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Chapter 20 The Macroevolutionary Puzzle

Chapter 20 The Macroevolutionary Puzzle - Chapter 20 The...

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Chapter 20 The Macroevolutionary Puzzle I. On Floods and Fossils A. Traditional explanations of rock formations and fossils relied on effects of the Deluge. B. Modern geologists view the same evidence as showing changes in geology and living organisms through time. II. Evidence of Macroevolution A. Macroevolution refers to the large-scale patterns, trends, and rates of change among higher-taxa groupings of species. 1. Evolution proceeds by modifications of organisms that already exist. 2. “New” species emerge as mutation, natural selection, and genetic drift change allele frequencies in reproductively isolated populations. B. The Fossil Record 1. A fossil is recognizable, physical evidence of an organism that lived long ago— skeletons, shells, leaves, seeds, tracks. a. For fossil formation, body parts or impressions must be buried in rock before decomposition. b. Fossil records vary according to type of organism (hard parts preserve well, soft parts do not), stability of the geographical region (sea floor vs. eroding hill), and quality of the specimen. 2. The fossil record is far from complete, but some lineages are extensive. C. Dating Fossils 1. Deepest rock strata are assumed to be the oldest, surface layers the youngest. 2. Fossils in these layers were the basis for dividing earth history into four eras: Proterozoic, Paleozoic, Mesozoic, Cenozoic (further research has added Archean before Proterozoic). 3. Radioactive dating methods have been used to assign dates to these eras. D.Comparative Morphology 1. Evolutionary history is reconstructed on the basis of observed patterns of body forms. 2. Stages of Development a. Different organisms may show similarities in morphology during their embryonic stages that often indicate evolutionary relationships. b. Such similarities are one of the reasons why fishes, amphibians, reptiles, birds, and mammals are said to belong to the same phylum. c. Some of the variation seen in adult vertebrates is due to mutations in regulatory genes that control the rates of growth of different body parts. 3. Homologous Structures a. These are body features that resemble one another in form or patterning due to descent through common ancestors.
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b. In morphological divergence, features have departed in appearance and/or function from the ancestral form (example: bones in forelimbs of vertebrates).
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