Lecture 6

Lecture 6 - Lecture 6 Phylogenetic trees Reading Ch 25...

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1 Lecture 6 Phylogenetic trees Reading Ch 25 Phylogentics and Systematics I. What is a phylogeny? II. How to read trees? III. How to build trees? A. characters B. reasons for similarities 1. homology 2. homoplasy D. tree building techniques 1. outgroup comparisons 2. parsimony 3. maximum likelihood IV. Is there such a thing as a fish? V. Using phylogenies in hypothesis testing I. Phylogeny = estimated evolutionary history of a group of species Phylogenetic tree – picture of that evolutionary history *branching diagram *shows relationships among taxa *recency of common ancestor Phylogenetic trees - must be inferred Systematists - those who study the relationships among organisms (systematics ) – Use all possible types of data (external and internal morphology (skeleton), behavior, cell structure, chromosome #, nucleotide sequences in DNA, etc.)
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2 Classification started before phylogenetics. Carolus Linneaus (1707-1778) Swedish botanist binomial naming system hierarchical categorization Not a depiction of order of evolutionary events Phylum : Arthropoda Class : Insecta Order : Diptera Genus : Musca Species : domestica Darwin gave this system an explanation: hierarchical classification reflects historical events (descent from a common ancestor) e.g., vertebrates all have backbones because descended from common ancestor with a backbone Phylum: Arthropoda Class : Insecta Order : Diptera Genus : Species : (1837) 1st -known sketch by Darwin of an phylogenetic tree For the most part - classification represents real history of evolution For the most part organisms that look more alike are more closely related But NOT always true - physical characters can be unreliable indicators of ancestry Example - birds and crocodiles more closely related (share more recent common ancestor) than crocodiles and lizards Examples of hypothetical trees Node - branching point, represents common ancestor of lineages Capital letters on top = species Time - vertical axis (branch length may or may not be informative ) III. How to read a phylogenetic tree Ridley fig. 15.1
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3 Cladograms • Imply the relative order of divergences of taxa, but provide no information about the amount of change, or the time-depth of various events Phylograms In a phylogram – The length of a branch in a phylogram reflects the number of genetic changes that have taken place in a particular DNA or RNA sequence in that lineage Figure 25.12 Drosophila Lancelet A m phibian F ish Bird H um an R at M ouse Ultrametric Trees In an ultrametric tree – The branching pattern is the same as in a phylogram, but all the branches that can be traced from the common ancestor to the present are of equal length. Branch lengths on an ultrameric tree represent a direct estimate of time D rosophila p h ib ia n B ird u a Cenozoic Mesozoic Paleozoic Proterozoic 542 251 65.5 Milions of years ago Sister taxa - 2 taxa that derive from an immediate common
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Lecture 6 - Lecture 6 Phylogenetic trees Reading Ch 25...

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