NOTES FOR BIOLOGY 1202 DR STEVEN POMARICO INSTRUCTOR 1 CHAPTER 26 Phylogeny and the Tree of Life The discipline of systematics classifies organisms based on shared relationships Systematics is the analytical study of the diversity and relationships of organisms both present day and extinct Systematists use fossil molecular and genetic data to infer these relationships Organizing or grouping things helps in dealing with them Example Grocery store or your closet The 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 with taxonomy Taxonomy is the branch of biology concerned with naming and classifying the diverse forms of life The origins of taxonomy date back to Aristotle There are many ways that organisms may be grouped Plants verses animals One 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 grouped with 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 The 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 2 Example Homo sapiens The genus name is capitalized and the species name begins with a lowercase letter The scientific name is 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 Kingdom Phylum Division Class Order Family Genus Species Use a phrase mnemonic device to help you remember the order of these categories Do Knowledgeable Political Candidates Often Forget General Subjects Many 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 means very small anatomical differences are used to distinguish between two species Some of these criteria can be misleading because very different organisms can have similar characteristics which are the result of convergent evolution Shark versus dolphin In 1969 Robert Whittaker proposed a means for easily determining which of the five kingdoms an organism belonged in based on observably different attributes that defined them 3 The five kingdoms and their defining characteristics Kingdom Cell Type Cell NumberMethod Nutritional Monera Protista Fungi Plantae Animalia Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic Unicellular Unicellular Multicellular Multicellular Multicellular Absorption photosynthesis Absorption photosyn ingestion Absorption Photosynthesis Ingestion In 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 improved in the 1980 s The techniques of molecular biology have aided the taxonomist in the classification of organisms by allowing the DNA sequences from two organisms to be compared The ability to examine the DNA sequences and genes of different organisms resulted in the kingdom Monera 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 are from 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 will see that the protest kingdom is also polyphyletic This and other issues in taxonomy are in the process of being resolved Linking systematics and phylogeny It was clear from very early that best systematic arrangement would group together organisms with a shared phylogeny Phylogeny describes a taxon whose members were derived from two or more ancestral forms not common to all members Grouping based on perceived evolution Systematists 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 the ancestral linage 4 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 when compared 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 shared ancestry 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 a common ancestor Cladograms are diagrams depicting patterns of shared characteristics among taxa Clades are groups of species that includes the ancestral species and all its descendants A group of organisms can be Monophyletic Paraphyletic Polyphyletic Only monophyletic groups made up of an ancestral species and all of its descendents qualify
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