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University of Toronto Mississauga
Angela B Lange

Chapter 27 Phylogenies and the History of Life Key Concepts o Phylogenies and the fossil record are the major tools that biologists use to study the history of life o The Cambrian explosion was the rapid morphological and ecological diversification of animals that occurred during the Cambrian period o 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 o Mass extinctions have occurred repeatedly throughout the history of life They rapidly eliminate most of the species alive in a more or less random mannerSection 271 Outline Tools for Studying History Phylogenetic TreesA Field Guide to Reading Phylogenetic Trees How Do Researchers Estimate Phylogenies How Can Biologists Distinguish Homology from Homoplasy Whale Evolution A Case HistoryThe evolutionary history of a group of organisms is called a phylogeny A phylogenetic tree shows ancestordescendant relationships among populations or speciesA Field Guide to Reading Phylogenetic TreesFigure 271 shows the parts of a phylogenetic treeBranches represent populations through time Adjacent branches are sister taxa a taxon is any named group of organismsNodes occur where an ancestral group split into two or more descendant groupsA polytomy is a node where more than two descendant groups branch offTips are the trees endpoints and represent living groups or a groups end in extinctionIn rooted phylogenies the most ancient node of the tree is shown at the bottomThe location of this node is determined using an outgroup a taxonomic group that diverged before the rest of the taxa being studiedAn ancestor and all its descendants form a monophyletic group also called a clade or lineageFigure 273 shows alternative phylogenetic trees for representing the same evolutionary relationshipsHow Do Researchers Estimate PhylogeniesMorphological andor genetic characteristics are used to estimate phylogenetic relationships among speciesThe phenetic approach to estimating trees is based on the overall similarity among populationsA tree is built that clusters the most similar populations and places more divergent populations on more distant branchesThe cladistic approach to inferring trees focuses on synapomorphies the shared derived characters of the species under studyA synapomorphy is a trait that certain populations or species have that exists in no othersWhen many such traits have been measured traits unique to each monophyletic group are identified and the groups are placed on a tree in the correct relationship to one another Figure 274How Can Biologists Distinguish Homology from HomoplasyProblems can arise with both cladistic and phenetic analysis because similar traits can evolve independently in two distant species rather than from a trait present in a common ancestorHomoplasy occurs when traits are similar for reasons other than common ancestry Figure 275aHomology occurs when traits are similar due to shared ancestry Figure 275bConvergent evolution occurs when natural selection favors similar solutions to the problems posed by a similar way of making a livingConvergent evolution is a common cause of homoplasyIf similar traits found in distantly related lineages are indeed similar due to common ancenstry then similar traits should be found in many intervening lineages on the tree of lifeParsimony is a principle of logic stating that the most likely explanation or pattern is the one that implies the least amount of changeConvergent evolution and other causes of homoplasy should be rare compared with similarity due to share descent so the tree that implies the fewest overall evolutionary changed should be the one that most accurately reflects what really happened during evolutionWhale Evolution A Case HistoryTraditionally phylogenetic trees based on morphological data place whales outside of the artiodactylsmammals that have hooves an even number of toes and an unusual pulleyshaped ankle bone astralagus Figure 277aDNA sequence data however suggest a close relationship between whales and hippos Figure 277bRecent data on short interspersed nuclear elements SINEs show that whales and hippos share several SINE genes that are absent in other artiodactyl groups Figure 277cThese SINEs are shared derived traits synapomorphies and support the hypothesis that whales and hippos are indeed closely relatedSection 272 Outline Tools for Studying History The Fossil RecordHow Do Fossils Form Limitations of the Fossil Record Lifes TimelineThe fossil record provides the only direct evidence about what organisms that lived in the past looked like where they lived and when they existedA 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 worldHow Do Fossils FormMost fossils form when an organism is buried in sediment before decomposition occursFour types of fossils are intact fossils compression fossils cast fossils and premineralized fossilsFossilization is an extremely rare eventLimitations of the Fossil RecordThere are several features and limitations of the fossil record that must be recognized habitat bias taxonomic bias temporal bias and abundance biasHabitat 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 habitatsTaxonomic bias is due to the fact that some organisms eg those with bones are more likely to decay slowly and leave fossil evidenceTemporal bias occurs because more recent fossils are more common than ancient fossilsAbundance 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 ephemeralPaleontologistsscientists who study fossilsrecognize that they are limited to asking questions about tiny and scattered segments on the tree of lifeYet analyzing fossils is the only way scientists have of examining the physical appearance of extinct forms and inferring how they livedLifes TimelineMajor events in the history of life are marked on the timeline shown in Figure 2710 which has been broken into four segments the Precambrian the Paleozoic the Mesozoic and the CenozoicThe Precambrian era encompasses the Hadean Archaean and Proterozoic eonsIn the Precambrian era almost all life was unicellular and hardly any oxygen was presentMany animal groupsincluding fungi land plants and land animalsappeared in the Paleozoic eraThe Mesozoic era also known as the Age of Reptiles ended with the extinction of the dinosaursThe Cenozoic era is known as the Age of Mammals Section 273 Outline The Cambrian ExplosionCambrian Fossils An Overview The Doushantuo Microfossils The Edicaran Faunas The Burgess Shale Faunas Did Gene Duplication Trigger the Cambrian ExplosionAnimals first originated around 565 million years ago Mya Soon after that animals diversified into almost all the major groups extant today This is known as the Cambrian explosionCambrian Fossils An OverviewThe Cambrian explosion is documented by three major fossil assemblages Figure 2712aThe presence of these exceptionally rich deposits before during and after the Cambrian explosion makes the fossil record for this event extraordinarily complete The Doushantuo MicrofossilsResearchers identified sponges cyanobacteria and multicellular algae in samples dated 570580 Mya They also found what they concluded were animal embryos in early stages The Edicaran FaunasSponges jellyfish comb jellies and traces of other animals dated 544565 Mya are found in fossils from the Ediacara Hills of Australia
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