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Chapter 27

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University of Toronto Mississauga
Fiona Rawle

Notes From Reading CHAPTER 27: P HYLOGENIES AND THE H ISTORY OF LIFE(PGS.571-595) 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 Introduction - In biology we must consider profound changes in the nature of life on Earth over immense periods of time. - Let’s first look at two major analytical tools that biologists use to reconstruct the history of life: phylogenetic trees and the fossil record. 27.1 Tools for Studying History: Phylogenetic Trees - Phylogeny – the evolutionary history of a group of organisms - Phylogenetic tree – shows ancestor-descendant relationships among populations or species - An ancestor and all its descendants form a monophyletic group (also called a clade or lineage) Reading a Phylogenetic Tree - Branches represent populations through time. Adjacent branches are sister taxa (a taxon is any named group of organisms). - Tips are the tree’s endpoints and represent living groups or a group’s end in extinction - The names at the tips can represent species or larger groups - Nodes occur where an ancestral group split into two or more descendant groups - A polytomy is a node where more than two descendant groups branch off - In rooted phylogenies the most ancient node of the tree is shown at the bottom - The location of this node is determined using an outgroup, a taxonomic group that diverged before the rest of the taxa being studied How Do Researchers Estimate Phylogenies? - Phylogenetic trees are an extremely effective way of summarizing data on the evolutionary history of a group of organisms - Researchers analyze morphological and/or genetic characteristics to infer phylogenetic relationships among species. - There are two general strategies for using data to estimate trees: the phenetic and the cladistic approaches. Notes From Reading CHAPTER 27: P HYLOGENIES AND THE H ISTORY OF LIFE(PGS.571-595) - The phenetic approach is based on computing a statistic that summarizes the overall similarity among populations. - A computer program then compares the statistics for different populations and builds a tree that clusters the most similar populations and places more divergent populations on more distant branches. - The cladistic approach to 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 no others - Said another way, a synapomorphy is a trait that certain groups of organisms have that exists in no others. - When many such traits have been measured, traits unique to each monophyletic group are identified and the groups are placed on a tree in the appropriate relationship to one another. Synapomorphies Identify Monophyletic Groups Distinguishing Homology from Homoplasy - Problems can arise with both phenetic and cladistic analyses because similar traits can evolve independently in two distant species rather than from a trait present in a common ancestor. - The issue is that traits can be similar in two species not because those traits were present in a common ancestor, but because similar traited evolved independently intwo distantly related groups - Homoplasy occurs when traits are similar for reasons other than common ancestry. Figure 27.2a shows an example comparing the similar traits of dolphins and extinct marine reptiles called ichthyosaurs. - Homology occurs when traits are similar due to shared ancestry. Figure 27.2b shows an example using the Hox genes. - Convergent evolution occurs when natural selection favors similar solutions to the problems posed by a similar way of life, as shown by the dolphin and ichthyosaur. - Convergent evolution is a common cause of homoplasy. - If similar traits found in distantly related lineages are indeed similar due to common ancestry, then similar traits should be found in many intervening lineages on the tree of life. Notes From Reading CHAPTER 27: P HYLOGENIES AND THE H ISTORY OF LIFE(PGS.571-595) - Parsimony is a principle of logic stating that the most likely explanation or pattern is the one that implies the least amount of change. - Convergent evolution and other causes of homoplasy should be rare compared with similarity due to shared descent, so the tree that implies the fewest overall evolutionary changes should be the one that most accurately reflects what happened during evolution. Whale Evolution: A Case History - Traditionally, phylogenetic trees based on morphological data place whales outside of the artiodactyls—mammals that have hooves, an even number of toes, and an unusual pulley- shaped ankle bone (astragalus). - DNA sequence data, however, suggest a close relationship between whales and hippos. - This tree would require two changes to the astragalus trait. - Recent data on gene sequences called 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 (synapomorphies) and support the hypothesis that whales and hippos are indeed closely related. 27.2 Tools for Studying History: The Fossil Record - A fossil is the physical trace left by 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 provides the only direct evidence about what organisms that lived in the past looked like, where they lived, and when they existed. 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 fossils. - Fossilization is an extremely rare event. Limitations of the Fossil Record - There are several limitations of the fossil record that need to be recognized: habitat bias, taxonomic bias, temporal bias, and abundance bias. - Habitat bias occurs because organisms that live in areas where sediments are actively being deposited are more likely to form fossils than are organisms that live in other habitats. - Taxonomic bias is due to the fact that some organisms (e.g., those with bones) are more likely to decay slowly and leave fossil evidence. - Temporal bias occurs because more recent fossils are more common than ancient fossils. - Abundance bias occurs because organisms that are abundant, widespread, and present on Earth for a long time leave evidence much more often than do species that are rare, local, or ephemeral. Notes From Reading CHAPTER 27: P HYLOGENIES AND THE H ISTORY OF LIFE(PGS.571-595) - Paleontologists—scientists who study fossils—recognize that they are limited to studying tiny and scattered segments on the tree of life, yet they also know that this is the only way to get a glimpse of what extinct life was like. Life’s Time Line - Major events in the history of life are marked on the timeline shown in Figure 27.8, which has been broken into four segments. - The Precambrian encompasses the Hadean, Archaean, and Proterozoic eons. This period spans from the formation of the Earth to the appearance of most animal groups about 542 million years ago (mya). - In the Precambrian era, almost all life was unicellular and hardly any oxygen was present. - The interval between 542 mya and the present is called the Phanerozoic eon and is divided into three eras: the Paleozoic, the Mesozoic, and the Cenozoic. These eras are further divided into periods. - The Paleozoic era covers the interval from 542 to 251 mya. - Many animal groups—including fungi, land plants, and land animals—appeared in the Paleozoic era. This era ends with the obliteration of almost all multicellular life-forms at the end of the
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