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The Tree of Life Chapter 26 Classification of Organisms More than 2000 years ago, Aristotle divided living things into animals and plants Later, basic units were called genera -Felis (cats) and Equus (horses) In the 1750s, Carolus Linnaeus instituted the use of two-part names, or binomials -Homo sapiens 2 Classification of Organisms Taxonomy is the science of classifying living things -A classification level is called a taxon Scientific names avoid the confusion caused by common names 3 The Linnaean Hierarchy Taxa are based on shared characteristics -Domain (most shared) -Kingdom -Phylum -Class -Order -Family -Genus -Species (least shared) 4 5 6 Limitations of the Hierarchy Many hierarchies are being re-examined based on the results of molecular analysis -Linnaean taxonomy does not take into account evolutionary relationships -Linnaean ranks are not equivalent -Legume family (16,000 species) -Cat family (36 species) The phylogenetic and systematic revolution is underway 7 Grouping Organisms Carl Woese proposed a six-kingdom system Prokaryotes Eukaryotes 8 Grouping Organisms Biologists are increasingly adopting a three-domain phylogeny based on rRNA studies -Domain Archaea -Domain Bacteria -Domain Eukarya Each of these domains forms a clade Archaea and Eukarya are more closely related to each other than to bacteria 9 During evolution, microbes swapped genetic information via horizontal gene transfer (HGT) 10 Viruses Are not organisms and so cannot be placed in any of the kingdoms Are literally “parasitic” chemicals -DNA or RNA wrapped in protein Can only reproduce within living cells Vary greatly in appearance and size 11