1. Graphically show how natural selection can change the physical makeup of a population over several generations.
2. Describe why natural selection is not the sole mechanism for changes in populations. 3. Explain how genetic drift has a larger effect upon smaller versus larger populations. 4. State a simple definition of evolution, natural selection, and genetic drift.
BACKGROUND & APPLICATIONS One fascinating aspect of all living creatures is that the phenotypes of the populations can change from generation to generation. Thus, through long periods of time, organisms can markedly change their appearance from their ancestors. If you consider this point and accept that changes in living creatures are an important theme in biology, you will realize that biologists should study and teach students how these changes occur. In short, biologists consider changes that occur in living creatures through several generations as evolution. One important aspect of biological change (evolution) is that changes in physical features (phenotype) over time must stem from changes in the genetic structure (genotype) of individuals from generation to generation. Thus, a definition for evolution is simply changes in the frequency (number) of a particular phenotype over several generations. Often, people are reluctant to discuss evolution because of the baggage the word carries with it. However, when people view evolution as biological change, they are quick to understand how it occurs in populations of living creatures. In many circumstances, people view evolution as primarily associated with human evolution. Contrary to this viewpoint, most evolutionary biologists focus their attention on the evolution of living creatures other than humans. Another major misconception is that evolution is “just a theory.” This is a serious misconception because when evolution is seen as changes in populations, one must understand how these populations change to really understand the biology of these organisms. Therefore, biologists consider the study of evolution as a fundamental part of the biological sciences. The theory of evolution is based on the reproductive success of some individuals over others. Sometimes a phenotype influences the reproductive success of an individual.
What may prevent some individuals from reproducing successfully? Evidence for Evolution: There is ample evidence that evolution (biological change) has occurred. First, consider yourself. You are a unique combination of genetic material from your mother and father. If you were to consider your ancestry, you would notice that the genetic makeup of the genes carried by your ancestors has changed due to new genetic material coming into your lineage from marriages and offspring from those marriages. After many generations, you may notice that the genetic make- up of your family has changed significantly from the make-up present in your ancestors.
Second, consider the domestication of crops and animals. If you were to examine the genetic architecture and physical characteristics of modern crop plants and farm animals, you would notice they are quite distinct from their nearest relatives found in the wild. You could say that man has caused evolution of these organisms to meet specific needs. This form of evolution is generally referred to as artificial selection. Last, consider the new forms of pesticide-resistant insect pests and antibiotic-resistant bacteria. Once again, humans have caused new forms of pests and bacteria to evolve because when pesticides or antibiotics have been widely applied for many years, many common forms were removed and only the resistant organisms survived. Thus, we are beginning to notice that many common pesticides and antibiotics are no longer effective. There are many more examples of evolution. What are other ways that populations can evolve?
Populations are the level that can show the effects of evolution. Individuals, during the course of a lifetime, cannot undergo the process of evolution. However, individuals can show the effects of acclimation to an environment. Because the focus is upon populations, one can consider evolution as the changes in physical and genetic makeup of a population over several generations. Natural Selection http://phet.colorado.edu/en/simulation/natural-selection rabbits http://www.techapps.net/interactives/pepperMoths.swf moths A major mechanism of evolutionary change is natural selection. Keep in mind that a population is comprised of many individuals that have variation in many characteristics. Charles Darwin argued that individuals better adapted to an environment tend to produce more offspring that survive and reproduce. Over several generations, a population can undergo a change in genetic structure (proportions of different alleles) that results in a change in physical structure (appearance) that reflects increasing adaptation to the environment. There are several requirements for populations to evolve via natural selection. Can you list some of these requirements?
Genetic Drift http://darwin.eeb.uconn.edu/simulations/drift.html Genetic Drift simulation http://www.pbs.org/wgbh/nova/evolution/evolution-action.html wimphogs While natural selection is a common mechanism of evolution, it is not the only mechanism for evolution (remember evolution is viewed as genetic or physical changes that occur over several generations). There are several other mechanisms of evolution. One is the effect of random circumstances that can change the genetic structure of a population. Over several generations, populations can undergo random changes in genetic structure because of random effects caused by the environment. For example, a large flock of snow geese can show changes in genetic structure if an unexpected hailstorm chances upon the flock. The survivors reflect a smaller portion of the total genetic material the flock once exhibited. Thus, the following generation would likely show a slightly different genetic structure than what previously existed in the population. These random changes in the genetic structure of the population define what is called genetic drift. These random changes affect smaller populations more than larger populations. Why do you think genetic drift affects smaller populations more than larger populations?
Exercise 1 Materials Obtain a 1 lb. bag of Elbow Macaroni (“grass”) one small bag of small shell-shaped pasta (‘small shell bugs’) one small bag of large shell- shaped pasta (“large shell bugs”). a container such as a box, baking pan, or serving tray about the size of your textbook and maybe a bit deeper. Simulate a predator feasting upon a population of larger and smaller prey. Over several generations, you may see a change in the phenotypic structure of the population due to selective feeding. What mechanism of evolution do you think this exercise focuses on?
Develop a Hypothesis: Consider the following question. How will a predator feeding on a population of prey affect the proportion of small phenotype to big phenotype over time? Why? Follow these instructions exactly as described…especially Steps 5, 6, and 7 Activity
1. Set up the grassland inside the container by adding all of the elbow macaroni to simulate “grass.”
2. Add 25 small shells and 25 large shells to the grassland. Shake the container thoroughly.
3. You are the predator. The predator “you” will use only your thumb and forefinger to collect its prey one at a time. The predator will have 30 seconds to collect as many prey as possible.
4. Record the number of each type of prey killed in Table 1. 5. Using the equation below, calculate the frequency of each prey type that remains in the population. Record the surviving frequencies in the table. (the two frequencies added together should equal 1.0)
Frequency surviving = surviving number of small or large
total number surviving
6. Simulate the survivors of the population reproducing and establishing the next generation. Use the survival frequencies you calculated in step 5 to determine how many of the next generation will be small and how many will be large. Calculate the new population of small and large shell bugs after reproduction by multiplying the survival frequency of each phenotype by 50. Enter these numbers as the initial number of the next generation.
7. Add enough small and large shell bugs to the population in the container to bring the Surviving # from the previous generation up to the Initial # of the next generation.
8. Repeat steps 3-6 for a total of 5 generations. 9. Remove any remaining shell bugs from the grass. Then remove the grass from the
container in readiness for the next exercise.
Table 1 Natural Selection Data Table.
Generation 1 Small Large Total Initial # # killed 25 25 50 # surviving Survival Frequency Generation 2 Initial # # killed # surviving Survival Frequency Generation 3 Initial # # killed # surviving Survival Frequency Generation 4 Initial # # killed # surviving Survival Frequency Generation 5 Initial # # killed # surviving Survival Frequency
Forces of Evolution Worksheet
1. What “force” or mechanism of evolution was being demonstrated in this exercise?
2. Write a conclusion to this exercise. Be sure to state your hypothesis. State whether the data you collected supported your hypothesis. Use examples from your data to back up the statement of support/nonsupport for the hypothesis. Explain why you think the results turned out the way they did.
3. Did your population evolve? Explain your answer
4. If you were to repeat this experiment again would you expect the results to be completely different, similar, very similar, or nearly exactly the same as the results you had this time? Explain your answer?
Exercise 2A: Small Population Genetic Drift. Here, you will determine if random fluctuations in prey types can cause substantial effects upon the genetic variety of a population. What mechanism of microevolution do you think this exercise focuses on? Make a prediction of how random removal of individuals from a population will affect the phenotype frequencies for that population. Will large populations be affected differently? How? Develop a Hypothesis: Consider the following question. How will the random removal of individuals from a small population affect the proportion of the tan phenotype to the colored phenotype? Why?
Table 2A. Effect of random events on a population of 50 Generation 1 Small Large Total Initial # 25 25 50
Activity # killed
1. Obtain 25 small and 25 large shells. No Grass this time
2. You will randomly distribute the population in the container.
3. Simulate a flood by removing 16 individuals from the left side of the area
4. Calculate the surviving frequency of each phenotype and reproduction as described in steps 5-7 of Exercise 1.
5. Randomly redistribute the population in the box
6. Simulate a grassfire striking the population by removing 16 individuals closest to the right side of the container.
# surviving Survival Frequency Generation 2 Initial # # killed # surviving Survival Frequency Generation 3 Initial # # killed # surviving Survival Frequency Generation 4 Initial # # killed # surviving Survival Frequency Generation 5 Initial # # killed # surviving Survival Frequency
50 50 50 50
7. Calculate the new surviving frequency, simulate reproduction, and redistribute the population.
8. Next, an earthquake shakes the area. Remove 16 individuals closest to the center of the area. 9. Calculate the new surviving frequency, simulate reproduction, and redistribute the population.
10. A severe hail storm causes damages the shell bug food source. Remove 16 individuals from the top of the area. 11. Calculate the new surviving frequency, simulate reproduction, and redistribute the population.
12. A tornado strikes. Remove 16 individuals from the bottom of the area. 13. Calculate a final population frequency.
Table 2B. Effect of random events on a population of 150
Exercise 2B: Large Population Genetic drift tends to be more of an issue with small populations; to illustrate this, we will repeat the earlier exercise with a larger population. Hypothesis: How will random events affect the proportion of tan phenotypes to colored phenotypes in a large population? Why? Activity 1. Obtain 75 small shell bugs and 75 large shell bugs toothpicks, and repeat the exercise exactly as described in Exercise 2A, Steps 3-14.
Generation 1 Initial # # killed # surviving Survival Frequency Generation 2 Initial # # killed # surviving Survival Frequency Generation 3 Initial # # killed # surviving Survival Frequency Generation 4 Initial # # killed # surviving Survival Frequency Generation 5 Initial # # killed # surviving Survival Frequency
Total 150 150 150 150 150
Forces of Evolution Worksheet
1. Graph your data from Exercise 2A below (both phenotypes, use different colored lines)
Affect of Genetic Drift on a Small Population
What are your observations regarding phenotype frequency in this population over time? Can you detect any constant trends with either phenotype?
2. Graph your data from Exercise 2B below (both phenotypes, use different colored lines)
Affect of Genetic Drift on a Large Population
What are your observations regarding the phenotype frequency in this population over time? Can you detect any constant trends with either phenotype?
3. What size population (large or small) is more likely to be influenced by genetic drift?
Why? Did your results support this? Why or why not?
4. Understand that the definition for genetic drift to be random changes that occur in a population’s gene pool. Should you be able to predict how these events will influence a population’s phenotype? Explain your answer?
5. How does the mechanism for removing individuals from the population differ from Exercise 1 to Exercise 2? (predator feeding….Random removal) Which of these would be more likely to drive evolution to favor one phenotype over the other? Explain your answer.
6. Write a definition for evolution as it pertains to a population.