|Target age or ability group:||High school biology, regular, honors, and advanced placement level.|
|Class time required:||Two 50-minute class periods. Day 1: Students work on counting amino acid differences between each pair of organisms and complete table #1, compare gross anatomy of selected primates from reference materials supplied by the teacher, and then begin the discussion questions and identification of organisms on the graph. Students then take home the activity to complete the discussion questions and the graph. On Day 2 the students meet in teams to discuss/debate their responses and the labeling of the graph, and then participate in a class discussion guided by the teacher.|
|Materials and equipment:||Student handout for each student and an overhead of each of the pages of the activity for the teacher to use during review. It is also recommended that the teacher provide references comparing the anatomy of the primates mentioned in this activity (see the student handout, question #3 for the list), a geologic time chart, and any phylogenetic tree that will serve as a good reminder to the students that they are discovering relationships that emphasize ancestry.|
|Summary of activity:||Working in groups of two, students begin by determining the number of amino acid differences in beta hemoglobin that occur between each of the pairs of organisms featured in the activity. The students do not know the names of the organisms studied until after they complete the amino acid comparisons and have begun the discussion. Students compare each possible pairing of organisms, looking for differences in the sequences, and discovering patterns in the degree of difference. The current view is that the number of differences for a particular molecule is an indicator of how long ago in the past any two organisms may have shared a common ancestor. By example, the relatively few differences between the beta hemoglobin of humans and gorillas is an indicator of a relatively recent common ancestor, whereas, a comparison of chimpanzee and lemur beta hemoglobin indicates a far more distant common ancestor. More striking, but less intuitive, is the observation that chimps, humans, and even monkeys are all approximately equidistant from lemurs.
This is because the common ancestor of all anthropoidea diverged from the common ancestor of lemurs at the same time, a very long time ago. Students use the data in table 1 to infer relative points of divergence since a common ancestor and, upon comparing gross anatomy to amino acid sequences for each of the 7 species, infer where each of the seven species belongs on the phylogenetic tree.
|Prior knowledge, concepts or vocabulary necessary to complete activity:||Students must have a good working knowledge of the structure and function of DNA. They must also be aware of the growing body of evidence that supports the scientific conclusion that life has a long history and has diversified widely through time. Students should have some exposure to phylogenetic trees and sequences in the fossil record.|
|Teacher instructions:||Begin by showing several overheads of phylogenetic trees (Strickberger, Evolution is an excellent source). Students will be most interested in phylogenetic trees for dinosaurs and other prehistoric organisms that are now extinct. Ask students to offer an explanation as to how scientists establish such evolutionary relationships. Most of their responses will likely focus on anatomical comparisons and fossil sequences. The teacher should then point out that living organisms also share common ancestry and that these ancestral relationships can be understood, in addition, by using molecular comparisons. The teacher might then put up an overhead (non-phylogenetic, merely comparative anatomy) which includes at least several of the primates mentioned in this activity and ask students why we group these organisms together. Most students will respond that these creatures share certain physical characteristics, especially a set of characteristics that they share with no other organisms classified outside of the order primates. Then ask the students why they share characteristics common to all vertebrates (ancestral characters - distant common ancestors)and characteristics common only to the order primates (derived characters - recent common ancestors). Here the teacher could talk about general patterns in the fossil record. The teacher should then pass out the 2 page student lab sheet along with a book or photocopies of the primates that are used in the activity. Working in 2 groups of 2, each lab station team will proceed through the activity. Each pair should work independently of the other pair, except when they hit a significant problem. The teacher should remind students that what they do not finish in class will need to be done by the student at home. On day 2 the teacher lets the students meet in their 4 person lab teams to discuss their answers. There will no doubt be loud debate and discussion. Allow 20 minutes. Students then present their findings in a discussion facilitated by the teacher (allow 30 minutes). The teacher should have overhead copies of both pages of the activity so that as answers are developed, all students can see the data, graph, and discussion questions/responses.|
Acknowledgment: I am grateful to Dr. Martin Nickels, Illinois State University, Normal, Illinois for his collaboration in the development of this module.
In Graph #1: the "letter" order is G,C,A,B,D,E,F, and the ≥name of organism≤ sequence is lemur, gorilla, human, chimpanzee, gibbon, rhesus monkey, squirrel monkey, and horse (for H). To scale the lower x axis as required in discussion question #9 merely draw a dotted line down from the first vertical split on the graph (at about 29 amino acid dissimilarities) to the lower axis and label that "43"(43 million years ago). Then mark the spot on the lower axis directly below the "0 dissimilarities" from the upper axis as "6" (6 million years ago).
43 - 6 = 37 million years from left to right boundary so each 10 units of amino acid dissimilarity is approximately equivalent to 12 million years. Merely subdivide so that 1 amino acid dissimilarity is equal to about 1.2 million years of elapsed time. Obviously your scale can be extended to the left beyond the 43 million year mark.
For the Discussion:
Studying the nucleotide sequences of DNA, and/or the amino acid sequences of proteins, gives scientists one of a growing number of ways to analyze relationships and infer ancestry for life on earth. The work of molecular biologists has been important in further clarifying and refining our understanding of not only family trees (phylogenies) for all life on earth, but also the possible rate of mutation, selection, and speciation.
Among the first proteins to yield its amino acid sequence was hemoglobin, and it remains today one of the most investigated of all proteins. The basic unit of hemoglobin consists of an iron-containing porphyrin (heme) that can reversibly bind oxygen attached to a globin polypeptide chain that is usually no less than 140 amino acids long. In vertebrates, hemoglobins are usually the primary protein of red blood cells, making them relatively easy to isolate and purify in large quantities. (Strickberger, M. W. Evolution. Jones and Barlett Publishers, 1990)
To examine the amino acid sequence differences for a specific protein (beta hemoglobin) in several primate species, and from this infer ancestral relationships.
Procedure Part A: Listed below is a comparison of the 146 amino acid beta chain of the hemoglobin molecule in 7 selected primate species. Notice that the amino acid position numbers may not be continuous. This is because those that are the same for ALL seven species have been left off the chart to save space. Count the number of amino acid differences between each of the possible pairs of organisms using the data below and then enter it in Table #1. Draw a diagonal line from the upper left to lower right of the grid and then cross out all of the squares in the upper right side of the table. No need to duplicate your work. You will enter in row H later.
Procedure Part B
The data that you have gathered and entered in Table #1 represents the amount of time (in amino acid substitutions) since one organism has diverged from a common ancestor with another organism. A fewer number of amino acid differences between any two creatures implies that the two organisms share a relatively recent common ancestor, whereas a large number of amino acid differences implies the two organisms share a relatively distant common ancestor. Scientists can take this information to construct a phylogenetic tree that shows the branching patterns of descent. Using the data from Table #1, enter the appropriate letter (A, B, C, D, E, F, or G) in each of the appropriate spaces to the right side of the phylogenetic tree in Graph #1 under "letter." Notice the horizontal time axis on the top of the graph. Then answer the discussion questions below.
Graph #1: Evolutionary tree for seven primate species based on a comparison of amino acid dissimilarities of beta hemoglobin
(Answer the essay questions on a separate sheet of paper and attach to this sheet).