27_Biological_Science_4e_IG

27_Biological_Science_4e_IG - Freeman Biological Science 4e...

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Freeman, Biological Science, 4e, Chapter 27 1 27 - Phylogenies and the History of Life Learning Objectives: Students should be able to ... Draw and interpret phylogenetic trees, and understand how they depict specific hypotheses of evolutionary relatedness. Describe how fossils form. List the major strengths and limitations of the fossil record. Name the three major eras of the Phanerozoic, and list the major evolutionary events and dominant taxa characteristic of each era. Describe why adaptive radiations occur, and give several examples. Explain the difference between mass extinction and background extinction. Describe what is (and isn't) known about the causes of the end-Cretaceous and end-Permian mass extinctions. Lecture Outline I. Tools for Studying History: Phylogenetic Trees A. Tree basics 1. A phylogeny is an evolutionary history of a group of organisms. 2. A phylogenetic tree diagrams the evolutionary relationships among a group of organisms. 3. Reading phylogenetic trees (see BioSkills 3 in Appendix A) a. Branches represent populations through time. b. The tips of the branches represent specific taxa. c. Nodes (forks) occur when an ancestral group split into multiple descendent groups. B. How do researchers estimate phylogenies? 1. Closely related species should share many characteristics, while more distantly related species should share fewer characteristics. 2. The phenetic approach is based on the overall level of similarity among populations. 3. The cladistic approach is based on synapomorphies (shared derived traits) that identify monophyletic groups. (Fig. 27.1) a. Ancestral and derived traits are relative. 4. The phenetic approach includes all traits in its calculation of overall similarity, but the cladistic approach uses only shared derived traits. C. How can biologists distinguish homology from homoplasy? 1. Homology is similarity in a trait due to common ancestry. 2. Homoplasy is similarity in a trait for some reason other than common ancestry, such as convergent evolution. a. Convergent evolution occurs when natural selection favors similar solutions to the problems posed by a similar way of making a living. Example: body form in dolphins and ichthyosaurs. (Fig. 27.2) 3. Evidence for homology: Hox genes in insects and invertebrates.
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