1323380835_CHAPTER_4_SPR

1323380835_CHAPTER_4_SPR - EVOLUTION/LECTURE1 Evolution...

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EVOLUTION/LECTURE1 file:///E|/CH4-TREES-SPRING-2008/CHAPTER%204_SPR_2008.HTML[12/8/2011 2:26:54 PM] Evolution ( PCB 4674 ). Chapter 4. Reconstructing evolutionary trees Main topics of lecture: I: Parsimony and phylogeny: II: The phylogeny of whales: 1.- Are whales closely related to hippopotamuses? 2.- Choosing characters: Morphology and molecules 3.- Alternatives to parsimony: Maximum likelihood and genetic distances III: Using phylogenies to answer questions: 4.- Classifying the diversity of life 5.- Coevolution 6.- The spread of AIDS I: Parsimony and phylogeny: I.1.- The evolutionary history of a group is called its phylogeny . A phylogenetic tree is a GRAPHICAL SUMMARY of this history. The tree describes the pattern, and in some cases the timing of branching events. It records the sequence of speciation and documents which taxa are more closely or distantly related I.2.- Because we do not have direct knowledge of evolutionary history, a phylogeny must be inferred indirectly from data !!!! . As its most basic level, the logic of estimating evolutionary relationships is simple: The most closely related taxa should have the most traits in common. We can say that the traits that we can use to reconstruct a phylogeny should be: i.- Independent of one another ii.- Heritable iii.- Variable among the taxa Examples of characters are DNA sequences, presence or absence of certain skeletal elements or flower parts. I.3.- In choosing characters to study, however, IT IS ESSENTIAL !!!
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EVOLUTION/LECTURE1 file:///E|/CH4-TREES-SPRING-2008/CHAPTER%204_SPR_2008.HTML[12/8/2011 2:26:54 PM] to realize that evolutionary relationships are ONLY!!! revealed by traits that are similar because they were derived from a common ancestor. Traits that are shared by a group of taxa because they were modified in a common ancestor are called synapomorphies . Mammals for example share fur and lactation as traits that were derived from their common ancestor (a population from a long-extinct group called the synapsids ) I.4.- Two ideas are key to understand the value of synapomorphies in phylogenetic reconstruction: i.- Synapomorphies identify evolutionary branch points because after a species splits into two lineages that begin evolving independently, some of their homologous traits undergo changes due to mutation, selection, and drift. These changed traits are synapomorphies that identify populations in the two independent lineages ii.- Synapomorphies are nested. That is, as you move back in time and trace an evolutionary tree backward from its tips to its root, each branching event adds one or more synapomorphies I.5.- Figure 13.1 shows how a series of these shared derived character states (that we have defined as synapomorphies) defines phylogenetic relationships among groups of birds that lived during the Mesozoic era. These synapomorphies identify trends in changes in several bones that undoubtedly are adaptations for flight. Figure 13.1.: Phylogeny of bird groups from the Mesozoic era.
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1323380835_CHAPTER_4_SPR - EVOLUTION/LECTURE1 Evolution...

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