BISC 495, Evolution, Spring 2017

Study guide for the first exam

The first exam will be Thursday, March 16, and will be worth 15 percent of your grade for the course. If you will be absent that day, e-mail me by the end of the day on Friday, March 10, so that we can schedule your makeup exam.

You may not use your notes during the exam. You will not need a calculator.

The exam will consist of 20 questions. Each of your answers should consist of one to a few sentences. You may include drawings in your answers if they help you make your point. While there is no strict length limit, you may get points off for writing long, rambling answers that say the same thing over and over.

Here are some practice questions. Try answering them, then highlight the invisible text inside the red box (drag your cursor across it) to see my answer.

1. McDonald (2013) studied the Gpi gene in the amphipod crustacean Megalorchestia californiana on the Washington, Oregon and California coasts. He found that the Gpi100 allele was very common in southern California and became less common as you go further north, and he concluded that cold temperatures selected against Gpi100. There is another possible explanation for the difference in allele frequency between the locations that doesn’t involve selection; what is it and how would it give these results?

Answer: Random genetic drift could give these results if the frequency of Gpi100 drifted up in an isolated southern population, or if it drifted down in an isolated northern population.
--->[If a question asks for one answer, and you can think of more than one possible answer, just put down the one that you think is most likely to be correct. You won't get extra credit for having both correct answers if only one was asked for, and if one of your answers is right and one is wrong, you'll get graded based on the wrong answer. For example, if you wrote: "Random genetic drift could give these results if the frequency of Gpi100 drifted up in an isolated southern population, or if it drifted down in an isolated northern population. It could also be that the warmer temperatures in the southern population caused the gene to mutate to the Gpi100 allele", you would get zero points, because the second half of your answer is very wrong.]

2. In one of the papers you read, Kong et al. (2012) estimated the mutation rate in humans. How did they do this? They found that the rate of new mutations varied among individuals; what was the main reason that some individuals had more new mutations than others?

Answer: They determined the DNA sequence of the genomes of parents and offspring; any allele that was present in the offspring, but not in either parent, was a new mutation. They found that offspring with older fathers had more new mutations that offspring with younger fathers.
--->[Kong et al. (2012) is one of the assigned readings; this question makes sure you've done the reading. There will be several questions based on the readings.]

3. You want to become a world's expert on random genetic drift in island populations of rats. What are the three most important things you would do to find scientific literature about this topic?

Answer: 1. Do a topic search for in the Web of Science, searching for something like genetic drift island rat.
2. Pick some of the articles from the topic search and look at their reference lists for good older papers.
3. Pick some of the best papers from steps 1 and 2 and use the Web of Science to find more recent papers that cite them.
--->[Note that if a question asks for more than one answer, be sure to give the correct number of answers. Numbering the answers, as I've done here, may help you keep track of this.]

4. The spadefoot toad, Scaphiopus couchii, lives in southern New Mexico. Some toads live on dark rock and are dark colored; some live on white sand and are light colored. You want to know whether this difference in color is genetic. You don’t know whether any genes are involved, much less which genes. What experiment could you do to tell whether the difference in color between populations of toads from dark rock vs. white sand is genetic? What would the result be if the difference is not genetic?

Answer: In a lab, raise some offspring of dark-colored toads in a dark-colored environment and some in a light-colored environment. Do the same for the offspring of light-colored toads. If the trait is not genetic, toads will match the color of their environment, not their parents.

5. In one of the papers you read, Berry et al. (1991) measured DNA sequence polymorphism on the fourth chromosome of two species of Drosophila and found that the fourth chromosome had much less polymorphism than other chromosomes. What is this evidence of? Why?

Answer: This is evidence for recent positive selection; because there is no recombination on the fourth chromosome, any positively selected mutation will "sweep" through the population, and all of the fourth chromosomes will be identical until new mutations occur.

6. Some people have wet, sticky earwax; other people have dry, crumbly earwax. This is caused by variation at a single gene with two alleles. The allele for wet earwax (W) is dominant over the dry allele (d). Right now, the frequency of W is about 90% in the United States. What do you expect to happen to the frequency of W in the United States over the next 100 years? Why?

Answer: I would expect the frequency of W to stay about the same, because even if there is selection on the earwax gene, I don't think it would be very strong. (If you say the frequency will stay exactly the same, you get a little off, because there's always random drift. If you say W will go up because it's dominant, you get 0 points and I will throw an eraser at you.)

7. Flies in the species Drosophila heteroneura have their eyes on the end of long stalks. Some individuals have longer stalks than others. Without using any kind of DNA data, what kind of experiments would you do to tell whether this variation in stalk length within a population is genetic? What result would you get if the variation is mostly genetic?

Answer: See how much variation there is among relatives (siblings, parents/offspring, etc.) compared to the variation among the total population. If the trait is mostly genetic, flies will be very similar to their relatives in stalk length.
Or rear the flies for several generations in the lab, impose strong selection (such as only let the flies with the longest stalks survive), and see how fast the average stalk length changes. If the trait is mostly genetic, the average length will change quickly.

8. Janet Weiss of Denton University is studying a genetic polymorphism in cats, the long hair locus. Cats with the dominant L allele have short hair, while cats that are homozygous for the recessive l allele have long hair. She observes the hair length (long or short) on 100 cats from Hockessin, Delaware and estimates the allele frequency of l is 0.60. How did she do that? (You don’t have to do the calculations, just describe how the basic principle she used works.)

Answer: She used the Hardy-Weinberg principle: the allele frequency of the recessive l allele is the square root of the frequency of the recessive homozygote phenotype (long haired cats).

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