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1MO3_Chapter 27.docx

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Phylogenies and the History of Life Key ConceptsPhylogenies and the fossil record are major tools that biologists use to study the history of lifeThe Cambrian explosion was the rapid morphological and ecological diversification of animals that occurred during the Cambrian periodAdaptive radiations are a major pattern in the history of life They are instances of rapid diversification associated with new ecological opportunities and new morphological innovationsMass extinctions have occurred repeatedly throughout the history of life They rapidly eliminate most of the species alive in a relatively unbiased manner IntroductionIn biology we must consider profound changes in the nature of life on Earth over immense periods of time Now looking at macro evolutionary processLets first look at two major analytical tools that biologists use to reconstruct the history of life phylogenetic tress and the fossil record Tools for Studying History Phylogenetic TreesThe evolutionary history of a group of organisms is called o phylogenyA phylogenetic tree shows ancestordescendants relationships among populations or species o The diagram provides an extremely effective way to summarise data according to criteria such as distance parsimony likelihood Bayesianism and may be interpreted as depicting the evolutionary history for the groupAn ancestor and its descendants from a monophyletic group also called a clade or lineage Bio skills 2 Reading a Phylogenetic TreeBranches represent groups through time Adjacent branches are sister taxa a taxon is any named group of organismsTips are the trees endpoints and represent living groups or a groups end in extinctionThe names at the tips can represent populations or higher taxaNodes occur where an ancestral group split into two or more descendant groups an inferencePolytomy is a node where more than two descendant groups branch off an inferenceIn rooted phylogenies the most ancient node of the tree is shown at the bottom an inferenceThe location of this node is determined using an outgroup a taxonomic group that is diverged before the rest of the taxa being studied How Do Researchers Estimate PhylogeniesPhylogenetic trees are an extremely effective way of summarizing data on the evolutionary history group of organisms Researchers analyze morphological andor genetic characteristics to infer phylogenetic relationships among species Other traits may be usedFor example to reconstruct relationships among fossil species of humans scientists analyze toot jaw And skull structures There are two general strategies for using data to estimate trees the phonetic and the cladistics approaches o Actually 4 distance parsimony likelihood Bayesianism The phenetic approach is based on computing a statistic that summarizes the overall similarity among populationsFor example researchers might use gene sequences to compute an overall genetic distance between two populations A genetic distance summarizes the average percentage of bases in a DNA sequence that differs between two 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 branchesThe cladistics approach to inferring trees focuses on synapomorphies the shared derived characters of the species under study o A synapomorphy is a trait that certain groups of organisms have that exists in no others Synapomorphies allow biologists to recognize monophyletic groups For example fur and lactation are synapomorphies that identify mammals as a monophyletic group 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 Distinguishing Homology from HomoplasyHomology samesource occurs when traits are similar due to shared ancestry Problems can arise with both phonetic and cladistics analyses because similar traits can evolve independently in two distant species rather than from a trait present in a common ancestoro For example Species 2 is not at all related to species Its ancestors may have had the sequence TAT GGT AGT which happened to change to AAC GCT ACT due to mutation selection and drift that took place independently of the changes that took place in the ancestors of species 1Homoplasy sameform occurs when traits are similar for reasons other than common ancestry Figure 272a shows an example comparing the similar traits of dolphins and extinct marine reptiles called ichthyosaurs Many other animals
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