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Chapter 27

Chapter 27 Notes - Phylogenies and the History of Life

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Christoph Richter

Notes From Reading CHAPTER 27:P HYLOGENIES AND THE H ISTORY OF LIFE Key Concepts  Phylogenies and the fossil record are the major tools that biologists use to study the history of life.  The Cambrian explosion was the rapid morphological and ecological diversification of animals that occurred during the Cambrian period.  Adaptive radiations are a major pattern in the history of life. They are instances of rapid diversification associated with new ecological opportunities and new morphological innovations.  Mass extinctions have occurred repeatedly throughout the history of life. They rapidly eliminate most of the species alive in a more or less random manner. 27.1 Tools for Studying History: Phylogenetic Trees  Phylogeny – the evolutionary history of a groups of organisms o Are usually depicted in the form of a phylogenetic tree  Phylogenetic Tree – shows the ancestor-descendant relationships among populations or species, ad clarifies who is related to whom  Branch – represents a population through time  Node – the point where two branches diverge (also known as a fork) o Represents the point in time when an ancestral group split into two or more descendant groups  Tip – the endpoint of a branch o Represents a group (a species or larger taxon) living today or one that ended in extinction How Do Researchers Estimate Phylogenies?  Phylogenetic trees are an extremely effective way of summarizing data on the evolutionary history of a group of organisms  The fundamental idea in phylogeny inference is that closely related species should share many of their characteristics, while distantly related species should share fewer characteristics  There are two strategies used to estimate trees: phenetic and the cladistic approach  Phenetic Approach – estimating trees based on the overall similarity among populations  A tree is built that clusters the most similar populations and places more divergent populations on more distant branches  Cladistic Approach – inferring trees focuses on synapomorphies, the shared derived characters of the species under study  A synapomorphy is a trait that certain groups of organisms have that exists in o others  Synapomorphies allow biologists to recognize monophyletic groups (also called clades or lineages) How Can Biologist Distinguish Homology from Homoplasy?  The issue is that traits can be similar in two species not because those traits were present in a common ancestor, but because similar traits evolved independently in two distinctly related groups  Homoplasy – occurs when traits are similar for reasons other than common ancestry  Homology – occurs when traits are similar due to shared ancestry  Convergent Evolution – occurs when natural selection favors similar solutions to the problems posed by a similar way of making a living Notes From Reading CHAPTER 27:P HYLOGENIES AND THE H ISTORY OF LIFE  Convergent evolution is a common cause of homoplasy  If similar traits should be found in many intervening lineages on the tree of life  Parsimony – is a principle of logic stating that the most likely explanation or pattern is that one that implies the least amount of change  Convergent evolution and other causes of homoplasy should be rare compared with similarity due to share decent  So the tree that implies the fewest overall evolutionary changed should be the one that most accurately reflects what really happened during evolution Whale Evolution: A Case History  Traditionally, phylogenetic trees based on morphological data place whaes outside of the artiodactyls – mammals that have hooves, an even number of toes, and an unusual pulley-shaped ankle bone  DNA sequence data, however, suggests a close relationship between whales and hippos  Recent data on short interspersed nuclear elements (SINEs) show that whales and hippos share several SINE genes that are absent in other artiodactyl groups  These SINEs are shared derived traits (snapomorphies) and support the hypothesis that whales and hippos are indeed closely related 27.2 Tools for Studying History: The Fossil Record  Fossil record provides direct evidence about what organisms that lived in the past looked like, where they lived and when they existed  Fossil – a piece of physical evidence from an organism that lived in the past  Fossil Record – the total collection of fossils that have been found throughout the world How Do Fossils Form?  Most fossils form when an organism is buried in sediment before decomposition occurs  Four types of fossils are intact fossils, compression fossils, cast fossils and premineralized
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