Biology Lessons Part 2: Population Biology

Lesson 2.8: How Do Populations Change Over Time?

Time To Ponder Supplies Objectives Introduction and Background Supplementary Resources AAAS Benchmarks

Exercise 1: Natural Selection Simulation Interpret Your Results Think About the Big Picture

Grade Level		Prospective and Practicing K-8 Teachers; may be adapted for use in K-12 classes
Time		Exercise 1 will take approximately 2 1/2 hours.
To Ponder		1.	Why do we see so much diversity in individual members of animal and plant populations?
		2.	What does it mean for a species to become extinct?
		3.	How do new species arise, according to evolutionary theory?
		4.	What is natural selection?
Supplies		These are some materials you may want for the sample experiments supplied with this lesson. For an entire class of 24 to 30 students: tiny particulate food, like cat food or cheerios large container for holding food six buckets to hold feeding mechanisms six calculators feeding mechanisms(you need about 15 of each total) straws pair of shishkabob sticks pencils tweezers clothespins spoons 24 to 30 margarine tubs with 2 inch dia. holes cut in lids a large grassy area For each group: one calculator one bucket full of one type of feeding mechanism For each student working in a group: one feeding mechanism one margarine tub with lid
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.
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.
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?
		3.	What produces genetic variation in a 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. Figure 1. Beak shapes in the imaginary bird population. Drinking straw-shaped   shishkabob-stick-shaped pencil-shaped clothespin-shaped tweezer-shaped spoon-shaped
Question		5.	How can genetics explain the observable differences in the shapes of the bird beaks?
		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: Cheerios cat food marshmallows raisins
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?
		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.
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.
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: one of the feeding mechanisms (beak) (same for the entire group a "mouth" (margarine tub with lid)
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" a. Food must be lifted with the feeding mechanism (beak) and placed into the "mouth" (margarine tub) held in the opposite hand. b. You must lift the food into the mouth. You can't take off the lid of the margarine tub and shove the food in by pushing it along the ground. c. You can steal food from another student if he/she is still trying to get into his/her mouth. Food in the tub that has been "eaten" already can't be stolen. IV. After the STOP signal is given: a. Count how many pieces of food you have collected. b. Tell your team captain how many pieces you collected. c. Return your food to the food collection container set out by the teacher. d. Wait while your captain calculates how many pieces of food your team gathered altogether and reports this to the teacher. Figure 2. Cat food in margarine tub.   V. If you are a captain: a. Add up how many pieces of food each team member captured. Record the number and report it to your instructor. b. The instructor (or one of the extra students) will add up the totals reported by each team to determine a class total. He/she will tell this total to you and the other team leaders. Calculate the number of players your team earned for the next generation by using this formula:
			c. If your team gathered relatively little food and thus earned fewer players than you started with, some players must turn in their beaks and join the group of "extras." If your team collected a lot of food and thus earned more players than you started with, additional players from the group of "extras" may join you.   d. To begin the next round of play (the next generation), give new members of your team feeding mechanisms (beaks). This symbolizes the birth of babies having the same trait as their parents.
		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).
Table 1. Survival Rate of Groups
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: a. add up the total number of players for each round b. divide by the total number of survivors from each group by the total number of people playing.
Table 2. Percentage of the Total Population Represented by Groups
		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?
		5.	What should the y-axis measure?
	Interpret Your Results
		1.	Did your group thrive or die out? Why? What significance does this have for the entire population of birds?
		2.	What other factors besides beak shape seemed to be important in the successful survival of a group?
	Think about the Big Picture
Question		1.	What determines how many adult birds having a particular beak shape will survive?
		2.	What determines how many baby birds will be born having a certain beak shape?
		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?
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?
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.
Related AAAS Benchmarks		Chapter 5 : The Living Environment Section 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. Section B: Heredity Grades K-2 Benchmark 1 (of 2) There is variation among individuals of one kind within a population. Chapter 6: The Human Organism Section 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 Perspectives Section 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.