The Making of the Fittest

The Everyday Math of Evolution: Chance, Selection, and Time

Read Chapter 2 and answer the following questions:

3. Describe the significance of William Castle’s experiments in 1914.
4. What two factors must be present for natural selection to work?
5. There are two colors of desert mouse, black and brown. The black mice have a selection coefficient, s = -0.15 when compared to the brown mice in a particular environment. Based on this
information, answer the following questions:
a. What does the selection coefficient a measure of? Is it a constant or relative term?
b. In this particular environment, which color of mouse would become more prevalent as time goes on?
c. If the environment changed, would the black mice still have the same selection coefficent compared to the brown mice?
6. If s=0.1 for a particular trait compared to not having that trait, how many generations would it take a population in which only 8/1000 had the trait to a point where 90% of the population had
the trait?
7. Why is it difficult to study “evolution in action”?
8. Despite the difficulties of studying most cases of “evolution in action” because of the reasons you answered in the previous question, there are some traits in some organisms that have such high
selection coefficients, it is easy to measure and watch. Describe the evolution of melanism in the peppered moth. Be sure to discuss the selection coefficient of the trait that was selected for,
what was it about the environment that caused s to get so high, and what were some of the agents of natural selection?
9. Researches studying the white-rump trait in pigeons hypothesized that the white-rump trait was selected for over the blue-barred trait because the white-rump feathers confused falcons as they
dove in for a mid-air attack against the pigeons, thus allowing white- rumped pigeons to avoid attacks more often. Answer the following questions regarding this hypothesis.
a. Although it wasn’t specifically mentioned in the book, state the two predictions the authors came up with regarding their hypothesis. Remember, scientific predictions are always in the form of,
“If I……., then I would expect to see……”
b. How did the authors test these two predictions (what experiment did they do)?
c. What were the results of these experiments?
10. The above examples rely on variation already being present in the population. For example, blue-barred and white-rump pigeon varieties already existed in the population. But how do variations
occur? For example, if there is a population of only-white furred mice, how does a new variant arise – like black furred mice? What is the source of the variation necessary for natural selection?
11. Of Mutation and natural selection, which is “blind” and which is not? Explain.
12. What are the four basic types of mutation? Which of these types leads to more genetic material?
13. The last universal common ancestor (LUCA), the first life form from which all species, extinct and extant, evolved had considerably less DNA in its genome than

humans. What is one of the ways discussed in the book that humans (and other organisms) got more genetic material (DNA)?
14. In your own words, list the four different situations in which a mutation will not cause serious harm to an organism.
15. The author runs through some math to determine the probability of a black-causing mutation to occur in a population of light-colored mice. How would this probability differ (increase, decrease,
or stay the same) when comparing light-colored mice living in a light-colored sandy environment with that of light-colored mice living in dark-colored environment? Explain.
16. Other than the number of base pairs in the genome, what factors can affect how long it takes for a particular mutation to appear in a population? Explain.
17. If we assume 10,000 mice are born in a population per year, how many years would it take a mutation to arise in the MC1R gene of a mouse to cause dark fur?
18. Once black-fur causing mutation arises, what determines how quickly (or slowly or not at all) it will spread in the population? Is this dependent on the environment in which the organism lives?
19. Assuming the selection coefficient for dark mice living on black lava flows is 0.01, how many generations of mice would it take for this entire population of mice to have dark-fur?
20. If lava flows never occurred and the mice only ever lived in light-colored sand, is it still possible for dark-furred mice to appear? Explain.
21. What is the biggest limitation on evolution, the amount of variation possible in a population or the ecology of a population?
22. Whenever a mutation occurs within a gene, a new version of the gene is created. These versions of genes are called alleles. Assume there are two different alleles of the X gene called allele
X-1 and allele X-2 that exist in a population. What are three possible relationships between X-1 and X-2 with regards to selection?
23. If all mutations within a gene were selectively neutral, that would mean that every possible allele of that gene should exist in a population in roughly equal frequencies. In reality, are all
alleles of a gene equally found within populations? Why or why not?

24. Humans show great variation in regards to tattoos. Some people have them, some people don’t. There is even great variation among people who have them in regards to the type, size, color, and
shape of the tattoo. Is the tattoo trait in humans subject to natural selection? Why or why not?

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