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NOTES FOR BIOLOGY 1202DR. STEVEN POMARICO, INSTRUCTORCHAPTER 26Phylogeny and the Tree of LifeThe discipline of systematics classifies organisms based on shared relationships.---Systematics is the analytical study of the diversity and relationships of organisms, bothpresent-day and extinctSystematists use fossil, molecular, and genetic data to infer these relationshipsOrganizing or grouping things helps in dealing with them.Example: Grocery store or your closetThe same idea holds true for the study of life.If organisms (or any items) are to be grouped then they need a name. In biology this begins withtaxonomy.--- Taxonomy is the branch of biology concerned with naming and classifying the diverse forms oflifeThe origins of taxonomy date back to Aristotle.There are many ways that organisms may be groupedPlants verses animalsOne of the early classification systems placed the animals in one group and the plants in another. The bacteria, fungi and many protists were considered plants while some of the protists were groupedwith the animals.Systematics is an inexact process.All grouping methods are subject to problems.Plants vs. animals was an imperfect system and as a result didn’t last.1The foundation for modern classification, binomial nomenclature, was developed by Linnaeus in the 1700's.Linnaeus also developed a scheme where organisms are classified or grouped into categories. These categories are further divided into smaller and smaller groupings creating an organizational hierarchy (see fig 26.3)The lowest two categories of the taxonomic hierarchy (genus and species) make up the scientific name for an organism.Example: Homo sapiensThe genus name is capitalized and the species name begins with a lowercase letter. The scientific nameis usually underlined or italicized.The major taxonomic categories from most inclusive (biggest set) to least inclusive (smallest set) are:Domain – this one was added later (see below)KingdomPhylum/DivisionClassOrderFamilyGenusSpeciesUse a phrase (mnemonic device) to help you remember the order of these categories. Do Knowledgeable Political Candidates Often Forget General SubjectsMany criteria are used by biologists to group organisms Morphology (size, shape, structure)Anatomy (organs, tissues)Developmental stage (compare the embryos) Cell structure (chromosome number and structure)Behavior (does it fly? Swim? Quack?)Lifecycle (How long does it live? How often does it reproduce?)Ecology (Where does it live? What does it eat?)Using systematic, the more categories two organisms have in common the more closely they are related.The more closely related two organisms are the smaller the differences between them. This often meansvery small anatomical differences are used to distinguish between two species. Some of these criteria can be misleading because very different organisms can have similarcharacteristics which are the result of convergent evolution.Shark versus dolphinIn 1969 Robert Whittaker proposed a means for easily determining which of the five kingdoms anorganism belonged in based on observably different attributes that defined them.2The five kingdoms and their defining characteristics:Kingdom Cell Cell NutritionalType NumberMethodMonera Prokaryotic Unicellular Absorption/photosynthesisProtista Eukaryotic Unicellular Absorption/photosyn./ingestion Fungi Eukaryotic Multicellular Absorption Plantae Eukaryotic Multicellular PhotosynthesisAnimalia Eukaryotic Multicellular IngestionIn this scheme each Kingdom is divided into either Phyla (plural of phylum) or Divisions.There were two problems with Whittaker’s system that became obvious once DNA technology improvedin the 1980’sThe techniques of molecular biology have aided the taxonomist in the classification of organisms byallowing the DNA sequences from two organisms to be compared.The ability to examine the DNA sequences and genes of different organisms resulted in the kingdomMonera being separated into two distinct groups which are polyphyletic.---Polyphyletic is the evolutionary history of a species or group of related species Carl Woese proposed that these two groups of prokaryotes are as different form each other as they arefrom the eukaryotes and that these differences arose very early in the process of evolution. In order to accommodate these differences and place them in the appropriate evolutionary context,Woese and others proposed an “evolutionary tree” with three main branches (See fig 18-6).These branches were termed domains (see fig 26.21)There are two prokaryotic domains; Bacteria and Archaea and a eukaryotic domain; Eukarya.The domain of Eukarya branches into the separate eukayotic kingdoms The second problem has yet to be resolved. If you examine the kingdoms within domain Eukarya you willsee that the protest kingdom is also polyphyletic. This and other issues in taxonomy are in the process ofbeing resolved.Linking systematics and phylogenyIt was clear from very early that best systematic arrangement would group together organisms with ashared phylogeny---Phylogeny describes a taxon whose members were derived from two or more ancestral formsnot common to all members Grouping based on perceived evolution.3Systematists depict evolutionary relationships in branching phylogenetic trees (see fig 26.4)Each branch point in the tree is a node represents the divergence of species. Nodes closer to the ancestral linage represent greater amounts of divergence than nodes farther from theancestral linage.A node with multiple lineages is a polytomy that requires more data to be resolved.Phylogenetic history can be inferred from similarities in homologous structures and genes whencompared among organisms.Generally, similar morphology and similar DNA sequences = closely related species.However analogous structures (a.k.a., homoplasies) may be due to convergent evolution, not sharedancestry.DNA similarities, known as molecular homoplasy can also occur (see fig 26.9)Shared characters are used to construct phylogenetic trees.Cladistics uses cladograms and clades to show phylogenetic relationships (see fig 26.10).--- Cladistics is the analysis of the resemblances among clades, or groups of species that share acommon ancestor --- Cladograms are diagrams depicting patterns of shared characteristics among taxa--- Clades are groups of species that includes the


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LSU BIOL 1202 - CHAPTER 26

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