A Case of Natural Selection 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
By the end of this case, you will:
• Understand the process of natural selection and the importance of environment-speciﬁc adaptations.
• Be able to use the terms variation, adaptation, natural selection, and evolution as they apply to this and other scientiﬁc studies.
• Gain experience with the scientiﬁc method and be able to propose hypotheses and justiﬁcations 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 ﬁgures and tables.
PART I—“I’M LOOKING OVER…”
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. The 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. The 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