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BGY Chapter_4.doc

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University of Toronto Scarborough
Biological Sciences

BGYB 51 Chapter 4 (pg.111-119) Estimating Evolutionary Trees The evolutionary history of a group of species is called it phylogeny and a phylogenetic tree is a graphical summary of this history. An evolutionary tree describes the patter, and in some cases the timing, of events that occurred as species diversified. It also records the sequence in which lineages appeared and documents which organisms are more closely or distantly related. 4.1. The Logic of Phylogeny Inference • The most closely related taxa should have the most traits in common • Many types of characters could qualify: the sequence of nucleotides in a particular gene, the presence or absence of specific skeletal elements or flower parts, or the mode of embryonic or larval development • Synapomorphies Identify Monophyletic Groups o The most fundamental principle of phylogeny inference is that only certain types of homologous characters are useful in estimating phylogenetic trees o A Synapomorphy is a homologous trait that is shared among certain species and is similar because it was modified in a common ancestor  They are shared, derived traits o Any group that includes an ancestor and all of its descendants is called a monophyletic group (or clade or lineage)  This means all synapomorphies are homologous traits but not all homologous traits are synapomorphies  For example, the genetic code helps identify bacteria and mammals (eukaryotes) as members of the same monophyletic group (homologous trait) but it does not help us distinguish bacteria from eukaryotes • Bacteria and mammals each have synapomorphies that identify them as distinct monophyletic groups • All bacteria have cell walls that contain pepitdoglycan (one synapomorphy) while all eukaryote cells contain a nuclear envelope (another Synapomorphy) o Two ideas are key to understanding why evolutionary relationships can be inferred by analyzing synapomorphies  Synapomorphies identify evolutionary branch points (speciation) • Speciation starts when two populations become genetically isolated • During speciation some of the homologous traits of the two independently developing species undergo changes due to mutation, selection and drift. • These changes become synapomorphies distinguishing the two new populations  Synapomorphies are nest • As you move through time and trace a tree from its root to its tips, each branching event adds one or more shared, derived traits • The hierarchy described by synapomorphies also describes
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