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Destination: Galapagos Islands Cyber Field Trip
FOR TEACHERS

Cyber Field Trip Teaching Guide
DNA Adventure: The Web of Life


Student activity available at
../galapagos/g44_dna.html

This interactive activity will be available for students beginning Thursday, December 10, 1998.

Introduction
National Science Education Standards
Objectives
Concepts and Terms
Materials
Procedure
Critical Thinking Questions
Related Activity




INTRODUCTION

On December 27, 1831, Charles Darwin set sail aboard the H.M.S. Beagle. He spent the next five years aboard the ship observing and collecting organisms from around the world, including the Galapagos Islands. The voyage of the Beagle was one of the most important events in science. It was on this voyage that Darwin laid the basis for his theories on natural selection. Darwin observed that species were constantly changing, and that they could evolve over time to produce new species.



NATIONAL SCIENCE EDUCATION STANDARDS

SCIENCE AS INQUIRY
5-8
9-12:
Ability to Make a Scientific Inquiry
LIFE SCIENCE
5-8: Structure and Function in Living Systems, Reproduction and Heredity, Diversity and Adaptation of Organisms
9-12: Molecular Basis of Heredity, Biological Evolution




OBJECTIVES

  1. To define the terms of natural selection, adaptation, adaptive radiation and species diversity.

  2. To identify the environmental and biological conditions necessary for adaptive radiation to occur.

  3. To use and understand a model of DNA hybridization in order to analyze the degree of relatedness of one species to another.

  4. To investigate homologies and analogies that exist in different species.


CONCEPTS AND TERMS

Earth's diversity ranges from the smallest microorganism to the blue whale, the largest animal of all. This voluminous diversity on Earth can be described as species diversity, or the variety of different organisms in a given area. Coral reefs and tropical forests are the ecosystems that have the highest species diversity on Earth.

Similarities in the structure of organisms, which indicate species relatedness, or related ancestry, are very important to the study of taxonomy. Many organisms have structures that are formed in similar ways during embryonic development and are said to be homologous organs, or homologies. The flipper of a whale, the arm of a human and the wing of a bat are all homologous organs. That is, the flipper, the arm and the wing develop in similar ways and have a similar structure.

In addition to anatomic structural similarities, scientists are increasingly using biochemical homologies to determine and understand species relatedness. Biochemical homologies include similarities in blood, proteins, DNA and RNA. The greater the similarities in DNA sequences, the more closely related two organisms are thought to be.

An adaptation is a hereditary characteristic that provides a selective advantage for survival and reproduction. For example, a bird with a beak that is long and slender may be able to obtain nectar from flowers more easily than birds with shorter or broader beaks. The long-beaked bird would then have an advantage over others in an area where nectar is one of the only food sources. The bird would thrive and reproduce, passing on its genetic traits to subsequent generations; the long, slender beak would be an adaptation for survival in habitats where nectar is plentiful. Camouflage is a protective adaptation that allows animals to blend into their surroundings. For example, lions on the Serengeti Plains blend in with the grasses to prevent them from being spotted by their prey.

Natural selection is the tendency of members of a population with successful adaptations to their environment to survive and reproduce.

Natural selection doesn't necessarily result in speciation, the formation of new species. For adaptation and speciation to occur, several factors must be present: the organism must have physical access to a new environment, the species must be capable of exploiting new niches or ecological roles, the species must possess a level of physical and physiological tolerance to enable it to gain a foothold in the new environment, an ecological niche must be available and competition must be absent or minimal. Darwin discovered that all of these conditions had been met on the Galapagos Islands, where geographic isolation acted as a physical barrier to separate populations.

Darwin identified 13 distinct species of finch on the Galapagos Islands that were different from those found on mainland South America. The finches on the islands all evolved from a common ancestor and were similar, but each had distinctive characteristics, or adaptations. An evolutionary pattern in which related species become dissimilar, or less alike, is called adaptive radiation. Darwin's finches are, perhaps, the most famous example of adaptive radiation.

In convergent evolution, unrelated organisms become more similar in order to survive in similar environmental conditions. For example, the fur seal and the King penguin are very different in that one is a mammal and the other is a bird. However, because they exist in similar environments, they both have streamlined bodies for swimming, webbed feet/forelimbs and a thick layer of insulating fat to protect against cold.




MATERIALS

  • 8 1/2 x 11 paper cut into five long strips.
  • 50 black jelly beans
  • 50 red jelly beans
  • 50 white jelly beans
  • 50 green jelly beans
  • needle and thread
  • paper and pencil (to create chart)


PROCEDURE

Students will "synthesize" and "hybridize" five strands of DNA, which represent the genetic codes for hemoglobin, the oxygen-carrying pigment in blood, for Human, Hominoid (common ancestor), Gorilla and Chimpanzee.

Use the following color codes for the bases that make up DNA:

adenine (A) = black
cytostine (C) = red
thymine (T) = white
guanine (G) = green
  1. Label the five strips of paper as Human DNA, Human cDNA, Hominoid DNA (common ancestor), Gorilla cDNA and Chimpanzee cDNA.

  2. Using the needle and thread, string the following sequences of jelly beans (each on a different thread):

Hominoid (common ancestor) DNA
A-G-G-C-C-G-G-C-T-C-C-A-A-C-C-A-G-G-C-C

Human DNA
A-G-G-C-A-T-A-A-A-C-C-A-A-C-C-G-A-T-T-A

Human cDNA
T-C-C-G-T-A-T-T-T-G-G-T-T-G-G-C-T-A-A-T

Gorilla cDNA
T-C-C-G-G-G-G-A-A-G-G-T-T-G-G-T-C-C-G-G

Chimpanzee cDNA
T-C-C-G-G-G-G-A-A-G-G-T-T-G-G-C-T-A-A-T

Note: cDNA, or complementary DNA, is a single strand of DNA produced by an individual member of a species -- in this case, a human, a gorilla and a chimpanzee.
  1. Place each "DNA strand" on its labled piece of paper.

  2. Keeping in mind that complementary nitrogen bases always pair in DNA as A-T (adenine to thymine) and G-C (guanine to cytosine), match the human DNA with the chimpanzee cDNA, base by base (complementary bases should touch, while those that do not match should form "loops," as shown in the graphic below). By doing this, you are creating a representation of a hybridization of human and chimpanzee DNA.

    illustration of loops


  3. Make a copy of the chart below. Count the number of loops, which represent bases that do not match, and record your findings in the chart.

Human DNA Hybridized to: Chimp cDNA Gorilla cDNA
Number of Loops    
Number of Base Differences    
  1. Now match all three cDNA strands (human, chimp, gorilla) with the hominoid or common ancestor strand by repeating steps 4 and 5, using the chart below.

  2. Make a copy of the chart below. Record your results.
Hominoid DNA Hybridized to: Human cDNA Chimp cDNA Gorilla cDNA
Number of Loops      
Number of Base Differences      




CRITICAL THINKING QUESTIONS

  • Based on your findings, which hemoglobin gene do you believe is most closely related to humans -- that of gorillas or chimpanzees?

  • Based on your hemoglobin comparison, which species do you conclude is the most closely related to humans?

  • Which cDNA is most similar to the common ancestor hominoid DNA?

  • What other methods of determining species relatedness might scientists use?

  • Scientists use the rate of mutations in DNA, which occur at regular rates, to predict when two organisms branched off from a common ancestor. Most scientists agree that the great apes -- chimpanzees, gorillas and orangutans -- share a common ancestor with humans, but there is fierce disagreement as to when the branching-off occurred. Some scientists believe that the split was simultaneous, while others believe that the first to branch away were the orangutans, then gorillas, then chimps and, finally, humans. In the branching debate, which is more plausible according to your findings?

  • Explain the difference between convergent and divergent evolution. What forces drive convergent evolution? Divergent evolution?

  • In terms of natural selection and adaptation, why are the Galapagos Islands unique?

  • Based on your findings, explain Darwin's discovery of 13 distinct finch species derived from one ancestor. What factors were involved in their speciation?


RELATED ACTIVITY

To learn more about Darwin's finches, visit Identifying Organisms with a Taxonomic Key in the More Galapagos Classroom Activities section.




 

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