THE TORTOISE AND THE CACTUS TEACHER HANDOUT. Grade Level: High-school biology Objectives: The students will:

Transcription

1 Grade Level: High-school biology Objectives: The students will: THE TORTOISE AND THE CACTUS TEACHER HANDOUT Identify selective pressures that act on organisms Identify adaptations and state how they help organisms survive and reproduce in the face of selective pressures Outline the steps of acquiring adaptations (these are the steps of natural selection) Describe how two species (Galápagos tortoise and Opuntia cactus) can be major selective pressures on each another Make inferences about the historical biogeography of the Galápagos tortoises based on the distribution of Opuntia forms Make predictions about how the growth forms of Opuntia may respond to the extirpation of tortoises on some of the islands Background Information: The students should already be familiar with the following topics: Hypothesis testing Selective pressures Adaptations The steps of acquiring adaptations (natural selection) Time Requirement: (Includes time for a short introduction to the Galápagos and a follow-up discussion) Two 70-minute blocks or three 45-minute class periods Materials: Student handouts (1 per student) Evaluation: Check student handout. Answers, even if technically incorrect, should be defensible. Teacher Handout: The tortoise and the cactus. Page 1

2 THE TORTOISE AND THE CACTUS TEACHER HANDOUT The Galápagos are dry volcanic islands, which arose from the seafloor west of South America sometime between 3 and 6 million years ago. They were never connected to South America, but have plants and animals that originally came from there. One example is the genus of cactus called Opuntia (which includes the prickly-pear cactus). Another example is the giant Galápagos tortoise (Testudo elephantopus). Figure 1.Cactus from South America (Opuntia dillenii) [Illustration from Watson, W. (1889)] Figure 2. Tortoise from South America (Geochelone chilensis) [Illustration from Wayne and Burke. (1997)] Question: How could the turtle have traveled from South America to the Galápagos Islands? Answers could include: Swimming Rafting o Some students may note that a pregnant /gravid female would have been most likely to establish a new population. Teacher Handout: The tortoise and the cactus. Page 2

3 Question: Scientists have proposed two ways that the cactus may have traveled to the Galápagos from South America. What do you think they are (Hint: One way involves an animal)? a. Floating b. Seed dispersal in the gut of an animal (e.g. bird) Some students may suggest that the turtle from the previous question carried it. Question: How could scientists test the hypothesis that the Galápagos tortoises (Testudo elephantopus) and the cacti (Opuntia species) originally came from South America? Answers will vary, but may include: Compare to similar species from South America Check the fossil record Figure 3. Map of South America and the Galápagos Islands [Illustration from Moen] Adaptations of the Opuntia The Galápagos tortoise is one of the main predators of Opuntia. The fruits and pads of the cactus provide both food and water for the tortoise. Although the cactus has populations on all of the islands, the turtles are only found on some of the islands. Question: Name two selective pressures that Opuntia have to deal with on the Galápagos Islands. a. Lack of water (from introductory paragraph) b. The predatory tortoises Teacher Handout: The tortoise and the cactus. Page 3

4 Listed below are some of the characteristics of the various Opuntia species on the Galápagos Islands. Some of the characteristics are found on Opuntia that live on the same islands as tortoises and some of the characteristics are found on Opuntia that do not live with tortoises. Put a checkmark next to the characteristics that you think are adaptations the Opuntia have for living with the turtles. Explain how these adaptations help the cactus survive and reproduce in the presence of turtles. Table 1. Adaptations the Opuntia have to living with turtles (Biggs, 1990). Adaptation Characteristic to turtles Plant grows tall (arborescent form) with the edible fruits and pads on top (See Figure 4) Spines are weak or absent Plant grows low along the ground (decumbent form. See Figure 5) Sharp spines on the lower part of the plant fall off as it grows. They are replaced with a thick, tough bark. How it helps the cactus survive/reproduce in the presence of turtles Protects them from being eaten by the tortoises Protects them from being eaten by the tortoises Young plants are very spiny and grow fast. Protects them from being eaten by the tortoises, even though they are within reach The seeds of the plant have a thick husk that can (and must) pass through an animal s intestines before germinating (sprouting). The fruits fall to the ground once ripened. The seeds of the plant cannot survive passing through an animal s intestines. The seeds are protected from being eaten by tortoises by their thick husks, even after landing on the ground. Teacher Handout: The tortoise and the cactus. Page 4

5 Figure 4. Arborescent form of the Opuntia [Illustration from National Center for Mathematics and Science (2002)] Figure 5. Recumbent form of the Opuntia [Illustration from National Center for Mathematics and Science (2002)] Choose two of the characteristics that you put a checkmark next to in Table 1. Write them in the spaces below labeled A and B. Fill in the Steps to getting adaptation column (from your notes). Fill in the steps for getting adaptation A and B. Table 2. How the Opuntia adapted to living with turtles. Characteristics A. Plant grows tall with pads and fruits Step Steps to getting adaptation B. Plants have seeds with thick husks on top 1 Organisms produce more offspring than Opuntia produce more offspring than Opuntia produce more offspring than 2 Offspring vary, and some of the variation is due to genetic differences. Opuntia vary in height, some of which is due to genetic differences 3 Selective pressures are present. Tortoise predation is a selective pressure 4 Organisms with beneficial heritable Taller Opuntia (with pads and fruits on variations are more likely to survive, top) are more likely to survive and reproduce, and pass the trait on to reproduce, and pass this trait on to offspring. offspring. Opuntia vary in husk thickness, some of which is due to genetic differences Tortoise predation is a selective pressure Opuntia with thicker seed husks are more likely to survive and reproduce (if eaten by a tortoise), and pass this trait on to offspring. Teacher Handout: The tortoise and the cactus. Page 5

6 Question: Assume that all the adaptations Opuntia have, for living with turtles, cost the cactus precious energy. Write a hypothesis that predicts which growth form (decumbent or arborescent) a cactus would have if it lived on islands that have never had turtles. Opuntia that live without tortoises should have a decumbent growth form, because it would save them precious energy. Question: Look at the map (Figure 6) and state whether or not you were correct. Explain how you could tell. Partially. All of the islands with turtles have arborescent forms of Opuntia. However, some of the islands without tortoises also have arborescent forms. It turns out that the turtles were hunted to extinction on four of the islands in the last two hundred years. Question: Which islands probably used to have turtles, but no longer do? How could you tell? Rabida, Pinzon, Santa Fe, and Santa Maria probably used to have turtles. Their Opuntia are the arborescent forms. T Question: Predict how the Opuntia on these islands may change now that they do not have to live with turtles. They may become decumbent because they no longer have the selective pressures to stay tall. T Illustration modified from Biggs (1990). Teacher Handout: The tortoise and the cactus. Page 6

7 Adaptations of the Testudines Although they are all fairly dry, some islands of the Galápagos have more moisture than others. The cooler, wetter islands have many plant species that the turtles can eat. On the drier islands, there is almost no ground vegetation and the turtles have to rely on eating the arborescent Opuntia in order to survive. Question: Name two selective pressures on Opuntia that are stronger on the drier islands than on wetter islands. Less water More predation by tortoises Question: Name two selective pressures on tortoises that are stronger on drier islands than on wetter ones. Less water Fewer plants to eat Arborescent Opuntia It is said that you can tell which island a Galápagos tortoise is from by the shape of its shell. There are two basic shapes: large dome-shaped shells (Figure 7) and smaller saddleback shells (Figure 8). The advantages of being a saddleback The shell of the saddleback tortoise is raised above the neck and flared around the back legs (see Figure 8). Question: How do you think this helps the saddleback tortoise? (Hint: Remember the selective pressures that they have to deal with.) Students should note that saddleback tortoises could better reach the arborescent Opuntia. Teacher Handout: The tortoise and the cactus. Page 7

8 Figure 7. Dome-shaped shell. Figure 8. Saddle-shaped shell Photograph from Humphrey (2003) Photograph from Humphrey (2003) Illustration from National Center for Mathematics and Science (2002) Illustration from National Center for Mathematics and Science (2002) Photo from Doubleday (2004) Photo from Townsend (1925) Teacher Handout: The tortoise and the cactus. Page 8

9 The Advantage of Being a Tall Opuntia Figure 9. Tall and short arborescent Opuntia. There are two heights of arborescent Opuntia: tall and short. The tall forms are more common on the drier islands. Question: Why would the Opuntia on the drier islands need to grow taller than those on the wetter islands? They need to keep their fruits out of reach of the tortoises. Illustration modified from National Center for Mathematics and Science (2002) Table 3. Fill in the following table. Steps to getting adaptation Tortoise: saddleback shell shape Opuntia: tall arborescent form 1 Organisms produce more offspring than Tortoises produce more offspring than Opuntia produce more offspring than 2 Offspring vary, and some of the variation is due to genetic differences. Opuntia vary in height, some of which is due to genetic differences 3 Selective pressures are present. 4 Organisms with beneficial heritable variations are more likely to survive, reproduce, and pass the trait on to offspring. Tortoise offspring vary. Because of genetic differences, some will have a more pronounced saddleback shell shape than others. Reaching the tall arborescent Opuntia is a selective pressure. Tortoises with a more saddleback shell shape will be more able to reach taller Opuntia. They are then more likely to survive, reproduce, and pass this trait onto their offspring. Teacher Handout: The tortoise and the cactus. Tortoise predation is a selective pressure Taller Opuntia (with pads and fruits on top) are more likely to survive and reproduce, and pass this trait on to offspring. Page 9

10 Question: On a separate sheet of paper, answer the following in paragraph form: How did the Opuntia adapt to living with turtles (summarize what you wrote in Table 1)? o Should include adaptations checked in Table 1. How did the amount of moisture affect the food available to the turtles? o Less water results in fewer plants, which is what turtles eat. How and why did the turtles on the dry islands develop saddleback shells (from Table 3)? o Why? to reach the major food source, arborescent Opuntia. o How? students should summarize the steps outlined in Table 3. How and why did the arborescent Opuntia on the dry islands get taller than those on the wet islands (from Table 3)? o Why? to keep their fruits out of reach of the hungry tortoises o How? students should summarize the steps they outlined in Table 2 Teacher Handout: The tortoise and the cactus. Page 10

11 References Biggs, A. L. (January, 1990). Coevolution in the Galápagos: an example for the biology classroom. The American Biology Teacher. Doubleday, E. (March, 2004). Galapagos Hawk on Galápagos Tortoise The Galápagos Islands Y.C.A.S. Photo Gallery Slide and Photo Contest. York County Audobon Society. Retrieved October 30, 2005 from Fritts, T.H. (1984). Evolutionary divergence of giant tortoises in Galápagos. Biological Journal of the Linnean Society, 21, Fritts, T.H Morphometrics of Galapagos tortoises: Evolutionary implications. In: R.I. Bowman, M. Berson, and A.E. Leviton eds. Patterns of Evolution in Galapagos Organisms, pp San Francisco, CA:Pacific Division, AAAS. Humphrey, R. (2003, August). A Sample of Fact Sheets Developed for the San Diego Zoological Society Intranet, Galápagos Tortoise, Geochelone nigra. Colorado.edu. Retrieved October 30, 2005, from Moen, J. Galápagos Islands. Worlatlas.com. Retrieved October 30, 2005, from National Center for Mathematics and Science. (2002). Modeling for Understanding in Science Education: Natural Selection. Wisconsin Center for Education Research, University of Wisconsin-Madison. Retrieved October 30, 2005 from Townsend, C.H. (1925). The Galápagos tortoises in their relation to the whaling industry [Online]. New York Zoological Society, New York Aquarium Nature Series, New York. Reprinted from Zoologica v.4, no.3, Retrieved October 30, 2005 from W. M. White, W.M (2001, January). Galápagos geology on the web. Cornell.edu. Retrieved October 30, 2005, from Watson, W. (1889). Cactus Culture for Amateurs Being Descriptions of the Various Cactuses Grown in this Country, with Full And Practical Instructions for their Successful Cultivation [Online]. L. Upcott Gill, London. Available online from Project Gutenberg.org. Retrieved October 30, 2005, from Wayne, K.F. and Burke, R.L. (eds.). (1997). Crocodilian, Tuatara, and Turtle Species of the World: An Online Taxonomic and Geographic Reference [Online].Association of Systematics Collections, Washington, D.C. Retrieved October 30, 2005 from Teacher Handout: The tortoise and the cactus. Page 11