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Classification of organisms is an important process for field biologists. Classification includes the investigation of genetic relationships among things, such as plants and animals, and the placement of these organisms into an organized system.
Charles Darwin spent most of his life doing research that involved the classification of plants and animals. Even as a young man he collected and classified all kinds of insects, especially beetles. During his 1835 exploration of the Galápagos, Darwin noted the variety of life forms that existed throughout the archipelago. In his description of the land birds in his journals, he attempts to classify the now famous Galápagos finches:
Of land-birds I obtained 26 kinds, all peculiar to the group [of islands] and found nowhere else, with the exception of one lark-like finch from North America (Dolichonyx oryzivorus), which ranges on that continent as far north as 54°, and generally frequents marshes. … The remaining land-birds form a most singular group of finches, related to each other in the structure of their beaks, short tails, form of body and plumage…
In this activity, students will classify many of the reptiles Darwin saw when he visited the Galápagos. Before they begin, introduce the concepts of classification, taxonomy, and dichotomous keys. Taxonomy is the branch of study that deals with the classification of organisms. One of the most helpful tools of the taxonomist is the dichotomous key. Keys are prepared by scientists who are experts on a particular group, to make it easier for others to identify members of that group.
People can use dichotomous keys to classify almost anythinganimals, plants, minerals, bacteria, etc.into specific categories. The structure of a dichotomous key is usually built upon dividing a group of objects into two categories based on a particular characteristic. Each group is subdivided by different criteria, until each group contains only a single entity.
To divide a group of organisms into two categories, students can begin with a questionfor example: Does the animal have legs? Animals that do have legs would then be classified in one group, and animals with no legs would be classified into a second group. You may wish to introduce this concept by using a dichotomous key to classify the students in your class. This classification tool is called a dichotomous key because the group is divided into two groups each time.
Discuss the process of classifying organisms. Introduce the concepts of taxonomy and the dichotomous key, explained above.
Divide your students into teams of three or four and have them visit the Online Resource
Classification Gallery. Each group should then carefully observe the images and descriptions of the Galápagos animals and take notes describing their different characteristics using the Student Printout Observation Chart.
Ask each team to make a dichotomous key to classify the animals found in the Classification Gallery. This key should consist of a series of questions. Each question should be used to divide the animals into two categories, until each category consists of only one animal. Students might begin by dividing the group into animals with legs and animals without legs. Then those with legs could be divided into animals with shells and those without, and so on. Here is one example of a dichotomous key for the Galápagos animals in the Classification Gallery:
Does the animal have legs? If yes, go to question #2. If no, then the animal is a Galápagos snake.
Does the animal have a shell? If yes go to question #3, If no go to question #6.
Does the animal have flipper-like legs? If yes, the animal is a green sea turtle. If no, go to question #4.
Does the animals shell have a dome-like shape? If yes, the animal is a dome-shaped giant tortoise. If no, go to question #5.
Does the animals shell have a neck opening that is highly arched and looks like a saddle? If yes, then the animal is a saddleback giant tortoise. If no, go to question #6.
Is the adult longer than 50 cm (.5 meters) from nose to tail tip and does it have distinct spines on its back? If yes go to question #6. If it is less than 50 cm (usually 15-25 cm) and has no spines or very small spines on its back, it is a lava lizard.
Is the animal mostly black, does it have a flat tail and a flat snout (nose), and does it swim in the sea? If yes, it is a marine iguana. If no, it is a Galápagos land iguana.
Go to number 2
Go to number 3
Go to number 5
Legs not flipper-like
Go to number 4
Green sea turtle
Shell shaped like a dome, front opening not highly arched
Dome-shaped giant tortoise
Shell saddle-shaped, front opening highly arched
Saddleback giant tortoise
Adult animal longer than 50 cm (0.5 meters) from nose to tip of tail; distinct spines on back
Go to number 6
Adult animal less than 50 cm (usually 15-25 cm) from nose to tip of tail; no or very small spines on back
Body color mostly black, tail somewhat flattened, snout (nose) flat, behavior includes swimming in ocean
Body color mostly tan, yellow on top, darker below; tail not compressed; inhabits land
Of course there can be any number of correct keys. The most important part of this activity is learning how to observe the many variations of organisms that have evolved throughout the archipelago.
Once each team has designed a dichotomous key to classify the animals, have them switch keys (leaving the answers blank) and try to classify the animals using another groups key.
What did you notice when using another groups key to classify the animals? Did the other group use different characteristics to create the dichotomous key?
Why is classification an important process for field biologists?
Discuss the importance of observing every detail of an animal when creating a dichotomous key.
Choose another group of animals found in your local habitat and create a dichotomous key for classifying them.
This activity uses the following principles of the National Science Education Standards for grades 5-8:
Use Appropriate Tools and Techniques to Gather, Analyze, and Interpret Data
The use of tools and techniques, including mathematics, will be guided by the questions asked and the investigations students design.
The characteristics of an organism can be described in terms of a combination of traits. Some traits are inherited and others result from interactions with the environment.
Diversity and Adaptations of Organisms
Millions of species of animals, plants, and microorganisms are alive today. Although different species might look dissimilar, the unity among organisms becomes apparent from an analysis of internal structures, the similarity of their chemical processes, and the evidence of common ancestry.
Biological evolution accounts for the diversity of species developed through gradual processes over many generations. Species acquire many of their unique characteristics through biological adaptation, which involves the selection of naturally occurring variations in populations. Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.
This activity is an online modification of a chapter from NSTAs publication Ecology and Evolution: Islands of Change. Visit the Ecology and Evolution Web page to learn more about the range of Galápagos-oriented activities that can be found in the full-length book.