hePhylogenetic Trees: Diagraming the Evolutionary History of a Group of Organisms
Adapted from lab by Dr. Rebecca Irwin and “Constructing Cladogram” from Biology: The Dynamic Science, 2nd edition by Russell, Hertz, and McMillan.

**It would greatly benefit you to read the “Constructing Cladogram” example in your text on pg. 514 -515 Goals: 1. Understand how to interpret a phylogeny 2. Understand how to construct a phylogenetic tree using cladistics Background Information: A phylogeny represents the evolutionary history of a group of organisms. Phylogenies are illustrated as phylogenetic trees. Phylogenetic trees are hypotheses that identify likely relationships among species or higher taxonomic groups (Kingdom, Phylum, Class, Order, Family, Genus). Like all hypotheses, they are constantly revised by scientist as they gather new data. Look at the phylogeny of vertebrates provided for you on your lab bench (and also on blackboard in a separate file). Notice that some of the lines are dashed representing a part of the phylogeny where there is not enough information to get a good idea about the evolutionary history of that particular group and how they fit into the overall phylogeny. Phylogenetic trees are often drawn along a time line and can be depicted either vertically or horizontally. If the phylogeny is vertical (like the example provided), the most ancient organisms and evolutionary events are at the bottom of the tree. Some phylogenies have explicit time scales meaning that length of each branch represents how long a group persisted before it diversified. However, many phylogentic trees used in textbooks (and those we will build today) do not have a precise time scale, but rather the sequence of nodes only indicates the sequence in which groups appeared. In other words, groups closer to the root of the tree are more ancient and those near the top of the phylogeny are more recent. As you are looking at the example, notice that not all of the lines reach our current time period. Those lines represent extinct groups of organisms. The common ancestor of all species included in the tree is described as the root of the tree. Branching points (where speciation occurred) on a phylogenetic tree are called nodes and all the branches emerging from that node are collectively called a clade. Most nodes have two branches protruding from them reflecting the evolution of two species from a common ancestor, however, for some groups, we have not yet discovered the pattern of branching that produced the diversity we see today and therefore you may see three or more branches coming from the same node. Scientists call these nodes unresolved and hopefully further research will help us portray the evolution of that group more precisely. When reading a phylogeny, it is important to note any clade can be rotated around a node without changing the meaning of the phylogenetic tree. For example, on the sample phylogeny, we could rotate the node from which Sphenodontia (Tuataras) and Squamata (Lizards and Snakes) evolved and not change the meaning of the phylogeny. In other words, the order that species (or groups) appear at the top of the phylogeny is not as important as the branching pattern leading to those groups. Phylogenies are built based on homologies (similarities that are the result of common ancestry). These homologies can be: Morphological (based on form, such as the bone structure found in the front appendage of eagles and bats)

Developmental (based on the patterns of development of the embryo; specifically, homologous characters from the same embryonic tissues in similar ways during development)

Genetic (use the sequence of nucleotides in particular genes to discern phylogenetic relationships) Cladistics Cladistics is one approach to building phylogenies and the one we will be using today in lab. A phylogenetic tree produced using this method is called a cladogram. Cladograms are produced by comparing different characters that groups may have. Each character can exist in two or more forms and these forms are described as character states. Evolutionary processes change characters over time from the original state, called the ancestral character state, to a newer, derived character state. In order to determine if a character is ancestral or derived, we use a technique called outgroup comparison. The outgroup is a closely related species that is not a member of the clade. Character states observed in the outgroup are considered ancestral, and those only observed in the clade (ingroup) are considered derived. Building a Cladogram: In this example we will be using diagrams of toucans as the ingroup and a diagram of a barbet as the outgroup (provided on your lab table and on blackboard). We will be comparing the different color patterns observed on different parts of the body to create a chart in which ancestral character states are denoted with a “-“ and derived character states are denoted with a “+.” As you choose characters to use in your table, make sure they are found in one or more of the toucan species, but NOT in the barbet (i.e. that it is a derived character state). Species Long colored stripe on tail Horizontal Stripe on Bill Feet blue or not Yellow Eye Color Color other than yellow under the wing tip + + +

Barbet (outgroup) Toucan 1 Toucan 2 Toucan 3 Toucan 4

+ + +

+ + +

+ + + -

+

Constructing the cladogram from the information in the table: Since the barbet is the outgroup and have only ancestral characters, we place them on their own branch to the left and all groups that have derived characters on the branch to the right.

Toucan 2, has only one derived character (based on the ones we choose for this table), so we will place them as the first branch on the left.

Toucans 1, 3, and 4 each have four derived characters. Toucans 1 and 3 have four derived characters in common, whereas toucan 4 has only three derived characters in common with them. Considering this, we might hypothesize that toucans 1 and 3 are more closely related to each (have a more recent common ancestor) than they are to toucan 2. This can be depicted in the cladogram below:

Remember that we can rotate the branches at any node. With that in mind, the cladogram below shows the same evolutionary relationships as the one above (i.e. both are correct and showing the same thing).

Today in lab you will be given a barbet (outgroup) and four toucans. You will need to come up with 5 character states (for today’s exercise, we will be using colors on different parts of the birds only) that you will use to create a cladogram (hypothesis) representing the evolutionary history of the group. Each character you choose should occur in one or more of the toucans, but NOT in the barbet. Choose at least 3 characters that occur in two or more toucans, but NOT the barbet to help you determine who might be more related. Remember that for the characters you choose, the outgroup represents the ancestral character (-) and any deviation from that trait is considered a derived character (+). Once you have built your first phylogeny, switch toucans with your neighbors at your table and build a second phylogeny for this new group of birds.