Lab 12 Evolution online Fall2020 PDF

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Harper College Bio110—Introduction to Biology and Society Lab 12: Evolution Name: ______________________________________________ Date: ___________ Objectives:  Explain how natural selection works to affect body coloration within rock-pocket mouse populations  Explain the role of mutations in evolution and in the rock-pocket mouse populations  Predict how populations may change over time due to natural selection  Explain what selective pressures are and how they can apply to human populations  Describe factors that influence the evolution of skin color in humans including UV radiation, folate production, and vitamin D production Introduction: Evolution can generally be described as the change in the traits of a population over time. Natural selection is one of the forces that leads to evolutionary change. Natural selection occurs when individuals have different chances of survival and reproduction based on their inherited traits and their interaction with the environment. Individuals that have traits that give them high rates of survival and/or reproduction will leave behind more offspring with those same traits. Those traits will increase in prevalence in the population over time. Individuals that have traits that give them low rates of survival and/or reproduction do not pass on their traits to future generations, so over time their traits (and the alleles associated with them) are removed from the population. Because there are differences between environments, traits that are advantageous for survival or reproduction in one environment may be a liability in a different environment. Thus, different environments select for different types of traits. This is one of the reasons why biologists say that natural selection can help account for the diversity of life on earth. Both lab exercises you will be completing in this lab are provided by BioInteractive at HHMI (Howard Hughes Medical Institute). Author information is provided at the end of this lab. Exercise 1: Color Variation Over Time in Rock Pocket Mouse Populations A typical rock pocket mouse is about 170 millimeters long from its nose to the end of its tail, shorter than an average pencil. And at just 15 grams, this tiny mouse weighs about as much as a handful of paper clips. Rock pocket mice, however, have had an enormous impact on science. What’s so special about them? You can find populations of rock pocket mice all over the Sonoran Desert in the southwestern United States. There are two common varieties of these mice — a light-colored variety and a dark-colored variety. There are also two major colors of substrate, or surface materials, that make up the desert floor. Most of the desert is covered in light-colored sand and rock. However, there are also patches of dark volcanic rocks that formed from cooling lava flows. These patches of dark-colored substrate are often separated by several kilometers of light-colored substrate. 

Examine the four illustrations in the link below. These illustrations represent snapshots of rock pocket mouse populations. Each illustration shows the color variation of the mice at two 1

different locations, A and B, at a particular moment in time. The illustrations may be out of order in terms of what the populations look like chronologically in time. https://www.biointeractive.org/sites/default/files/MouseColorVar-Illustrations-act.pdf **Note: You should be able to rotate the illustrations, so they appear right-side up, by using a circular arrow icon at the top right side of the PDF. 1. Count the number of light-colored and dark-colored mice present at each location at each moment in time. Record your counts in table 1 below. Table 1: Number of rock pocket mice at two different locations A and B in four different illustrations

Location A

Location B

Number of light-colored mice Number of dark-colored mice Number of light-colored mice Number of dark-colored mice

1 11

Illustration Number 2 3 10 10

4 11

1

2

2

1

2

10

6

2

10

2

6

10

2. Place the illustrations in what you think is the correct order from oldest to most recent. This is a prediction. In this space below, write the numbers of the illustrations in the order you decided. 2,4,3,1 3. Explain how you decided which illustration represents the most recent rock pocket mouse population and why you positioned the others in the order that you did.

light colored mice decreased over time while dark colored mice increased. . 

Watch the BioInteractive short film “The Making of the Fittest: Natural Selection and Adaptation” located at: https://www.biointeractive.org/classroom-resources/making-fittestnatural-selection-and-adaptation

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As you watch the video, answer the following questions: 2

4. Why are some mice light-colored and some mice-dark colored? Mice living on light-colored sand tend to have light-colored coats, while mice living on patches of darkcolored rock have mostly dark-colored coats

5. Does fur color provide any selective advantage or disadvantage (i.e. does it provide any advantage or disadvantage in terms of survival and/or reproduction)? Light colored pocket mice were at an advantage because they can hide easier from predators. Dark colored mice on the other hand were at a disadvantage because they were easily exposed to these visual predators.

6. What role does the rock pocket mouse play in the desert food web? a rock pocket mouse's color influences its overall fitness. Remember that "fitness" is defined by an organism's ability to survive and produce offspring in its environment.

7. Are mutations a random process? Mutation, since it is simply a random error in copying the DNA, is the random part of evolution

8. Is natural selection a random process? Evolution is not a random process. The genetic variation on which natural selection acts may occur randomly, but natural selection itself is not random at all. 9. Using what you learned by watching the film, confirm or change the order in which you arranged the illustrations. Once you are satisfied that the order is correct, fill out table 2 below using the counts you recorded for the illustrations. Table 2: Number of mice at different location placed in chronological order Illustration Order Oldest Second Third Most Illustration oldest oldest recent illustration illustration illustration Illustration Number Location A

10 Number of light-colored mice Number of 2 dark-colored

11

10

11

1

2

1 3

Location B

mice 10 Number of light-colored mice Number of 2 dark-colored mice

9

6

2

3

6

10

10. Explain why a rock pocket mouse’s color influences its overall fitness. Remember that “fitness” is defined by an organism’s ability to survive and produce offspring in its environment. Student explanations should include coat color as an important means of camouflage for the rock pocket mice

11. Explain the presence of dark-colored mice at location A. Why didn’t this phenotype become more common in the population?

The dark-colored mice arose in the population at location A by random mutation. The phenotype did not become more common because it did not afford a selective advantage to the mice

12. Write a scientific summary that describes changes in the rock pocket mouse populations at location B. Your summary should include: a. a description of how the population has changed over time b. an explanation of what caused the changes c. a prediction that describes what the population will look like 100 years in the future i. Base your prediction on trends in the data you have organized. You can assume that environmental conditions do not change over the 100 years. * Originally, location B had a sandy-colored substrate. In this environment, light-colored mice had a selective advantage because they could better avoid predation. * Location B became covered in dark-colored volcanic rock, which means that darkcolored mice now had an advantage over light-colored mice in that environment. * Over time, dark-colored mice became more common at location B because more of their offspring survived to reproduce and pass on their genes, including genes for fur color. * In 100 years, the population at location B will likely consist of mostly dark-colored mice. There may be a small number of light-colored mutants. 4

13. Use the data and what you have learned about evolution to explain how mutation is a random process, but natural selection is not random.

However, the dark-colored phenotype became more common once there was a selective advantage for it, which indicates that selection is not random

Exercise 2: Human Skin Color: Evidence for Selection Our closest primate relatives have pale skin under dark fur, but human skin comes in a variety of shades from pinkish white to dark brown. How did this variation arise? Many biological traits have been shaped by natural selection. To determine whether the variation in human skin color is the result of evolution by natural selection, scientists look for patterns revealing an association between different versions of the trait and the environment. Then they look for selective pressures that can explain the association. In this lesson, you will explore some of the evidence for selection by analyzing data and watching the film “The Biology of Skin Color,” featuring anthropologist Dr. Nina Jablonski. A. Is There a Connection Between UV Radiation and Skin Color? In this section, you’ll discover the particular environmental factor correlated with the global distribution of skin color variations. 

Watch the film “The Biology of Skin Color” from the beginning to time stamp 5:49 minutes. The video can be found at: https://www.biointeractive.org/classroom-resources/biology-skincolor



Pause the video when Dr. Nina Jablonski asks the question, “Is there a connection between the intensity of UV radiation and skin color?”

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In this segment of the film, Dr. Jablonski explains that the sun emits energy over a broad spectrum of wavelengths. In particular, she mentions visible light that you see and ultraviolet (UV) radiation that you can’t see or feel. (Wavelengths you feel as heat are in a portion of the spectrum called infrared.) UV radiation has a shorter wavelength and higher energy than 5

visible light. It has both positive and negative effects on human health, as you will learn in this film. The level of UV radiation reaching Earth’s surface can vary depending on the time of day, the time of year, latitude, altitude, and weather conditions. 

The UV Index is a standardized scale that forecasts the intensity of UV radiation at any given time and location in the globe; the higher the number, the greater the intensity. Examine Figure 1 below and answer questions 14-19 below.

Figure 1: Ultraviolet Radiation Index Across the World. The colors on this map of the world represent Ultraviolet (UV) Index values on a particular day in November 2020. The UV Index is a standardized scale of UV radiation intensity running from 0 (least intense) to 18 (most intense). The y-axis values are degrees of latitude, which range from the equator (0°) to the poles (90° north and −90° south). The x-axis values are degrees of longitude, which range from the prime meridian (0°) to the antimeridian (180° east and −180° west). (Source: European Space Agency, http://www.temis.nl/uvradiati on/UVindex.html .)

14. Describe the relationship between the UV Index (the colored bar in Figure 1) and latitude (y-axis). The primary UV Index value in Florida on September 24, 2015 is between 8 and 9. 15. How do you explain the relationship between the UV Index and latitude? In other words, why does UV intensity change with latitude? As latitude grows more extreme (further from equator), the UV index decreases, as less radiation from the Sun is able to penetrate the Earth’s atmosphere due to the Earth’s spherical shape and the tilt of its axis.

16. Find your approximate location on the map. What is the primary UV Index value of your state on this particular day in November? 6

17. Look at the regions that receive the most-intense UV (light pink). Site a specific piece of evidence from the map that a factor other than latitude was contributing to UV intensity on this day. The Andes Mountain Range, Chile: The high altitude of the mountains results in a thinner atmosphere, which allows more UV rays to pass through. The same is true in the Himalayas

18. In the film, Dr. Jablonski explains that melanin, located in the top layer of human skin, absorbs UV radiation, protecting cells from the damaging effects of UV. Genetics determines the type of melanin (i.e., brown/black eumelanin or red/brown pheomelanin) and the amount of melanin present in an individual’s cells. Based on this information, write a hypothesis for where in the world you would expect to find human populations with darker or lighter skin pigmentation (i.e., different amounts of melanin). Darker-skinned populations would be expected to live near the equator because this is the latitude that is consistently exposed to the most UV radiations and these populations require the most protection. The opposite is true for light-skinned populations that far from the equator, as there is a lesser threat of physical harm by UV rays, and, as a result, this trait is not selected against.

19. Explain how scientists could test this hypothesis.



You will now look at another figure that has to do with skin color. One way to measure skin color is by skin reflectance. Scientists can shine visible light on a portion of skin (typically the inside of the arm) and then measure how much light is reflected back. Dark skin reflects less visible light than does light skin. The lower the reflectance value, therefore, the darker the skin. Examine Figure 2 and answer Questions 20–22.

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20. Why do you think that reflectance data are collected from a subject’s inner arm?

21. Describe the relationship between skin reflectance (y-axis) and latitude (x-axis). Consider both the direction and steepness of the lines’ slopes. As latitude approaches zero, skin reflectance decreases (reaching a minimum of about 25% reflectance). The shape of the graph is a result of the distribution of the world’s population; many more people live above the equator than below it, possibly resulting in the discrepancy. It is also possible that some external factor present in the Southern Hemisphere has resulted in lower skin reflectance than expected.

22. Do these data support your hypothesis from Question 18? Justify your answer. Yes, these data support the response to Question 4. Distance from latitude has a direct correlation to skin reflectance, and melanin levels.



Watch the film from time stamp 5:49 minutes to 9:08 minutes. Pause when Dr. Jablonski says, “That suggests that variation in human skin melanin production arose as different populations adapted biologically to different solar conditions around the world.” 23. Based on what you know about skin pigmentation so far, suggest a mechanism by which UV intensity could provide a selective pressure on the evolution of human skin color. In other words, propose a hypothesis that links skin color to evolutionary fitness (i.e. ability to survive and reproduce and pass on traits) in high UV regions.

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B. What was the selective pressure? In this section, you’ll come to understand the specific selective pressures that have shaped the evolution of skin color. 

Watch the film from time stamp 9:08 minutes to 12:19 minutes. Pause when Dr. Jablonski says, “For that reason, though it might cut your life short, it’s unlikely to affect your ability to pass on your genes.” 24. What does it mean for a trait, such as light skin coloration, to be under negative selection in equatorial Africa? Relate negative selective pressure to what we know about MC1R allele diversity among African populations. It is disadvantageous for a species to have light skin (high skin reflectance) in equatorial Africa, as this trait is not favored by natural selection, due to its inability to adequately protect the body from harmful UV rays. This negative selection pressure, that results in darker skin, can be seen in the lack of diversity of the MC1R allele in African populations, as all variants are selected against.

25. Why does Dr. Jablonski dismiss the hypothesis that protection from skin cancer provided selection for the evolution of darker skin in our human ancestors? because skin cancer does not usually arise until after an individual's peak reproductive years. To be affected by natural selection, a trait must have an effect on an individual's ability to survive and pass on its genes.

26. Revisit your hypothesis from Question 18. Based on the information you have now, does this seem like a more or less probable hypothesis than when you first proposed it? Provide evidence to support your reasoning. The greater amount of eumelanin in darker skin protects folate from being broken down by UV radiation and thus increases fitness among populations in high-intensity UV areas.



Watch the film from time stamp 12:19 minutes to 13:32 minutes. Pause when Dr. Jablonski says, “That is what melanin does.”



In this segment of the film, Dr. Jablonski references a paper she had read about the connection between UV exposure and the essential nutrient folate (a B vitamin), which circulates throughout the body in the blood. The paper, published in 1978, describes how the serum (blood) folate concentrations differed between two groups of light-skinned people. You will now look at one of the figures from that paper. Examine Figure 3 and answer Questions 27–30. 9

27. Describe the relationship between folate levels and UV exposure. Use specific data from the graph to support your answer. UV exposure lowers folate levels (breaks down folate), as is evidenced by the statistically significant data (p = 0.005 < 0.05). The error bars of the data do not overlap, and the average Serum folate level of roughly 7.5 ng/mL in the control group was much higher than that of 4 ng/mL in the patients treated with the intense UV light. 28. Dr. Jablonski describes learning that low folate levels are linked to severe birth defects as a “eureka moment.” Explain what she means by this. Dr. Jablonski saw a connection between phenotype (skin color), environment (UV intensity), and fitness (folate levels and the risk of severe birth defects and low sperm counts). This connection provides an alternative hypothesis for the selective pressure that drove the evolution of darker skin.

29. Based on this new information, revise your hypothesis to explain the selective pressure on the evolution of human skin color. The greater amount of eumelanin in darker skin protects folate from being broken down by UV radiation and thus increases fitness among populations in high-intensity UV areas.

30. Can the effects of UV light on folate explain the full variation of human skin color that exists among human populations today? Explain your reasoning. No. The effect of UV radiation on vitamin D levels must also be considered. While UV radiation lowers folate levels, it increases vitamin D levels, resulting in an evolutionary trade off that the body seeks to keep in balance.

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C. Why Aren’t We All Dark Skinned? In this section you will explore other selective pressures that can affect skin color in populations. 

Watch the film from time stamp 13:32 ...