Rock Pocket Mouse Quiz Answer Key2019 PDF

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Evo-Short 1 Quiz

BIO B51: Evolutionary Biology

Answer Key: Natural Selection in the Rock Pocket Mouse* *Quiz modified from materials related to HHMI BioInteractive teaching materials.

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Question 1 Define “mutation.” Answer: A mutation is a change in the sequence of nucleotides in the DNA of an organism.

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Question 2 Is the following statement true or false? Justify your answer in one or two sentences: “Mutations are caused by selective pressure in the environment.” Answer: False. Mutations are not caused by the environment or by selection. Mutations occur randomly. (Additional Explanatory note: Although mutations are not induced or caused by selection, they do generate new phenotypes on which selection can act. So mutations lead to new phenotypic variants, and once these variants exist, natural selection can act to make them more or less common in the population over generations.)

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Question 3 Is the following statement true or false? Justify your answer in one or two sentences: “The same mutation could be advantageous in some environments but deleterious in others.” Answer: True. If a mutation affects the phenotype, then it may be advantageous or deleterious depending on whether the new phenotype allows organisms that possess it to perform better (increase their lifetime reproductive success) relative to others in the population in that particular environment. Thus the mutation that causes black colouration is advantageous in habitats where lava flows have made the surface dark, but is deleterious in habitats where light-coloured sand is more common.

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Question 4 Recall Darwin's postulates (see Lecture 2 if you need a reminder). Does the example of evolution of furcolour in the Rock Pocket Mouse fit Darwin’s postulates? State each postulate and briefly describe how this example relates to each postulate.

Answer: Yes, this example fits Darwin's postulates. Postulate 1. There is variation in the population. This example: Some mice have light-coloured fur, and some have dark-coloured fur.

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Postulate 2. Some of the variation in inherited. This example: dark-coloured mice have different alleles than do light-coloured mice. (Additional Explanatory note: the particular alleles that cause dark colouration may differ between populations, as described in the video. In the New Mexico population, a mutation in the MC1R gene causes dark colouration, in other populations in the Southwest, different alleles cause the same outcome. If you image that dark colouration arises through a series of physiological steps, different mutations that affect different steps may yield similar phenotypic outcomes).

Postulate 3. Not all individuals survive and reproduce. This example: Some mice are killed by predators and do not reproduce, or produce fewer offspring than others which survive. Postulate 4. Individuals with particular traits have greater success in the struggle to survive and reproduce. This example: Mice with fur colour that allows them to be cryptic (hard to see) in their environment survive and reproduce at a higher rate than those that contrast with their environment. (Additional note: Since the particular traits that confer higher fitness are also heritable, in this case we predict natural selection will lead to evolutionary change, and this has occurred, as shown in the video). 

Question 5 As you saw in the film, rock pocket mice evolved to have darkcolored fur in certain habitats. In three to five sentences, explain how this trait increased in frequency in the population. Include the following key terms: “fitness” (or “fit”), “survival” (or “survive”), “selection” (or “selective”), and “evolution” (or “evolve”). Answer: Populations of rock-pocket mice included many individuals with light-coloured fur, and a few with dark-coloured fur. Rock pocket mice with dark-coloured fur were more fit on dark-coloured volcanic rock because visual predators could not see them well. This means that natural selection favoured individuals with darkcoloured fur in these habitats; that is, more of the dark-coloured mice survived and reproduced. Since dark-coloured fur is caused by a different allele than light-coloured fur, this resulted in more dark-coloured mice being born in the next generation. This process was repeated over many generations in areas with dark-coloured volcanic rock. This caused the population of rock pocket mice to evolve from one that was mostly mice with light-coloured fur, to one where most individuals had dark-coloured fur.

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Question 6 Near the end of the film, Dr. Sean B. Carroll states that “while mutation is random, natural selection is not.” In your own words, explain how this is possible. Answer: Natural selection acts on phenotypic traits as a function of whether they allow

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individuals to survive and reproduce at a higher rate than others in a population. So the traits favoured by natural selection are not random, they are specifically those that confer higher fitness. However, natural selection can only act if there is variation in the population, and mutation is a source of that variation. Mutation leads to a variety of phenotypes, not just those that are most fit in a given environment. So mutation is random with respect to the traits that would increase fitness in a given environment. 

Question 7 Suppose you are studying a recently discovered population of rock pocket mice with darkcolored fur that lives on volcanic rock. You take a DNA sample from a member of this new population and determine the DNA sequence of a gene known to play a role in fur color. The sequence you get is identical to that of the same gene in another rock pocket mouse population with darkcolored fur that lives on a different patch of volcanic rock. Which of the following could explain this observation? a. The mice in the two populations evolved from the same ancestral population. b. The volcanic rock caused the same mutation in each rock pocket mouse population, resulting in dark coloration. c. The same mutation spontaneously arose in the two different populations. d. Both (a) and (c) are possible. e. All of the above are possible. Answer: Both (a) and (c) are possible. (Additional Explanatory Note: This question asks which answers could be consistent with your observation, not which you think is more likely. Thus the similar genes could either be the same due to inheritance [i.e., they may be homologous], or they may have arisen independently by chance [i.e., they may be homoplasious or analogous—the result of convergent evolution]. We will discuss homology & homoplasy in future lectures, but you may also recall these terms from your high school or first year introductory courses, and recall that homoplasy or analogy is expected to be less common than homology. Nevertheless, both are consistent with these data).

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Question 8 For rock pocket mice, which of the following contributes to selection favoring dark-colored fur? There may be more than one correct response. a. The presence of visually-hunting predators. b. Genetic mutations affecting fur colour. c. Colour of the rock on which the mouse lives d. Distribution of food plants that determine the types of pigments produced during development.

Answers:

The presence of visually-hunting predators Colour of the rock on which the mouse lives

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(Additional Explanatory Note: Genetic mutations produce the variation on which selection acts, but in itself, mutation does not cause selection. That is, the change in gene sequence (=mutation) does not yield differences in lifetime reproductive success; rather, gene expression can lead to variation in phenotype, and a changed phenotype under a given set of conditions may lead to changes in lifetime reproductive success) 

Question 9 Suppose you are studying a new population of rock pocket mice in Arizona. These mice live on a recently discovered patch of dark-colored volcanic rock. You observed the following numbers of light- and dark-colored mice on this new patch of rock over several years of observation.

Describe what the data show in one or two sentences, then outline one possible hypothesis that could explain the observed data. Be sure to include the following key words in your answer: “selection” (or “selective”), “fitness” (or “fit”), and “survival” (or “survive”).

Answer: The data show that both light and dark-coloured mice are maintained in the population in similar numbers. It appears that both dark and light coloured mice have similar fitness, so natural selection does not change the frequency of these traits. Hypothesis: I hypothesize that, in this new population, fur colour does not affect survival because there are fewer (or no) flying predators that hunt using vision. [there may be other reasonable hypotheses proposed here, as long as they focus on why there is no advantage to black or to white fur, they may be good answers. Post to the discussion board or come to my office hours if you have questions about whether your hypothesis is a good answer]. Additional Note: ‘Does not change the frequency’ means ‘does not lead to

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statistically significant changes in the frequency of the traits’. There can be fluctuations in the frequency of the different morphs from year to year (as seen most clearly here for light coloured mice), but these are not the same as substantial, systematic changes in frequency. All biological systems (all natural systems) show fluctuations around average values. We will discuss how to determine whether there is statistically significant change when we talk about methods of evolutionary analysis and population genetics. Question 10 You decide to move a group of the light-colored rock pocket mice from the newly-discovered population in Arizona to the population in New Mexico that was described in the video. Recall that the mice from New Mexico also live on dark-colored volcanic rock. You also move a group of darkcolored mice from the New Mexico colony to the Arizona colony. You monitor the populations for five years and observe the following.

Summarize the data in two or three sentences. Then provide an explanation for these observations in a few sentences. Be sure to include the following key words in your answer: “selection” (or “selective”), “fitness” (or “fit”), and “survival” (or “survive”).

Answer: Description: In New Mexico, the number of dark mice increases while the number of light mice decreases over time, (or: the proportion of dark mice in the population increases over time). In Arizona, the number of light mice and dark mice remain constant over time (or the proportion of light and dark mice remains the same over time). Explanation: In New Mexico, there is a selective advantage to dark fur since dark mice are less visible

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to predators and visually-hunting predators are common there. Dark mice are thus more fit than light mice in this habitat and are more likely to survive and have the opportunity to reproduce. In Arizona, there is apparently no selective advantage to dark-coloured fur. Therefore dark and light coloured mice are equally fit which means they survive and reproduce at similar rates and their frequency in the population does not change significantly over time. Additional note: the fact that the dark mice in Arizona have a larger population size overall does not mean that dark fur is favoured by selection in that population. Remember that you took some white mice and moved them out of Arizona (thus reducing the number of white mice) and that you took some black mice from New Mexico and moved them to Arizona (thus increasing the number of white mice). Essentially—you created ‘migration’. Once the migration is complete, in the absence of selection (or other mechanisms of evolution), the proportion of white and black mice should remain constant over time (as observed here).

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