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Systematics: Seeking Order Amidst Diversity Chapter 18 Systematics Systematics is the branch of biology concerned with Reconstructing phylogeny (evolutionary history) Naming organisms and placing them into hierarchical categories based upon their evolutionary relationships Major Categories of Classification The eight major categories of classification, in order of decreasing inclusiveness are Domain Kingdom Phylum Class Order Family Genus Species Scientific Names The scientific name of an organism is formed from the genus and species The genus Sialia (bluebirds) includes three species: Sialia sialis (the eastern bluebird) Sialia mexicana (the western bluebird) Sialia currucoides (the mountain bluebird) The Origin of Classification Aristotle (384-322 B.C.) Was among first to develop a standardized language for naming organisms Classified about 500 organisms into 11 hierarchical categories based on various characteristics Carolus Linnaeus (1707-1778) Laid the groundwork for the modern classification system Placed organisms into hierarchical categories based on their resemblance to other organisms Introduced the scientific name composed of genus and species Charles Darwin (1809-1882) Published On the Origin of Species, which demonstrated that all life is related by common ancestry Evolutionary Relationships Biologists realized that taxonomic categories should reflect evolutionary relatedness The more categories two organisms share, the closer their evolutionary relationship All organisms share certain similarities Similarities result from common ancestry or convergent evolution Present-Day Classification Systematists determine evolutionary relationships based on similarities due to common ancestry Similarities may be anatomical or molecular Anatomical similarities Systematists examine similarities in external body structure Systematists examine similarities in internal body structures, such as skeletons and muscles Anatomical Similarities Systematists examine microscopic similarities to discern finer details Number and shape of the “teeth” on the tonguelike radula of a snail Shape and position of the bristles on a marine worm External structure of pollen grains of a flowering plant Molecular Similarities Systematists examine genetic similarities between: DNA nucleotide sequences Chromosome structure It has been estimated that 99% of the chimpanzee genome is identical to that of humans The Two-Kingdom System Before 1969, all forms of life were classified into two kingdoms Animalia Plantae (included plants, bacteria, fungi and photosynthetic eukaryotes) The Five-Kingdom System Proposed by Robert H. Whittaker (1969) Kingdoms include Monera (all prokaryotes) Plantae Fungi Animalia Protista (eukaryotes that are not plants, fungi, or animals) The Three-Domain System Introduced by Carl Woese (1990) Discovered that kingdom Monera included two very distinct groups (Bacteria and Archaea) based on nucleotide sequences of ribosomal RNA Domains include Bacteria (prokaryotic) Archaea (prokaryotic) Eukarya (eukaryotic) Kingdom-Level Classification Systematists have yet to reach a consensus about the precise definitions of new prokaryotic and eukaryotic kingdoms Figure 18-6, p. 364, shows the evolutionary relationships among some members of the domain Eukarya… How Many Species Exist? Biodiversity is the total number of species in an ecosystem Number of named species is currently about 1.5 million (biased toward large organisms in temperate regions) 5% prokaryotes and protists 22% plants and fungi 73% animals Estimated that 7 million to 10 million species may exist How Many Species Exist? Between 7,000 and 10,000 new species are identified annually, mostly in the tropics Tropical rain forests are believed to be home to two-thirds of the world’s existing species, most of which have yet to be named Because tropical rain forests are being destroyed so rapidly, species may become extinct before we ever knew they existed