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Unformatted text preview: Im Looking Over a White-Striped Clover by Evarts, Krufka, & Wilson Page Case Objectives By the end of this case, you will: Understand the process of natural selection and the importance of environment-specifi c adaptations. Be able to use the terms variation, adaptation, natural selection, and evolution as they apply to this and other scientifi c studies. Gain experience with the scientifi c method and be able to propose hypotheses and justifi cations to explain the distribution of the two variants of white clover. Design experiments to test hypotheses and describe data that would support these hypotheses. Understand and synthesize information in fi gures and tables. P ART II M L OOKING O VER White clover (Trifolium repens) , a small perennial plant, is found throughout the world, and has two forms. One variant has entirely green leaves (plain) and the other has green leaves with a prominent white stripe (striped). Both variants of white clover (plain and striped) are found along the coast of Long Island, New York. Most of Long Island is only a few feet above sea level. A series of low grass-covered hills separated by shallow depressions covers the area behind the oceanfront dunes. Th e shallow depressions reach to the water table, so they tend to be permanently moist year round and do not freeze in winter. Water drains away quickly from the low hills, which tend to dry out many times over the year and freeze in the winter. Th e habitat in the shallow depressions is more hospitable to molluscs (snails and slugs) that feed on clover. One type of clover is more common in shallow depressions while the other type is more likely to be found on low hills. At the end of the case, we will come back to New York and ask you to predict which type of white clover is most abundant in each microhabitat. But fi rst, lets consider the abundance of these two types of clover on a larger scale. Figure , below, shows the relative frequency of white clover variants in Minnesota and North Carolina. by Susan Evarts , Department of Biology, University of St. Thomas Alison Krufka , Department of Biological Sciences, Rowan University Chester Wilson , Department of Biology, University of St. Thomas Im Looking Over a White-Striped Clover: A Case of Natural Selection Im Looking Over a White-Striped Clover by Evarts, Krufka, & Wilson Page Table provides additional information on Minnesota and North Carolina. Table 1 Minnesota North Carolina Latitude 4349 N 3436 N Mean elevation 0.365 km 0.213 km Ave. monthly temp. range 19.4 to 28.6 C 2.6 to 31.3 C High temperature 45.6 C 43.3 C Low temperature 51 C 37 C Mean # days with high above 32 C* 14 38 Mean # of days with low below 0 C* 154 75 Ave. yearly precipitation 6676 cm 107117 cm Presence of herbivores (molluscs such as snails, slugs) smaller population, not present in winter larger, more active population,...
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