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Classification Chapter 14 Thousands of new species are discovered every year. In order for biologists to study all of the organisms possible, it is necessary to classify them into groups. Taxonomy is the branch of biology that groups organisms according to their characteristics and evolutionary history. Taxonomy started over two thousand years ago with the Greek thinker Aristotle. Aristotle classed organisms as either plant or animal. He grouped animals into land, water, or air dwellers. He groups plants as herbacious, grass, or woody stems. Aristotle’s main criteria was external morphology. As more organisms were discovered, biologists found that Aristotles groups were not adequate. They didn’t show relationships. And common names could be confusing. In the eighteenth century, Carolus Linnaeus devised a hierarchical system of categories to classify organisms. For the most part, Linnaeus used an organism’s morphology to classify it. Linnaeus’s categories follow: Domain Kingdom Phylum-- Division Class Order Family Genus Species In Linnaeus’s system, the species name or scientific name has two parts. The first part is the genus and the second part is the species or species identifier. Because of these two part names, the system is called the Binomial System of Nomenclature. The scientific names are latinized so that they are the same in every language. We placed in text, they are underlined or printed in italics. Botanists sometimes split plant species into varieties and zoologists sometimes separate animal species into subspecies. The Biological Species Concept states that a species is a group of individuals that can produce fertile offspring and are reproductively isolated from other species. Taxonomists now consider a field of evidence in classification: Morphology Phylogeny DNA and Embryology Cladistics Systematics or Phylogeny is a family tree that shows evolutionary or phylogenetic relationships thought to exist among organisms. This places organisms into distinct, related groups. Evidence used in systematics includes the fossil record. The fossil record provides some distinct framework but careful systematic investigation is necessary to fill in the gaps. Homologous and analogous characteristics are important morphology structures to study. This connects to embryology. Homologous structures have the same embryonic origin even though they may perform different functions. Analogous structures have different embryonic origins even though they may perform the same function. Embryological patterns of development also show relationships on the phylogenetic tree. Shortly after the zygote forms in development, mitosis forms a hollow ball of cells called the blastula. Shortly, an indentation forms in the blastula called the blastopore. In most animals, this forms the anterior part of the digestive system. However, in echinoderms and vertebrates, the blastopore forms the posterior part of the digestive system. This would put these two groups closer to each other on the phylogenetic tree. A relatively new field of systematic classification is called cladistics. Cladistics uses certain features of organisms called shared derived characters to establish relationships. A derived character is a trait that exists only in the group of organisms being considered and not in other groups. This method of classification can be used to make cladograms to show ancestry. Kingdoms (Chapter 19) The kingdom system of classification has been changed by some modern biologists. By studying ribosomal RNA, they have divided the organisms of the world up into three large domains. The domains include: • Archaea • Bacteria • Eukarya Modern biologists use six kingdoms to classify organisms: Archaebacteria Eubacteria Protista Fungi Plants Animals The Domain Bacteria contains one kingdom, the Eubacteria. This kingdom includes the bacterial decomposers, soil components, science specimens, and pathogens that we know today. Bacterial cell walls are composed of peptidoglycan. Archaebacteria These are unicellular prokaryotes with distinctive cell membrane and other biochemical properties different from all other forms of life. Some are autotrophic. These bacteria live in extreme environments such as hot springs and salt lakes. They may produce high levels of methane and live in anaerobic environments such as the intestines of animals. These organisms probably flourished before the earth had an oxygen rich atmosphere. Archaebacteria may be classified as extreme thermophiles, extreme halophiles, or methanogens. Protista—The kingdom Protista is made up of a wide variety of eukaryotic, single celled organisms. Some may be colonial. The members of this group are not exactly plants, animals, or fungi. The exact reproductive cycle of many is not known. Some, such as Euglena, may live as an autotroph or a heterotroph. The most ecologically important protists are probably algae. They form plankton in the ocean and make up the base for many food chains. Fungi—Fungi are both unicellular and multicellular heterotrophs that absorb their nutrients rather than eating. The exact reproductive cycle of many of them is unknown but most are able to involve genetic recombination. Members include mushrooms, morels, rusts, smuts, molds, and yeast. Many forms of fungus clump together with strands called hyphae. Plantae—These are a wide variety of modern, multicellular organisms that use photosynthesis to organize their own nutrients. Most plants live on land and have a sexual life cycle based on meiosis. They include mosses, ferns, conifers, and flowering plants. One of the most important developments of plants was the development of vascular tissue. Animalia-- Animals are modern multicellular heterotrophs that have symmetrical body organization and move about their environment. Most all animals have a sexual reproductive cycle showing recombination of genes. Ninety-nine percent of all animals are invertebrates.