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1 Evolution by Natural Selection Living things that are well adapted to their environment survive and reproduce. Those that are not well adapted don’t survive and reproduce. An adaptation is any characteristic that increases fitness, which is defined as the ability to survive and reproduce. What characteristic of the mice is an adaptation that increased their fitness? The table below gives descriptions of four female mice that live in a beach area which is mostly tan sand with scattered plants. According to the definition given for fitness, which mouse would biologists consider the fittest? Color of fur Black Tan Tan and Black Cream Age at death 2 months 8 months 4 months 2 months # pups produced by each female 0 11 3 0 Running speed 8 cm/sec. 6 cm/sec. 7 cm/sec. 5 cm/sec. If a mouse's fur color is generally similar to its mother’s color, what color fur would be most common among the pups? A characteristic which is influenced by genes and passed from parents to offspring is called heritable. Over many generations heritable adaptive characteristics become more common in a population. This process is called evolution by natural selection. Evolution by natural selection takes place over many, many generations. Evolution by natural selection leads to adaptation within a population. The term evolution by natural selection does not refer to individuals changing, only to changes in the frequency of adaptive characteristics in the population as a whole. For example, for the mice that lived in the beach area with tan sand, none of the mice had a change in the color of their fur; however, due to natural selection, tan fur was more common for the pups than for the mother mice. 2 In summary, a heritable characteristic that helps an animal or plant to have more offspring which survive to reproduce will tend to become more common in a population as a result of evolution by natural selection. Notably, not all characteristics which contribute to longer life become more common in the population. Some characteristics contribute to long life, but not more offspring. For example, a female cat which is sterile and cannot have any offspring may live longer because she will not experience the biological stresses of repeated pregnancies. Thus, a characteristic like this which contributes to a long life, but with few or no offspring, would not become more common as a result of evolution by natural selection. Simulation of Natural Selection We will now play a simulation game to demonstrate how natural selection works. A simulation is a good way to simplify the problem in such a way that we can observe how evolution by natural selection may work in a real population. This simulation involves rabbits that can reproduce. The main concerns our bunnies have is the presence of predators (wolves) and finding food. All we need is a system that has three necessary conditions for evolution by natural selection. i. Variation in characteristics: For natural selection to occur, different individuals in a population must have different characteristics. In our simulation, rabbits vary in color; they are white or brown. They also vary in tooth-size, having either short or long teeth. ii. Differences in fitness: For natural selection to occur, the different characteristics of different individuals must contribute to differences in fitness (i.e. differences in ability to survive and reproduce). For example, variation in rabbit color may influence the probability that a rabbit is snatched up by a hungry wolf. Also, different tooth lengths may vary in their success in competing for food. These differences contribute to survival and therefore success in reproducing. iii. Heritability of characteristics: For natural selection to occur, the characteristics that affect fitness must be heritable (i.e. passed by genes from one generation to the next). In our simulation, a rabbit that is born into the population will have the color and tooth length determined by the genes it receives from its parents. To run the simulation and conduct the experiments: a. Go to http://phet.colorado.edu/sims/natural-selection/natural-selection_en.jnlp to run the java application. You see a simple representation of a forest near the equator. The rabbit population you’ll be working with is white, and you start with one rabbit. Take a few minutes to familiarize yourself with how the simulation software works. Be sure to notice the following: The population graph under the environment. This is where you will get the number of each type of rabbit. You can zoom in or out to get a more accurate reading of population size. The generation bar, play, and pause buttons. These give you a time before each breeding period. The pause button allows you to stop the simulation so you can read the graph and take your data. 3 NOTE: you can always start over by pressing the “RESET ALL” button b. For the population to grow, there must be two rabbits. Click the “Add a friend” button to give your rabbit a mate. As time continues, notice what happens to the rabbit population. c. Without any selection factor, the rabbit population grows exponentially and “takes over the world”. Rerun the simulation (don’t forget to “Add a friend”), but this time, around the 4th generation, select “Wolves” under Selection Factor. Notice what happens to the population. Experiment 1. d. Now we will run an experiment to test the hypothesis that the proportion of traits within a population can change as a result of selection factors. We will test this first using predation as the selection factor. Restart the simulation and click “Add a friend”. Watch the generation bar and let it run twice, then hit pause. Record the data from the graph for generation two in data table 1. e. Once you’ve recorded the data, click on “Brown fur” in the add mutation section of the simulator. Using the edit genes section of the simulator change brown fur to a recessive trait. f. 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 data table 1. g. Now add wolves as a selection factor and start the simulation. Let the simulation run for three more generations. Hit pause and collect the data for generation 7 on data table 1. h. Click play and let the simulation run for three more generations. Pause the simulation and record the data for generation 10 in data table 1. Data table 1 Graphical Data Percentage calculations Generation number Number of Brown Number of White Total % Brown Rabbits % White Rabbits 2 4 7 10 Analysis: Calculate the percentage of each color of rabbit using the following method: % Brown rabbit = (Brown rabbits / Total rabbits) X 100 % White rabbit = 100% - % brown rabbits. 1. Does the data in the table above support the hypothesis for this experiment? Be sure to use your data in an explanation of why the data does or does not support the hypothesis. 4 Experiment 2. i. Now we will run a second experiment to test the effect of a mutation that changes how the rabbits obtain food. We will test the same hypothesis as Experiment 1, this time using food availability as the selection factor instead of predation. Click the “Reset All” button in the lower right hand corner of the simulator. When you are ready to start your second experiment click on the “Add a friend” button to start the simulation. Watch the generation bar, let it run twice, then hit pause. Record the data from the graph for generation two in data table 2. j. Once you’ve recorded the data click on “long teeth” in the add mutation section of the simulator. This mutation should already be designated as the dominant trait but check the edit genes section of the simulator to be sure. k. 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 data table 2. l. Now add food as a selection factor and start the simulation. Let the simulation run for three more generations. Hit pause and collect the data for generation 7 on data table 2. m. Click play and let the simulation run for three more generations. Pause the simulation and record the data for generation 10 in data table 2. Data table 2 Graphical Data Percentage calculations Generation number Number of Short teeth Number of Long teeth Total % Short Tooth Rabbits % Long Tooth Rabbits 2 4 7 10 Analysis: Calculate the percentage of each tooth type, as you did for Experiment 1. 2. Does the data in the table above support the hypothesis for this experiment? Be sure to use your data in an explanation of why the data does or does not support the hypothesis. 5 3. Given the current definition of evolution being a change in allele frequency over time, did evolution by natural selection occur in each rabbit population from Experiments 1 and 2? In other words, did one rabbit color or tooth size become more common over time while the other traits became less common? Did any individual rabbits change color or grow longer teeth? If not, then why did the final population differ from the colors of the rabbits in the original populations? Experiment 3. We will run a final experiment to test how a changing environment can influence these interactions. Rerun Experiment 1. However, after recording the data for generation 7, change the environment to Arctic. Allow the simulation to run for 3 more generations, and record the results for generation 10 in data table 3. Data table 3 Graphical Data Percentage calculations Generation number Number of Brown Number of White Total % Brown Rabbits % White Rabbits 2 4 7 10 Analysis: Calculate the percentage of each tooth type, as you did for Experiment 1. 4. How did the habitat change from equatorial to Arctic influence the rabbit population? 5. Suppose that natural selection over many generations had resulted in only brown rabbits surviving in the equatorial forest before the environment changed. Would natural selection for rabbit color occur? Based on this example, explain why evolution by natural selection cannot occur if there is no variation in a characteristic. 6 6. Explain why evolution by natural selection cannot occur if the variation in a characteristic does not contribute to differences in fitness. Suppose, for example, that all the predators in the simulation were blind and could only find rabbits by smell. Would you expect evolution by natural selection in the color of the rabbits? Assignment: Turn in (via SafeAssign on Blackboard) a typed document that shows the data tables for Experiments 1-3 and fully (but succinctly) answers the numbered questions 1-6.

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2.
In this experiment we limited the food supply available to the rabbit population. . The rabbits had varied teeth length; initially all rabbits had short-teeth, but a mutation in later generations introduced into the population rabbits with longer teeth. During the 4th generation there were roughly 4 times as many short-teeth rabbits than there were rabbits with long-teeth.

As food became a limiting factor, the rabbit population allegedly experienced natural selection. Though the population was originally predominantly short-teeth, long-teeth rabbits overtook the population. That is, by the 7th generation the predominance reversed, and there more than 4 times as many long-teeth rabbits than there short-teeth rabbits.

Much like in the former experiment, the empirical evidence corroborates the hypothesis; the proportion of teeth length changed in response to a selective force. Long-teeth rabbits were better adapted to the environment, and underwent positive selection.

3.
It seems likely that fur color and teeth size alike, were modeled after genes with two alleles. That said, the results of the experiments are in line with such a model – and showed change in trait/allelic frequencies due to selective forces – as one rabbit trait became more common on the expense of another, over time. Therefore it could be said that each rabbit population experienced natural selection, under the current definition.

No one individual rabbit changed color or grew longer teeth over time, but rather since one heritable trait had higher fitness, and provided better survivability and more progeny than the other - the higher fitness trait grew more common as more of its progeny survived and spread.

As far as the fur color, since apparently white color fur rabbits were more susceptible (to being devoured by wolves), they experienced a negative selection pressure, gave less progeny, and thus were less prominent in the final population....
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