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Lecture 6

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Biological Sciences
Maydianne Andrade

Lecture 6- September 27 Chapter 4 (pg. 111-119) & chapter 10 (pg. 376-379) Phylogeny – evolutionary history of a group of species Phylogenetic tree – graphical summery of the evolutionary history of a group of species 4.1 the Logic of Phylogeny Inference The most closely related taxa should have the most traits in common Some of these characters would be: sequence of nucleotides in a gene, presence or absence of a specific skeletal element, or the mode of embryonic development Synapomorphies identify monophyletic Groups Homologies that are useful in estimating phylogenies = synapomorphies Synapomorphy – homologous trait that is shared among certain species and is similar because it was modified in a common ancestor (shared, derived traits) Genetic code is a homologous trait that is shared by all organisms Monophyletic group – and group that includes an ancestor and all of its descendants Synopomorphies can be identified at whatever taxonomic level a researcher might be interested in: population, species, genera, phyla …. Synomorphies identify evolutionary branch points 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  hierarchy of branching events Outgroup analysis – character state in the group of interest is compared to the state in a very close relative that clearly branched off earlier A phylogenetic tree inferred by clustering synapomorphies = cladogram Problems in reconstructing phylogenies Some similar traits may have evolved independently in different groups of species (not derived from a common ancestor) Not all similar traits are homologous Octopusese share derived traits with squids, clam and mussels But they have an image-forming camera eye (similar to fish & vertebrate animals) both hippos and crocs have eyes on the top of their heads but crocs are classified, as reptiles while hippos are mammals. Convergent evolution – natural selection favors similar structures as solutions to problems posed by similar environments The octopus and vertebrate developed the camera eye independently (both must avoid danger and hunt for food) The hippos and crocs developed their eyes on top of their heads independently because they both spend most of their time submerged in water and must look out for predators or prey. Same similarities can also occur at a cellular level There may be changes in the nucleotide sequ
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