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Lesson 2.8 Teachers' Guide: How Do Populations Change Over Time? |
Grade Level |
Prospective and Practicing K-8 Teachers; may be adapted for use in K-12 classes | ||||||||
Time
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Exercise 1 will take approximately 2 1/2 hours. | ||||||||
To Ponder
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1. | Why do we see so much diversity in individual members of animal and plant populations? Genetic variation-- hereditary differences between individuals -- provides the diversity of appearances, capabilities, and behaviors from which the environment selects the parents for the next generation. The more genetic diversity that exists in a species, the better that species will be able to adapt to changes in its environment. | |||||||
| 2. | What does it mean for a species to become extinct? When organisms can no longer survive or reproduce normally, more individuals die than are replaced by reproduction. When the last breeding pair of a species in the entire world disappears, the species becomes extinct. Even mild extinctions of a few species have far-reaching biological repercussions, because an entire ecosystem is affected. | ||||||||
| 3. | How do new species arise, according to evolutionary theory? New species arise when physical or genetic barriers divide one population into separate populations that do not interbreed in nature and that follow different evolutionary pathways. | ||||||||
| 4. | What is natural selection? Natural selection leads to the increased survival and reproduction of individuals better adapted to the environment. Natural selection is the sum of all the forces that affect the survival of living things. It includes biological factors such as disease agents, predators, and food resources, and physical fctors such as temperature, water quality and availability, and shelter. | ||||||||
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Supplies
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These are some materials you may want for the sample experiments supplied with this lesson. For an entire class of 24 to 30 students:
For each group:
For each student working in a group:
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Objectives |
Once you have completed this lesson you should be able to: | ||||||||
| 1. | Define natural selection, adaptation, genetic diversity, and carrying capacity. | ||||||||
| 2. | Describe how genetic variation allows a population to survive during environmental changes. | ||||||||
| 3. | Explain how natural selection controls the genetic diversity found in populations. | ||||||||
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Introduction |
This lab demonstrates the process of natural selection. We will see how organisms cope with changes in the environment by noticing how the environment directs the preferential survival of certain traits in populations over others. Specifically, we perform a simulation of a population of fictional birds as they try to survive through several generations after a drastic change in their environment has occurred. | ||||||||
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Background Information |
Part 1. Background | ||||||||
| 1. | Pretend that you are a member of a population of seed-eating birds. A great deal of genetic variation exists in this population. We'll focus on the variation seen in the beak shapes of these birds. Because of the different shapes of their beaks, some birds in this population eat in different ways. | ||||||||
| Question | 2. | What is a population? Are all of these birds members of the same species? A population is a group of individuals of the same species living in a particular area.
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| 3. | What produces genetic variation in a species? Genetic variation in a population exists as a result of variations in genes. Different gene forms, called alleles, are responsible for a multitude of observable differences within a single species. | ||||||||
| Background | 4. | There are many different types of beaks observed in this population of birds. Figure 1 presents beak phenotypes that might be observed in this population of birds.
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| Question | 5. | How can genetics explain the observable differences in the shapes of the bird beaks? Genes largely determine the beak shapes of these birds. Mutations occur to produce different alleles of one or more genes involved in the development of beaks. This produces a wide range of beak shapes. Natural selection will then favor some beak shapes over others. In this lab we look at competition for food resources as the primary selective force within natural selection.
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| 6. | These birds get all of their nourishment from eating certain food. Right
now, conditions are good and there are many different types of food available
in this environment. Below is a list of foods that these birds normally
eat:
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| Question | 7. | Though these birds can eat any of these foods to survive, some birds prefer to eat certain things. This is because birds with particular beak shapes have an easier time obtaining and eating certain foods. Think of some examples of birds from the real world. What are some birds which have beaks that are well-suited to eating particular things? Students may suggest: woodpeckers, whose beaks are well-adapted to boring holes in tree trunks to scavenge for bugs; hummingbirds, whose long, slender beaks are adapted to sip nectar from flowers; or pelicans, whose deep pocketed beaks can capture fish. | |||||||
| 8. | Now, think about the imaginary birds in our lesson (see Figure 1). Can you think of some examples of beaks that seem well-suited to eating certain kinds of food? Use your imagination. Answers may vary. For example, students might suggest that the shish-kebab beaked birds have an easier time eating Cheerios because the birds can slip their slender beaks into the holes of Cheerios to pick up many of them at a time. | ||||||||
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Exercise 1 |
Natural Selection Simulation | ||||||||
| Background | 1. | Disaster has struck this population of birds! Sudden changes in the climate have altered the environment so that now only cat food is available and the entire population of birds must eat cat food in order to survive. Unfortunately, the cat food is in short supply. If birds having a certain beak shape fail to obtain food and therefore die, they obviously won't be able to produce offspring. The genetic traits of the parent birds will not be passed on to the younger generations.
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| To Do | 2. | Setting up: I. If the class contains 24 to 30 students, your teacher will assign six students to be team captains. Team captains will be given calculators and will be in charge of keeping track of the number of food pieces obtained by their team members. II. Your teacher will assign three or four additional students to work with each team captain. All teams should be the same size. Extra students will have the chance to participate during later hunts. III. Each group of students represents a group of birds having the same beak phenotype (feeding mechanism). Each member of the group should get:
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| To Do | 3. | The Experiment: I. You and your group will try to get food using your "beak" during 5 rounds of play. You will be competing with the other groups of birds to capture cat food. II. The food will be thrown onto a grassy area by a teacher. You should face away from the hunting area while the food is being thrown out. III. When the teacher gives the signal, you will try to capture cat food as fast as you can until STOP is called. You will have about 45 seconds to capture food during each round. Rules of the "hunt"
IV. After the STOP signal is given:
V. If you are a captain:
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| 4. | Results: Below is a table showing the amount of food obtained and how many players each team earned during each of the five generations (rounds of play).
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| To Do | 1. | Return to the classroom. Complete this table (which should be posted on the board). Each student should record this information in their lab book, filling in Table 1 completely. First, write in the number of players each group began with in the column labeled "Generation 0". Next, write in the number of pieces of food each group gathered in each round and the number of members of each generation. | |||||||
| 2. | Then, fill in Table 2 below. First, copy the number of birds in each group from Table 1. Next, calculate the percentages of each beak shape in the total population over the five generations. To find the percent of the population having a particular beak shape for each generation, first:
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| 3. | Graph the results, showing how the percentages of beak shapes in the total population changed over time. Data for each group will be represented by a line. All groups will be shown on the same graph. You may use Figure 3 or graph paper to do your work. Be sure to label both the x and the y-axis. Use the following questions to guide you. | ||||||||
| Question | 4. | Graphs typically have the time variables along the horizontal axis. What points should be along the horizontal axis? Each of the five generations. | |||||||
| 5. | What should the y-axis measure? The percentage of the total population having a particular beak shape.
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Interpret Your Results | |||||||||
| 1. | Did your group thrive or die out? Why? What significance does this have for the entire population of birds? Answers may vary, but students should hypothesize as to why their beaks were successful or unsuccessful at picking up cat food. Students should recognize that the success or failure of their beakto collect food in a competitive environment led directly to their group thriving or dying out. | ||||||||
| 2. | What other factors besides beak shape seemed to be important in the successful survival of a group? Some answers might be: speed and vitality of individuals in a group, clever strategies for using beaks, social cooperation, learning, and practice.
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Think about the Big Picture | |||||||||
| Question | 1. | What determines how many adult birds having a particular beak shape will survive? How many cat food pieces (food resources) a group is able to collect during each generation. This is determined by the efficiency of the beak and by the eating strategy employed by the group members. | |||||||
| 2. | What determines how many baby birds will be born having a certain beak shape? How many parent birds having a particular beak shape survive to produce offspring. | ||||||||
| 3. | Imagine that changes in the environment had made it so that only Cheerios remained. Predict how this might change the percentages of bird beak phenotypes seen in the population after five generations. Would these results differ from the results we got in this demonstration? Why or why not? Probably we would discover that the percentages of beak shapes in the population after five generations of hunting Cheerios would differ from the results we got in our experiment with cat food. Perhaps different beak shapes would be better at hunting Cheerios than the beak shapes most successful at hunting cat food. | ||||||||
| Background | 4. | Remember that each group is a sub-set of the entire population of imaginary birds. These sub-groups represent variations in beak phenotypes in a single species. It is important to note that even if your group died out during the simulation the population as a whole survived by hunting cat food. However, the overall genetic variation in the population was reduced. | |||||||
| Question | 5. | How does genetic diversity(having a lot of genetic variation) allow a species to better survive in a changing environment? The variation of organisms within a species increases the likelihood that at least some members of the species will survive under changed environmental conditions. As we saw in our demonstration of natural selection, the dramatic changes in the availability of food did little to affect the number of birds; rather, it caused a change in the frequency of alleles specifying beak shape. Because the population of birds had a high degree of genetic diversity, it was able to withstand a dramatic change in the environment. This illustrates how natural selection works, focusing on one of many variables, intraspecific (within species) competition.
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Supplementary Resources |
Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc. Gamlin, Linda. Evolution (Eyewitness Science). DK Publishing. 1993. | ||||||||
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Related AAAS Benchmarks |
Chapter 5 : The Living EnvironmentSection A: Diversity of Life Grades 9-12 Benchmark 1 (of 2) The variation of organisms within a species increases the likelihood
that at least some members of the species will survive under changed environmental
conditions, and a great diversity of species increases the chance that at
least some living things will survive in the face of large changes in the
environment. Grades K-2 Benchmark 1 (of 2) There is variation among individuals of one kind within a population. Chapter 6: The Human OrganismSection A: Human Identity Grades K-2 Benchmark 1 (of 3) People have different external features, such as the size, shape, and color of hair, skin, and eyes, but they are more like one another than like other animals. Chapter 10: Historical PerspectivesSection H: Explaining the Diversity of Life Grades 9-12 Benchmark 1 (of 6) The scientific problem that led to the theory of natural selection was how to explain similarities within the great diversity of existing and fossil organisms. Grades 9-12 Benchmark 2 (of 6) Prior to Charles Darwin, the most widespread belief was that all known species were created at the same time and remained unchanged throughout history. Some scientists at the time believed that features an individual acquired during its lifetime could be passed on to its offspring, and the species could thereby gradually change to fit its environment better. Grades 9-12 Benchmark 3 (of 6) Darwin argued that only biologically inherited characteristics could be passed on to offspring. Some of these characteristics were advantageous in surviving and reproducing. The offspring would also inherit and pass on those advantages, and over generations the aggregation of these inherited advantages would lead to a new species. Grades 9-12 Benchmark 4 (of 6) The quick success of Darwin's book Origin of Species, published in the mid-1800's, came from the clear and understandable argument it made, including the comparison of natural selection to the selective breeding of animals in wide use at the time, and from the massive array of biological and fossil evidence it assembled to support the argument. Grades 9-12 Benchmark 5 (of 6) After the publication of Origin of Species, biological evolution was supported by the rediscovery of the genetics experiments of an Austrian monk, Gregor Mendel, by the identification of genes and how they are sorted in reproduction, and by the discovery that the genetic code found in DNA is the same for almost all organisms. Grades 9-12 Benchmark 6 (of 6) By the 20th century, most scientists had accepted Darwin's basic idea. Today that still holds true, although differences exist concerning the details of the process and how rapidly evolution of species takes place. People usually do not reject evolution for scientific reasons but rather because they dislike its implications, such as the relation of human beings to other animals, or because they prefer a biblical account of creation.
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