Chapter 18 Systematics Seeking Order Amidst Diversity 18 1 How Are Organisms Named and Classified The branch of biology that is concerned with naming and classifying organisms is known as taxonomy Modern taxonomy was established by the Swedish naturalist Carolus Linnaeus Linnaeus introduced the two part scientific name to all organisms The two part scientific name of an organism is formed from the genus and species A genus is a group that includes a number of very closely related species A species within a genus includes populations of organisms that can potentially interbreed under natural conditions The genus Sialia bluebirds includes three species Sialia sialis the eastern bluebird Sialia mexicana the western bluebird Sialia currucoides the mountain bluebird Three Species of Bluebird Fig 18 1 Scientific names are always underlined or italicized The first letter of the genus name is always The first letter of the species name is always The species name is always paired with its capitalized lowercase genus name The Linnaean classification system includes eight major taxonomic categories each category including all the categories beneath it Domain Kingdom Phylum Class Order Family Genus Species Modern classification emphasizes patterns of evolutionary descent as biologists realized that taxonomic categories should reflect evolutionary relatedness The more categories two organisms share the closer their evolutionary relationship Today the process of classification focuses on reconstructing phylogeny or evolutionary history The science of reconstructing phylogeny is known as systematics Systematists identify features that reveal evolutionary relationships All organisms share certain similarities Biologists look at many kinds of characteristics in the search for informative similarities Anatomy plays a key role in systematics Systematists look carefully at similarities in both external body structure and internal structures such as skeletons and muscles Homologous structures such as the finger bones of dolphins bats seals and humans provide evidence of a common ancestor Molecular similarities are also useful for reconstructing phylogeny Systematists examine genetic similarities between DNA nucleotide sequences It has been estimated that 96 of the chimpanzee genome is identical with that of humans Similarities in actual chromosome structure also can be used to establish relationships between organisms Human and Chimp Chromosomes Are Very Similar HC HC HC HC HC HC HC HC 1 HC 2 HC 3 HC 4 HC 5 HC 6 7 X HC HC HC 8 9 10 11 12 13 14 15 HC HC HC HC HC HC HC HC 16 17 18 19 20 21 22 Y Fig 18 4 18 2 What Are the Domains of Life Before 1970 systematists divided all species into two groups Animalia Plantae including plants bacteria fungi and photosynthetic eukaryotes As knowledge expanded of life s evolutionary relationships it became clear that a two domain approach was an oversimplification A three domain system more accurately reflects life s history Carl Woese discovered that prokaryotic organisms included two very distinct groups Bacteria and Archaea Our current three domain system Bacteria prokaryotic Archaea prokaryotic Eukarya eukaryotic These three split very early in life long before animals and plants evolved The Tree of Life BACTERIA ARCHAEA EUKARYA animals fungi plants protists Fig 18 6 Within the domain Eukarya there are four different groups of organisms Kingdoms Animalia animals Plantae plants Fungi fungi Protists eukaryotic organisms that are not animals plants or fungi 18 3 Why Do Classifications Change Science is a PROCESS not a list of facts Classifications change when new information is discovered Changes at the Domain and Kingdom level are rare but there are frequent changes in species level It was previously thought that there were only two species of elephant African and Indian More recently the African elephant species has been divided into two species the savannah elephant and the forest elephant Genetically these two African species have no more in common than lions and tigers 18 4 How Many Species Exist Biodiversity is the total number of species in an ecosystem The number of named species is currently about 1 5 million biased toward large organisms in temperate regions 5 are prokaryotes and protists 22 are plants and fungi 73 are animals It is estimated that 7 million to 10 million species may exist Between 7 000 and 10 000 new species are identified every year Tropical rainforests are believed to be home to two thirds of the world s existing species most of which have yet to be named Because tropical rainforests are being destroyed so rapidly species may become extinct before we even know they exist The unexplored continent of the deep sea floor may have hundreds of thousands of unknown species to be discovered
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