In the mid 1800s a Charles Darwin discovered and developed a mechanism for evolution that he called natural selection. Natural selection simply stated that within any population with variation there would be a struggle for existence that would ultimately lead to the survival of the fittest. Darwin then went on to say that those traits that were the most fit would be passed on to the next generation more frequently. This continual selection for certain traits and against others over billions of years led to what Darwin called descent with modification. Descent with modification says that different environments selecting for certain traits over long periods of time could have caused an ancient ancestor of all life to be modified over and over again until it had evolved from a single species into all of the species that have lived on Earth since.
While Darwin defined the mechanism of evolution fairly accurately, there were two major components he was missing. Darwin had little to no idea what caused the variation he saw in nature and he could not explain how the variations he saw were passed from parent to offspring. It was not until the Hershey-Chase experiment in 1952, which showed DNA to be the hereditary molecule, and the discovery of the structure of DNA by Watson and Crick and Franklin in 1953 that these questions could be answered. This discovery would change the way science studied evolution. Now that science knew it was DNA that caused the variation seen in nature and DNA that caused those traits to be passed from parent to offspring, they could focus on it as the root of evolution by natural selection. This meant that instead of approaching evolution by looking at how an organism’s phenotype (physical features) changed over time, scientists could study the evolution of an organism’s genotype (allele combinations). This leads to our modern definition of evolution: The increase or decrease in the frequency of an allele over time.
In this simulation you will be studying how mutations in the DNA of an organism can lead directly to the evolution of a population. This will be done by tracking how allele frequency is dependent upon the core principles of natural selection.
Click on the link below to go to the simulation. It is found under Simulations: Biology and is called Natural Selection.
Simulation courtesy ofPhET Interactive Simulations, University of Colorado, underCC BY 4.0
(MAKE SURE YOU CHOOSE "LAB", NOT "INTRO" AT THE BOTTOM OF THE SCREEN.)
Follow the directions below to collect data. Take a few minutes to familiarize yourself with how the simulation software works. Be sure to notice the following:
The population graph under the picture.This is where you will get the number of each type of rabbit.
The generation bar (x axis), play, pause and reset buttons.The pause button allows you to stop the simulation so you can read the graph and take your data.
Experiment 1:
1. In this experiment you will be examining the effect of arecessive mutationthat changes the color of the organism and alters how it avoids predators.
2. When you are ready to start your first experiment make sure the environment is set to “equator” (the sun icon at the top right in the picture) then click on the “add a mate” button in the bottom centre of the picture. This will start the simulation. Watch the generation bar and let it run for 2 generations, then hit pause. Record the number of bunnies after the 2 generations you just created in a table. (Add two more columns to your table for the next step: Bunnies with Brown Fur (%), and Bunnies with White Fur (%).)
3. In the Add Mutations section of the simulator, click on (brown) fur as a recessive trait.
4. Start the simulation and let it run for 2 more generations. At this point pause the simulation and record this data in the same table. The table should now have number of generations, total bunnies, bunnies with brown fur (%) and bunnies with white fur (%), for both generations 2 and 4.
5. Now add Wolvesas a selection factor and start the simulation. Let the simulation run for 3 more generations. Hit pause and enter the data for generation 7 in your table.
6. Click play and let the simulation run for three more generations (unless you have already run out of one colour of bunny). Pause the simulation and record the data for generation 10 .
Experiment 2:
In this experiment you will be examining the effect of adominant mutationthat changes how the organism obtains food.
1. Click the reset all button in the lower right hand corner of the simulator.
2. When you are ready to start your second experiment click on the “add a mate” button. This will start the simulation. Watch the generation bar, let it run twice, then hit pause. Record the data from the graph for generation two in a second table.
3. Once you’ve recorded the data, click on “long teeth” as a dominant mutation in the Add Mutations section of the simulator. (This mutation will make it easier for the bunnies to eat tough food.)
4. Start the simulation and let it run for two more generations. At this point pause the simulation and use the graph to fill in generation 4 data in your second table.
5. Now add Tough Food as a selection factor and start the simulation. Let the simulation run for three more generations. Hit pause and record the data for generation 7.
6. Click play and let the simulation run for three more generations. Pause the simulation and record the data for generation 10.
Graded Discussion Questions
Submit your tables and answers to these questions to Turnitin.
1. Given a current definition of evolution being a change in allele frequency over time, did either of the mutations above fail to cause the population to evolve? If so which one? Use your data to explain how you know.
2. Predict what would have happened in simulation 1 if you had switched the environment from equator to arctic. Do you think the brown rabbits could have been completely bred out of the population like the white ones may have been in simulation 1? Why or Why not?
3. Compare your data from simulation 1 and simulation 2. In simulation 1 the mutation was recessive; in simulation 2 the mutation was dominant. What was different about how recessive and dominant traits evolved after the predators were introduced (generations 7 and 10)? Why do you think this occurred? You may need to do a little research, and if you do, remember to demonstrate that you understood it well by putting it in your own words (then referencing to acknowledge where you found it.)