’ ancestors, as well as where lineages share a common ancestry. In addition, the tree can be used to study entire groups of organisms.
Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Information is used to organize and classify organisms based on evolutionary relationships in a scientific field called systematics. Data may be collected from fossils, from studying the structure of body parts or molecules used by an organism, and by DNA analysis. By combining data from many sources, scientists can put together the phylogeny of an organism; since phylogenetic trees are hypotheses, they will continue to change as new types of life are discovered and new information is learned.
Review Question:
Match the term with the appropriate definition.
| 1) polytomy | A) groups of organisms that are more closely related to each other than to any other groups |
| 2) rooted | B) a diagram in which all of the included organisms are thought to have arisen from a common ancestor |
| 3) basal taxon | C) branch on a phylogenetic tree that has not diverged significantly from the root ancestor |
| 4) branch point | D) a point in the phylogenetic tree that indicates the last common ancestor of different groups |
| 5) sister taxa | E) multiple lineages that arise from a common branch point |
Limitations of Phylogenetic Trees
Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for the 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. Figure 5 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. This particular 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 this tree, 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.
Review Question:
What are somethings that you cannot determine from looking at a phylogenetic tree
A) the actual evolutionary ages of the species in the tree
The Levels of Classification
The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species.
Review Question:
Link to Learning: Visit this Nova website (Classifying Life) to try your hand at classifying three organisms—bear, orchid, and sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the bear or the Launch Interactive button.
Summary
End of Section Review Questions:
Attribution:
Text: Modified from OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. License: CC BY: Attribution
Figure 1. credit: modification of work by John Beetham
Figure 7. (credit “plant”: modification of work by “berduchwal”/Flickr; credit “insect”: modification of work by Jon Sullivan; credit “fish”: modification of work by Christian Mehlführer; credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck; credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog”:
C
A, B, D
The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship.
asexual; binary fission
natural polymers of high molecular weight secreted by microorganisms into their environment
B, C