BIOL 1107 Natural Selection Simulation

Transcription

1 BIOL 1107 Natural Selection Simulation Introduction Biological evolution can be defined as the change in the allelic frequencies in a population over time. There are several recognized mechanisms of evolution, including evolution by natural selection. Charles Darwin and Alfred Russell Wallace first described the process of natural selection in the 1850s. Darwin outlined several requirements necessary for natural selection to occur, including heritable variation within the population, the presence of more individuals than the environment can support, and an environment that favors certain traits in the population over others. If all of these conditions are present, individuals in the population that have the favored traits are more likely to survive and successfully reproduce, and thus pass down those traits to their offspring. The reproductive success of an individual relative to others in the population is known as the organism s biological fitness. Any heritable trait that increases the fitness of an individual is known as an adaptation. Individuals that are best adapted to their environment have higher fitness and thus more offspring than others in the population. Subsequent generations will therefore have a higher frequency of the successful traits. In this manner, the entire population evolves and becomes better adapted to the environment. In this lab, we will simulate the process of natural selection and track the allele frequencies for a particular trait in a population over several generations. Simulation This simulation studies the effects of predation pressure on a population of mice in three different environments. Grains of rice will represent the mice. Within the mouse population there are three possible phenotypes for fur color, determined by two alleles. BB = black Bb = reddish- brown bb = white Populations of mice will be placed in three different environments, represented by different colored towels. You will serve as the predator, capturing and eating the mice by picking them up with forceps and transferring them to your food cache (cup). You will complete the simulation four times in each environment (four generations) and allele frequencies will be tracked in each generation. Each new generation of mice will be determined by the survivors of the previous generation. The simulation assumes random mating (i.e. any surviving mouse can mate with any other surviving mouse), and follows the rules of Mendelian inheritance.

2 Run the Simulation 1. Spread out the white towel on your desktop. This will be Environment 1. Carefully count out 20 grains of each type of rice and spread them out evenly on the towel. (You have now populated your environment with 20 black, 20 brown, and 20 white mice). 2. Choose two students to be the predators and one student to time the simulation. The predators will use forceps to capture as many mice as possible in 30 sec. To successfully capture a mouse, you must pick up one of the rice grains with the forceps and transfer it to the designated food cache (cup). The cup must remain on the table at all times. Only one mouse can be captured at one time. 3. Run the simulation (30 sec to eat as many mice as possible). 4. After the 30 sec are up you will count the surviving mice of each phenotype. To do this, carefully shake off the towel into the tray provided. (Make sure you get all of the survivors!). Record the number of each color rice grain in Table 1. You will now calculate the allele frequencies of the survivors in order to determine the make- up of the next generation. Allele frequencies can be calculated by examining the number of mice of each genotype. Each of the black mice is homozygous dominant and thus has two B alleles. Reddish- brown mice are heterozygous and have one of each allele. White mice are homozygous recessive and thus have two b alleles. Therefore, the number of each allele in a generation can be determined as follows. # B alleles = [# black (BB) mice x 2] + [# brown (Bb) mice x 1] # b alleles = [# white (bb) mice x 2] + (# brown (Bb) mice x 1) The frequency of these alleles is then calculated by dividing each number by the total number of alleles present in the population. The sum of the two frequencies should always equal 1. Example: The allele frequencies of the initial population (20 of each phenotype) are determined as follows. # B alleles = [20x2] + [20x1] = 60 # b alleles = [20x2] + [20x1] = 60 Total alleles in population = = 120 Frequency of B allele = 60/120 = 0.50 Frequency of b allele = 60/120 = 0.50

3 5. Calculate the frequency of each allele for the survivors of Round 1. Record results in Table 2. Allele frequencies of the initial population have already been recorded for you. These allele frequencies will now be used to determine the make- up of the next generation of mice for Round Open the Excel file called Natural Selection Simulation on the laptop provided. Enter your allele frequencies into the designated columns for Environment 1 Round 1. Excel will automatically calculate the expected genotype frequencies in the next generation based on the allele data. Those frequencies are then used to determine how many of each color mice will populate the environment in Round 2. **Excel is calculating expected genotype frequencies using the Hardy- Weinberg equation. This assumes random mating among all survivors. Hardy- Weinberg Equation: p 2 + 2pq + q 2 = 1 p = frequency of dominant allele q = frequency of recessive allele p 2 = frequency of homozygous dominant genotype 2pq = frequency of heterozygous genotype q 2 = frequency of homozygous recessive genotype 7. Count out the appropriate number of each color mouse for Round 2 using the numbers generated by the Excel program. Spread them out on Environment Run the simulation again (Round 2), with the predators capturing as many mice as possible in 30 sec. Choose different students to be the predator and timekeeper this time. 9. Count survivors and analyze the allele frequencies as you did in Round Repeat the simulation two more times in Environment 1, recording your allele frequency data each time. Rotate which students serve as predators for each round. 11. When finished with Environment 1, repeat the same simulation in the other two environments (red towel and black towel). You will do four rounds for all environments.

4 Data Tables Table 1. Survivors of Each Round Environment 1 (White) Environment 2 (Red) Environment 3 (Black) Black Brown White Black Brown White Black Brown White Initial Round 1 Round 2 Round 3 Round 4 Table 2. Allele Frequencies of Each Generation Environment 1 (White) Environment 2 (Red) Environment 3 (Black) B b B b B b Initial Round 1 Round 2 Round 3 Round 4 Graph the Data When finished, your instructor may want you to graph your allele frequency data. If so, follow these instructions Open Microsoft Excel and create three graphs (one for each environment) displaying the change in allele frequencies over time. Instructions are listed below: a. Insert data in three columns like shown: (do NOT put any empty columns between the data) b. Highlight the data, including column headings c. On the Insert tab, select the chart that says Scatter with straight lines and markers from the drop- down menu under the Scatter chart. d. You should now have your graph and an accurate legend. e. Use the chart Layout tab to insert proper axis and chart titles. f. Repeat for the other two environments. g. Your can now save your Excel file to a flash drive or it to yourself.

5 Answer the following questions. 1. Which phenotype had the highest level of fitness in the Environment 1? (i.e. Which color survived best and thus contributed more of their alleles to the next generation?) 2. Which phenotype had the highest fitness in the other two environments? Was it the same or different from the Environment 1? 3. Does the environment play a role in determining which traits are most fit? 4. Did the frequency of each allele change in the environment 1 over time? Did it change in the Environment 2 or Environment 3? 5. Did evolution occur in these populations? Explain. 6. Did the populations become better adapted to their environment over time? Explain. (Did it become harder for you to find the insects in the latter generations?)