Chapter 6

ACTIVITY 4

Investigating Common Descent:
Formulating Explanations and Models

Standards-Based Outcomes

This activity provides opportunities for all students to develop abilities of scientific inquiry as described in the National Science Education Standards. Specifically, it enables them to:

• formulate descriptions, explanations, predictions, and models using evidence,
• think critically and logically to make relationships between evidence and explanations, and
• recognize and analyze alternative explanations and predictions.

In addition, the activity provides all students opportunities to develop fundamental understandings in the life sciences as described in the National Science Education Standards. Specifically, it conveys the following concepts:

• In all organisms, the instructions for specifying the characteristics of the organism are carried in DNA, a large polymer formed from subunits of four kinds (A, G, C, and T). The chemical and structural properties of DNA explain how the genetic information that underlies heredity is both encoded in genes (as a string of molecular "letters") and replicated (by a templating mechanism).
• The millions of different species of plants, animals, and microorganisms that live on earth today are related by descent from common ancestors.
• Biological classifications are based on how organisms are related. Organisms are classified into a hierarchy of groups and subgroups based on similarities that reflect their evolutionary relationships. The species is the most fundamental unit of classification.

One of the most common misconceptions about evolution is seen in the statement that "humans came from apes." This statement assumes that organisms evolve through a step-by-step progression from "lower" forms to "higher" forms of life and the direct transformation of one living species into another. Evolution, however, is not a progressive ladder. Furthermore, modern species are derived from, but are not the same as, organisms that lived in the past.

This activity has extensive historical roots. Few question the idea that Charles Darwin's Origin of Species in 1859 produced a scientific revolution. In essence, Darwin proposed a constellation of ideas that included: organisms of different kinds descended from a common ancestor (common descent); species multiply over time (speciation); evolution occurs through gradual changes in a population (gradualism); and competition among species for limited resources leads to differential survival and reproduction (natural selection). This activity centers on the theory of common descent.

The theory of common descent was revolutionary because it introduced the concept of gradual evolution based on natural mechanisms. The theory of common descent also replaced a model of straight-line evolution with that of a branching model based on a single origin of life and subsequent series of changes—branching—into different species.

Based on his observations during the voyage of the H.M.S. Beagle, Darwin concluded that three species of mockingbirds on the Galapagos Islands must have some connection to the single species of mockingbird on the South American mainland. Here is the intellectual connection between observations and explanation. A species could produce multiple descendent species. Once this idea was realized, it was but a series of logical steps to the inferences that all birds, all vertebrates, and so on, had common ancestors.

Common descent has become a conceptual backbone for evolutionary biology. In large measure, this is so because common descent has significant explanatory power. Immediately, the idea found supporting evidence in comparative anatomy, comparative embryology, systematics, and biogeography. Recently, molecular biology has provided further support, as the students will discover in this activity. See Chapter 3 of this document and page 185 of the National Science Education Standards for more discussion of this topic.

This activity also introduces students to scientific evidence, models, and explanations as described in the accompanying excerpt drawn from the National Science Education Standards.

Evidence, Models, and Explanations 2 Evidence consists of observations and data on which to base scientific explanations. Using evidence to understand interactions allows individuals to predict changes in natural and designed systems. Models are tentative schemes or structures that correspond to real objects, events, or classes of events, and that have explanatory power. Models help scientists and engineers understand how things work. Models take many forms, including physical objects, plans, mental constructs, mathematical equations, and computer simulations. Scientific explanations incorporate existing scientific knowledge and new evidence from observations, experiments, or models into internally consistent, logical statements. Different terms, such as "hypothesis," "model," "law," "principle," "theory," and "paradigm," are used to describe various types of scientific explanations. As students develop and as they understand more science concepts and processes, their explanations should become more sophisticated. That is, their scientific explanations should more frequently include a rich scientific knowledge base, evidence of logic, higher levels of analysis, greater tolerance of criticism and uncertainty, and a clearer demonstration of the relationship between logic, evidence, and current knowledge.

Materials and Equipment

For each student:

• Notebook
• Pencil

• 4 sets of black, white, green, and red paper clips, each set with 35 paper clips

• Overhead transparencies of Characteristics of Apes and Humans, Table 1, and Morphological Tree, Figure 1
• Overhead projector

Engage — Ask the students: When you hear the word "evolution," what do you think of first? Have the students explain what they understand about evolution. For many people, the first thing that comes to mind is often the statement "Humans evolved from apes." Did humans evolve from modern apes, or do modern apes and humans have a common ancestor? Do you understand the difference between these two questions? This activity will give you the opportunity to observe differences and similarities in the characteristics of humans and apes. The apes discussed in this activity are the chimpanzee and the gorilla.

Explore — Review Table 1, Characteristics of Apes and Humans, with the class. Make sure the students know that gibbons, chimpanzees, gorillas, and orangutans are four groups included in the ape family. Chimpanzees and gorillas represent the African side of the family; gibbons and orangutans represent the Asian side of the family. We focus only on the chimpanzee and gorilla in this activity. The only modern representative of the human family is Homo sapiens, although paleontologists have found fossil remains of other members, such as Australopithecus afarensis ("Lucy") and Homo sapiens neandertalensis.

Next discuss how the students can use the data to determine the relationships between humans, apes, and other animals. It might not be obvious that closely related organisms share more similarities than do distantly related organisms. Guide the students to the idea that structures might be similar because they carry out the same functions or because they were inherited from a common ancestor. Only those similarities that arise from a common ancestor can be used to determine evolutionary relationships.

Evolutionary relationships among organisms derived from comparisons of skeletons and other characteristics.

Use the transparency of the Morphological Tree, Figure 1, for this discussion. Diagrams called branching trees illustrate relationships among organisms. One type of branching tree, called a morphological tree, is based on comparisons of skulls, jaws, skeletons, and other structures. Look carefully at the morphological tree.

Explain — Ask the students to find the part of the morphological tree that shows the relationships between gorillas, chimpanzees, and humans. They will notice that there are no lines showing relationships. They should work with partners and develop three hypotheses to explain how these organisms are related. On a sheet of notebook paper, they should make a diagram of their hypotheses by drawing lines from Point A to each of the three organisms (G = gorilla, C = chimpanzee, H = human, A = common ancestor).

Allow the students to develop their own hypotheses. Give them help only if you see they are not making any progress. Three hypotheses the students might propose are shown below (although not necessarily in the same order).

Possible evolutionary relationships:

Instructional Strategy: Part II

Elaborate — Modern research techniques allow biologists to compare the DNA that codes for certain proteins and to make predictions about the relatedness of the organisms from which they took the DNA. Students will use models of these techniques to test their hypotheses and determine which one is best supported by the data they develop.

Procedure Step 1. Working in groups of four, "synthesize" strands of DNA according to the following specifications. Each different color of paper clip represents one of the four bases of DNA:

black = adenine (A) green = guanine (G)

white = thymine (T) red = cytosine (C)

Students should synthesize DNA strands by connecting paper clips in the proper sequence according to specifications listed for each group member. When they have completed the synthesis, attach a label to Position 1 and lay your strands on the table with Position 1 on the left.

Each student will synthesize one strand of DNA. Thirty-five paper clips of each color should provide an ample assortment. To save time, make sure all strands are synthesized simultaneously. Emphasize to the students that they are using models to test the hypotheses they developed in the first part of the investigation. Following are directions for the respective groups: