CHAPTER 26: Phylogeny 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, 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.o 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 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 theanimals in one group and the plants in another. - The bacteria, fungi and many protists wereconsidered plants while some of the protists weregrouped with the animals.Systematics is an inexact process.- All grouping methods are subject to problems.- Plants vs. animals were an imperfect system andas a result didn’t last.The foundation for modern classification, binomialnomenclature, was developed by Linnaeus in the1700's.- Linnaeus also developed a scheme whereorganisms are classified or grouped intocategories. These categories are further dividedinto smaller and smaller groupings creating anorganizational hierarchy (see fig 26.3)- The lowest two categories of the taxonomichierarchy (genus and species) make up the scientific name for an organism. 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- Class1- Order- Family- Genus- Specieso Use a phrase (mnemonic device) to help you remember the order of these categories. o 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 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.o 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.The five kingdoms and their defining characteristics:Kingdom Cell Type Cell Number Nutritional Method Monera Prokaryotic Unicellular Absorption/photosynthesis Protista Eukaryotic Unicellular Absorption/photosyn./ingestion Fungi Eukaryotic Multicellular Absorption Plantae Eukaryotic Multicellular Photosynthesis Animalia Eukaryotic Multicellular 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 describes a taxon whose members were derived from two or more ancestral forms not common to all members2Carl 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 phylogenyIt was clear from very early that best systematic arrangement would group together organisms with a shared phylogeny---Phylogeny is the evolutionary history of a species or group of related speciesGrouping based on perceived evolution.3Systematists depict evolutionary relationshipsin branching phylogenetic trees (see fig26.4)Each branch point in the tree is a noderepresents the divergence of species. Nodes closer to the ancestral linage representgreater amounts of divergence than nodesfarther from the ancestral linage.A node with multiple lineages is a polytomythat requires more data to be resolved.Phylogenetic history can be inferred fromsimilarities in homologous structures and genes when comparedamong organisms.Generally, similar morphology and similar DNA sequences = closelyrelated species.However analogous structures (a.k.a., homoplasies) may be due toconvergent 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 phylogeneticrelationships (see fig 26.10).--- Cladistics is the analysis of the resemblances among
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