Learning Goals

By the end of this reading you should 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
Figure 1. The life of a bee is very different from the life of a flower, but the two organisms are related. Both are members of the domain Eukarya and have cells containing many similar organelles, genes, and proteins. (credit: modification of work by John Beetham)

Introduction

The bee and Echinacea flower in Figure 1. could not look more different, yet they are related, as are all living organisms on Earth. By following pathways of similarities and changes—both visible and genetic—scientists seek to map the evolutionary past of how life developed from single-celled organisms to the tremendous collection of creatures that have germinated, crawled, floated, swam, flown, and walked on this planet. In scientific terms, the evolutionary history and relationship of an organism or group of organisms is called a phylogeny. Phylogeny describes the relationships of an organism, which organisms it is thought to have evolved from, which species it is most closely related to and so forth. Phylogenetic relationships provide information on which species it is most closely related to and so forth. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.

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.

Figure_20_01_01.jpg
Figure 2. Both of these phylogenetic trees show 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)
Scientists call such trees rooted (Fig. 2), which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Unrooted trees, on the other hand, don’t show a common ancestor but do show relationships among species. Notice in the rooted phylogenetic tree that the three domains — Bacteria, Archaea, and Eukarya — diverge from a single point and branch off. In a rooted tree, the branching indicates evolutionary relationships.
Figure 3. The root of a phylogenetic tree indicates that an ancestral lineage gave rise to all organisms on the tree. A branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is a basal taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy.

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.

Branches.png
Figure 4. The rotation of the branch point that connects B, D & E does not alter their evolutionary connections
Another point to mention on the phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point was rotated and the taxon order changed, this would not alter the information because the evolution of each taxon from the branch point was independent of the other (Fig. 4).

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

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. 
Figure 5. This ladder-like phylogenetic tree of vertebrates is rooted in 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.
For example, the phylogenetic tree to the right  (Fig. 5) shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs visually appear more similar to us than lizards and rabbits.

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

B) patterns of phenotypic similarities
C) patterns of evolutionary descent
D) whether sister taxa evolved from each other

The Levels of Classification

Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. Think about how a grocery store is organized. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. This organization from larger to smaller, more specific categories is called a hierarchical system.

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.

Figure 6. The taxonomic classification system uses a hierarchical model to organize living organisms into increasingly specific categories. The common dog, Canis lupus familiaris, is a subspecies of Canis lupus, which also includes the wolf and dingo. (credit “dog”: modification of work by Janneke Vreugdenhil)
The kingdom Animalia stems from the Eukarya domain. For the common dog, the classification levels are shown to the right. The full scientific name of an organism technically has eight terms. For the dog, it is Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and Canis lupus. Notice that each name is capitalized except for the second term in the species name (called the specific epithet), and the genus and species names are italicized. Scientists generally refer to an organism only by its species, which is its two-word scientific name, in what is called binomial nomenclature. Therefore, the scientific name (or species) of the dog is Canis lupus. The name at each level is also called a taxon. In the case of dogs Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are considered separate subspecies due to geographic or behavioral isolation or other factors.
Figure 7. At each sublevel in the taxonomic classification system, organisms become more similar. (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”: modification of work by “digital_image_fan”/Flickr)
Figure 7 shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology developed, more precise phylogenies have been determined. 

Review Question:

Recall that phylogenetic trees are hypotheses and are modified as data become available. Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, changes and updates must be made as new discoveries occur. In addition, classification historically has focused on grouping organisms mainly by shared characteristics and does not necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example, despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the closest living relative of the whale.

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

Scientists continually gain new information that helps us understand the evolutionary history of life on Earth. Each group of organisms went through its own evolutionary journey, called its phylogeny. Each organism shares relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the evolutionary pathways of all life on Earth. Historically, organisms were organized into a taxonomic classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary relationships.

End of Section Review Questions:

1)question description
Which of these are sister taxa?
A) Sand Tiger shark and Tiger Shark
B) Lemon Shark and Bull shark
C) Blue Shark and Caribbean reef Shark
D) Bull Shark and Blacknose shark

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”:

definition

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To the extent possible under law, s2jrmoor has waived all copyright and related or neighboring rights to VCU BIOL 152: Introduction to Biological Sciences II, except where otherwise noted.

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