Abstract Patterns of geographic variation in phenotype or genotype may provide evidence for natural selection. Here, we compare phenotypic variation in color, allele frequencies of a pigmentation gene (the melanocortin-1 receptor, Mc1r), and patterns of neutral mitochondrial DNA (mtDNA) variation in rock pocket mice (Chaetodipus intermedius) across a habitat gradient in southern Arizona. Pocket mice inhabiting volcanic lava have dark coats with unbanded, uniformly melanic hairs, whereas mice from nearby light-colored granitic rocks have light coats with banded hairs. This color polymorphism is a presumed adaptation to avoid predation. Previous work has demonstrated that two Mclr alleles, D and d, differ by four amino acids, and are responsible for the color polymorphism: DD and Dd genotypes are melanic whereas dd genotypes are light colored. To determine the frequency of the two Mclr allelic classes across the dark-co1ored lava and neighboring light-colored granite, we sequenced the Mclr gene in 175 individuals from a 35-km transect in the Pinacate lava region. We also sequenced two neutral mtDNA genes, COIII and ND3, in the same individuals. We found a strong correlation between Mclr allele frequency and habitat color and no correlation between mtDNA markers and habitat color. Using estimates of migration from mtDNA haplotypes between dark- and light-colored sampling sites and Mclr allele frequencies at each site, we estimated selection coefficients against mismatched Mclr alleles, assuming a simple model of migration-selection balance. Habitat-dependent selection appears strong but asymmetric: selection is stronger against light mice on dark rock than against melanic mice on light rock. Together these results suggest that natural selection acts to match pocket mouse coat color to substrate color, despite high levels of gene flow between light and melanic populations. Show
Journal Information Evolution, published for the Society for the Study of Evolution, is the premier publication devoted to the study of organic evolution and the integration of the various fields of science concerned with evolution. The journal presents significant and original results that extend our understanding of evolutionary phenomena and processes. Rights & Usage This item is part of a JSTOR Collection. This page is a draft and is under active development. 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. You can find populations of rock pocket mice all over the Sonoran Desert in the southwestern United States. There are two common varieties—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 landscape consists of light-colored sand and rock, but patches of dark volcanic rocks that formed from cooling lava flows are found, separated by several kilometers of light colored substrate. Activity 2Watch the video at biol.co/rockpock and answer the questions. 1. How caused the unusual landscape at the Valley of Fire?
2. Predators of the pocket mice hunt using what sense?
3. Why did dark-colored rock pocket mice first appear in a population of light-colored rock pocket mice?
4. Why do dark-colored rock pocket mice on dark lava flows have white bellies?
5. Mutations are always
6. Dark pocket mice are found in locations that have dark substrate. Genetics revealed what surprising fact about mice in different locations?
7. Now that you have watched the video, go back to your set of cards and arrange them in the order you think they happened, starting with the oldest. You may change your order from your original idea. Once you are satisfied with the order, complete the table.
8. How did you revise your sequence from your initial idea? Explain how you decided on the sequence. Activity 3: Data AnalysisUse colored pencils to make a BAR GRAPH showing the numbers and colors of mice at each location. Be sure to label the X and Y axis and each of the bars on the graph. Activity 4: Summarize the Data1. Compare how the graph looks at Location A to how it looks at Location B. What is the obvious difference between the two? 2. Explain why a rock pocket mouse color influences its overall fitness. Remember that “fitness” is defined by an organism’s ability to survive and produce offspring. 3. Explain the presence of dark-colored mice at location A. Why is this phenotype (appearance) not more common in the population at that location? 4. What is meant by this statement: “Mutation is random, but natural selection is not random.” Additional Discussion Questions1. Consider the dark and light colored rock pocket mice. Human skin color also comes in different shades. One hypothesis is that dark skin protected humans from intense UV radiation (sunlight). Based on this hypothesis, where do you think more darker skinned people lived? Suggest a reason why light skin may be an advantage for people living in other regions. 3. Animals develop many types of defenses to avoid being eaten. Camouflage is just one type of defense. What are other types of defenses that prey species might use? 4. Predators must be able to obtain food and overcome the defenses of prey. How do predator species in the desert survive? What adaptations do they have to make them good hunters? **Other Animals that Model Natural Selection - For discussion in Class
Evolution by Natural Selection - VIDA Chart - In the evidence section, write a list of ways in which the rock pocket mouse illustrates the idea posed in the description. Use what you have learned in the activity and the film we watched on rock pocket mice. ~ 5-10 min
What caused the dark variation in the rock pocket mice?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.
How did rock pocket mice evolve to have dark fur when they are living on rock lava flows?Why did dark-colored rock pocket mice first appear in a population of light-colored rock pocket mice? Individuals change color to blend in with the environment. There is dark lava rock in the area where they live. They have a genetic mutation that affects their fur color.
What caused the difference in fur color in the rock pocket mice?Fur color is controlled by many genes (4:29). Most genes are identical, but dark and light rock pocket mice differ in one gene (Mc1r; 4:55). Data from Data Set 2 show that a mouse's genotype for the MC1R gene affects their fur color. Mice with two copies of allele 2 have the darkest fur.
Why are darkRock pocket mice are generally light-colored and live on light-colored rocks. However, populations of dark (melanic) mice are found on dark lava, and this concealing coloration provides protection from avian and mammalian predators.
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