Chapter 26 - Chapter 26 Phylogenies and the History of Life...

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Chapter 26: Phylogenies and the History of Life Life has existed on Earth for some 3.8 billion years 26.1 TOOLS FOR STUDYING HISTORY: PHYLOGENIES AND THE FOSSIL RECORD Biologists have two tools to use when studying the past: phylogenies and the fossil record A phylogeny is the evolutionary history of a group of organisms, and it is usually summarized and depicted in the form of a phylogenetic tree A phylogenetic tree shows the ancerstor-descendant relationship among populations or species, just as a genealogy or family tree shows the ancestor-descendant relationships among a group of individuals A fossil , in contrast, is a physical trace of an organism that lived in the past The fossil record is the total collection of fossils that have been found throughout the world The fossil record is housed in thousands of private and public collections Fossils document what organisms looked like in the past and when various groups existed Phylogenies clarify who is related to whom Using Phylogenies How do biologists read a finished tree, and how are trees put together in the first place? A Field Guide to Reading Phylogenetic Trees They are an effective way of summarizing data on the evolutionary history of a group of organisms They are unusual diagrams, however, and it can take practice to interpret them correctly Note that a phylogenetic tree consists of branches, nodes, and tips Branches represent populations through time Nodes occur where an ancestral group splits into two or more descendant groups If more than two descendant groups emerge from a node, the node is called a polytomy Tips are the tree’s endpoints, which represent groups living today or a dead end—a branch ending in extinction Groups that occupy adjacent branches on the tree are called sister taxa A taxon is any named group of organisms and can be a single species, Homo sapiens, or a large group of species, such as Primates They phylogenetic trees in this text are all rooted —meaning the bottom, or most basal, node on the tree is the most ancient To determine where the root on the tree occurs, biologists include an outgroup species when they are collecting data to estimate a particular phylogeny An outgroup is a taxonomic group that is known to have diverged prior to the rest of the taxa in the study A monophyletic group consists of an ancestral species and all of its descendants. Monophyletic groups may also be called clades or lineages
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If estimating the past 5-6 million years, you might use chimpanzees or gorillas as an outgroup Chimps and gorillas would then be the first groups to have split off in the resulting tree, with the various species of humans forming a monophyletic group descended from the common ancestor at the tree’s bottom node How Do Researchers Estimate Phylogenies?
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