Phylogeneticslabexercise03

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Unformatted text preview: Species Identification and Phylogenetic Inference Species Classification Background: A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, and fish. Keys consist of a series of “either or” choices that lead the user to the correct name of a given item. A dichotomous key is a listing of characteristics, such as structure and behavior, organized in such a way that an organism can be identified or classified. Dichotomous means "divided into two parts". Therefore, dichotomous keys always give two choices in each step. For example, a question in a dichotomous key for trees might be something like, "Are the leaves flat or needle ­like?" If the answer was "needle ­like," then the next question might be something like, "Are the needles in a bunch or are they spread along the branch?" Eventually, when enough questions have been answered, the identity of the tree is revealed. This lab uses the identification of some common types of North American fish as an example of how to use a dichotomous key. Phylogenetic Reconstruction The process of evolution has generated the diversity of life forms we see all around us. As we study evolution, scientists often ask questions pertaining to the origins of new species or the process of speciation. Over time spans of hundreds of thousands or millions of years, a series of speciation events can lead to the development of new groups of organisms, all derived from a single common ancestor. The sum of these speciation events for a group of organisms is called a phylogeny. The phylogeny of a group represents all of the events starting with a single common ancestor that led to the development of each of the members or taxa within that group. The taxa could be species within a genus or within a family. Scientists show the evolutionary relationships by drawing phylogenetic trees. If you trace the process of speciation on paper, you end up with a branching pattern referred to as a cladogram. Each branch in the tree represents a speciation event, when one species diverged into two different species. A cluster of branches within the cladogram is often referred to as a clade. The clade simply represent...
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