ISCI 104 Lab 5 Evo Dots Forces of Evolution Answers PDF

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ISCI 104 Bio Anthropology Lab - Lab 5 answers...

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ISCI 104 Bioanthropology Lab 5 EvoDots Tutorial – Sample Answers Simulation 1: First, the baseline condition: Dot size is variable and heritable, survival is selective. 1. Predict how the population of dots will evolve in response to predation. Explain your reasoning. This would be whatever you predicted before running the simulation. Generally, a reasonable prediction would be that the larger dots should decrease in frequency first, because they are easier to click. After the first round of predation: 2. Compare the survivors to the staring population. Has the distribution of colors changed? How? The survivor population should have had a lower frequency of the larger dots – so, fewer yellow, orange, and red (possibly fewer green too). Some folks said that there were now more of the cool-colored dots (purple, blue), but this isn’t quite accurate – before reproducing, there won’t actually be any more such dots, but they will now make up a larger proportion of the survivors than they did of the original population before predation. 3. Click on the Run button again, and eat 25 more dots as fast as you can. Again, compare the survivors to the starting population. Has the distribution of colors changed again? How? Once again, you should observe still fewer of the red, orange, and yellow dots – it’s likely that at least one of these colors will no longer be represented. 4. Was the prediction you made in question 1 correct? Why or why not? Usually, the outcome matched your predictions. If your original prediction was not correct, I graded this question on your explanation, looking for evidence that you understood what was happening in the dot population. 5. Continue for a few more rounds of reproduction and predation. How many generations does it take for your population of dots to reach a point at which it can no longer evolve? For most people, it took about 4-5 generations to eliminate all the variation in the population, finishing up with only the smallest sized dots.

The requirements for evolution by natural selection Simulation 2: Size heritable but not variable 6. Note that each new population of dots you create contains considerable variation in size (and color, which is coded to indicate size). Do you think the population of dots would evolve if there were no variation in the starting population? If there’s no initial variation, there cannot be any evolution. 7. Test your hypothesis. Next to the label Size of dots is: click on the checkbox labeled Variable. There should no longer be a check in the box. Now create a new population. All the dots are the same size (and color). Go through a few rounds of predation and reproduction. Does the population evolve? This population does not evolve. With no variation, there’s no opportunity for differential survival and reproduction among different variants. Simulation 3: Size variable but not heritable 8. As we noted above, when the dots reproduce, each mother dot produces two daughters identical in size to each other and to their mother. In other words, size is heritable: It is passed from parents to offspring. Do you think the population of dots would evolve if size were not heritable? If size is not heritable, evolution by natural selection will not take place. This is because the survivors will not be able to pass on the traits that helped them survive; useful traits will be no more likely to be passed on to offspring than the traits that get you eaten.

9. Test your hypothesis. Next to the label Size of dots is: click on the checkbox labeled Heritable. There should no longer be a check in the box. Create a new (variable) population, click on the Run button, and eat 25 dots. Now click on the Reproduce button and watch closely what happens. Each mother dot produces two daughter dots whose size is chosen at random. They may or may not be identical to each other or their mother. Go through a few rounds of predation and reproduction. Does the population of dots evolve? If so, does it evolve the same way it does when size is heritable? Most people noticed some change in the population, but it’s random change, unlike the situation when size is heritable. It’s not possible to select for a particular size. Basically, you’re getting fluctuation in the proportions of different colors of dots, based on the fact that “parent” dots produce “baby” dots that don’t resemble themselves. Simulation 4: Size variable and heritable, but “predation” pressure is random 10. Until now, when you have eaten dots you have done so selectively. Because smaller dots are harder to catch, the smaller dots are much more likely to survive than the larger dots. If you were to eat the dots at random, instead of selectively, do you think the population would still evolve? If the predation pressure is random, then evolution by natural selection will not occur – deaths are random with respect to dot size/color. 11. Test your hypothesis. Next to the label Survival is: click on the checkbox labeled Selective. There should no longer be a check in the box. Create a new population (in which size is variable and heritable). Click on the Run button and eat 25 dots. Notice that when you click the mouse button, you kill not the dot you are pointing at, but a dot selected at random. (In fact, clicking anywhere inside the EvoDots window will kill a randomly selected dot.) Go through a few rounds of random predation and reproduction. Does the population of dots evolve? If so, does it evolve in the same way it does when survival is selective? Again, most people noticed some change in the population from generation to generation, but this again is rather random change, rather than the kind of directional change we see when natural selection occurs. Occasionally people observed that colors with the lowest numbers initially vanished first, but since survival was random, this was due to chance. Summary question. 12. Are the results of your experiments consistent with Darwin’s mechanism of evolution? Explain. The dot predation experiments are consistent with Darwin’s natural selection mechanism. There is no adaptive change in the dot populations unless (a) the population is variable, (b) variation is inherited, and (c) survival with respect to the “predation” selection pressure is nonrandom. Simulation 5: The source of variation among individuals – the role of mutation. 13. Go through a few rounds of selection and reproduction. Try to make the population evolve toward small dots as quickly as you can. Is there a limit to how far you can drive the population? Why? Yes, there is – it’s only possible to drive the dots down to the size of the smallest individual present in the original population. This is because under the conditions of this experiment, it’s not possible for new size variants to appear – you’re stuck with what you had in the original population. 14. Now note the label at the lower right that says Size of dots is variable and heritable. Click the box next to the label with mutation. The box should now be checked. Make a new population, and go through a few rounds of selection and reproduction. After each round of reproduction, examine the dots carefully. Can you spot the mutants? Try, again, to make the population evolve toward small dots. Can you drive the population further than you could before? Why or why not? What makes populations evolve? Now it should be possible to drive the population to an overall smaller average dot size than before; although some mutants are larger than the desired size, others are smaller than the original size range, and can

now be favored by natural selection (in this case, via predator action). Populations evolve through the joint actions of mutation (which adds new variation) and natural selection (which acts through the differential survival and reproduction of individuals to produce populations possessing adaptations that are useful in a particular environment). Simulation 6: Another, non-selective evolutionary force – Genetic drift 15. This simulation resembles Simulation 4, in that survival is random with respect to size (i.e., you can’t preferentially kill dots), although the number of different sizes of dots is lower. Do you expect to see any evolutionary change in this population? There might be some fluctuations in the proportions of different dot colors, but no directional evolution. 16. Click on Run and eat half of the dots (24), then hit Stop. As with Simulation 4, you’ll notice that survival is random – that dots died randomly with respect to your efforts to eat them. Note the number of dots per color after predation, then hit Reproduce and note the number of dots per color in the table above. Do a few more rounds of predation and reproduction (til you’ve filled in the table). What change, if any, has happened to the population? Answers varied, but some teams observed one (or more) dot colors becoming very rare or disappearing. Now, change the starting population size to 24, and click the New Population button. When you prey upon this population, eat 12 dots in each round. 17. Run enough rounds of predation and reproduction to fill in the table below. What happens to the population?

Our expectation would be that the smaller starting population would show a greater impact due to genetic drift – it’s more likely that you might have had a dot “extinction,” for instance, than with the larger starting population. Since it’s a random process, though, results varied. Reflect on your experiments with EvoDots and consider the following issues: The first three questions deal primarily with the first 5 simulations (natural selection and mutation); the last three also deal with Simulation 6 (genetic drift). 18. After they were “born,” did the individual dots ever change their size or color? If the individuals didn’t change, how was it possible for the population to change (Simulation 1)?

Individual dots don’t change size or color. The population changed due to the differential survival and reproduction of the individuals which were not selectively preyed upon. 19. When new sizes appeared in the population, was it because the dots needed them in order to survive? If not, where did new sizes come from (Simulation 5)?

New sizes did not appear in response to need – remember, when mutation was turned off, the population could not produce any new sizes no matter how strong the selection pressure. Mutation, which is random, is what created the new dot sizes. 20. What role did predation play in causing the population of dots to evolve? Did it create a need for the dots to change? Or did predation simply determine which dots survived to reproduce and which didn’t?

The predators created a situation where it was beneficial for individuals to be small in size, but the dots did not perceive a “need” to change. The predators simply determined which dots survived to reproduce and which didn’t. 21. What differences do you notice in the outcome of the genetic drift simulation (Simulation 6) between the larger (48 dots) and smaller (24 dots) starting population sizes? As mentioned above, we would expect that drift would have a stronger effect in a smaller starting population.

22. How much time do you think it would take for one dot color/size to go to fixation (to become the only type present in the population) in genetic drift as opposed to natural selection (slower or faster)? In genetic drift, would a single type go to fixation all the time? Why or why not?

Under the conditions of the simulation, you would expect that one color would go to fixation more quickly under natural selection, since you can fairly quickly remove the larger dots. In genetic drift, you wouldn’t necessarily see the same type go to fixation all the time, because the process is random. 23. How do the results for the genetic drift simulation (Simulation 6) differ from the standard natural selection simulation (Simulation 1)? Comparing the tables you’ve made will be useful here. Which force seems to be more powerful (genetic drift or natural selection) in changing dot type frequencies? The genetic drift simulation probably didn’t result in fixation of just one type of dot, whereas in the first, natural selection, simulation, the goal was to eliminate all but a single dot type. Non-surviving dot types also tend to vanish quicker under natural selection. Therefore, in these simulations, natural selection seems to be more powerful in changing dot type frequencies....