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OLE 6.12D Taxonomic Classification: Classifying Organisms How do Biologists Classify Organisms? Classification is the process of organizing and grouping things. The scientific study of how organisms are classified is called taxonomy. Taxonomy is useful because once a scientist knows the classification of an organism, he or she/the scientist already knows a lot of information about that organism. Biologists initially grouped organisms based on their similarities. Modern classification systems group organisms according to how they are related to each other. Naming Organisms Taxonomy involves naming organisms. In the 1730s, the Swedish botanist Carolus Linnaeus devised a system of naming organisms that is still used today. Linnaeus placed organisms in groups based on similarities in their observable features. Each organism was given a unique, two-part scientific name. This system is called binomial nomenclature. The first word in an organism’s scientific name is its genus. A genus (plural genera) is a classification grouping that contains similar, closely related organisms. For example, pumas and house cats are both classified in the genus Felis. Organisms in the genus Felis share characteristics such as sharp, retractable claws, and behaviors such as hunting other animals. The second word in a scientific name often describes a distinctive feature of an organism, such as where it lives or its appearance. Together, the two words form the scientific name of an organism. All organisms with the same scientific name belong to the same species. A species (SPEE sheez) is a group of similar organisms that can mate with each other and produce offspring that can also mate and reproduce. What Are the Levels of Classification? The classification system that scientists use today is based on the contributions of Linnaeus. But today’s classification system uses a series of many levels to classify organisms. The Major Levels of Classification Organisms are classified by their characteristics and relatedness. The diagram below shows the levels most biologists use. First, an organism is placed in a broad group, which in turn is divided into more specific groups. A domain is the highest level of organization. Within a domain, there are kingdoms. Within kingdoms, there are phyla (singular phylum). Within phyla are classes. Within classes are orders. Within orders are families. Each family contains one or more genera. Finally, each genus contains one or more species. The more classification levels two organisms share, the more closely related they are. How are Organisms Classified into Domains? Today, biologists commonly use a three-domain system to classify organisms. The three domains are Bacteria, Archaea, and Eukarya. Organisms are placed into domains based on their cell type and their chemical composition. Domain Bacteria Members of the domain Bacteria are prokaryotes. Prokaryotic cells have no nuclei. All prokaryotes are single-celled organisms. Some bacteria are autotrophs (they make their own food), while others are heterotrophs (they eat other organisms or the remains of organisms). Bacteria usually reproduce asexually through a process called binary fission, in which one cell divides to form two identical cells. Domain Archaea Members of the domain Archaea, like bacteria, are unicellular prokaryotes that often reproduce through binary fission. And like bacteria, some archaea are autotrophs and others are heterotrophs. Archaea are classified in their own domain because their chemical makeup differs from that of bacteria. Bacteria and archaea also differ in the structure of their cells. Some archaea are found in harsh,
extreme environments. These harsh places include vents deep under the ocean, oil wells, and hot springs, where the temperature can be above 80°C. Domain Eukarya Organisms in the domain Eukarya are eukaryotes, organisms with cells that contain nuclei. Within this domain, organisms have many different characteristics. How are Organisms Classified into Kingdoms? The domains Bacteria and Archaea represent two kingdoms, Bacteria and Archaea. The domain Eukarya, however, contains four kingdoms: protists, fungi, plants, and animals. Organisms are placed into kingdoms based on characteristics such as their ability to make food, the number of cells in their bodies, and how they reproduce. Protists A protist is any eukaryotic organism that cannot be classified as a fungus, plant, or animal. Because its members are so different from one another, the protist kingdom is sometimes called the “odds and ends” kingdom. For example, some protists are autotrophs, while others are heterotrophs. Most protists are unicellular, but some, such as seaweeds, are multicellular. Protists can reproduce sexually, but most reproduce asexually. Fungi Mushrooms, molds, and mildew are all fungi. The majority of fungi are multicellular, but a few, such as the yeast used in baking, are unicellular. Fungi are found almost everywhere on land, but only a few live in fresh water. All fungi are heterotrophs. Most fungi feed by absorbing nutrients from dead or decaying organisms. Fungi reproduce both asexually and sexually. Plants Plants are all multicellular eukaryotes, and most live on land. Also, plants are autotrophs that make their own food. Plants do not move from place to place on their own. Their life cycles include both asexual and sexual reproduction. Animals All animals are multicellular eukaryotes. In addition, all animals are heterotrophs. Animals have different adaptations that allow them to locate, capture, eat, and digest food. Members of the animal kingdom live in diverse environments throughout Earth. Most animals can move from place to place for at least part of their life cycle. Most animals reproduce sexually, though some, such as sponges, jellyfish, and worms, can reproduce asexually. The table below summarizes the characteristics of the major groups of organisms. DOMAIN Bacteria Archaea Eukarya KINGDOM Bacteria Archaea Protists Fungi Plants Animals CELL TYPE Prokaryotic Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic NUMBER OF Unicellular Unicellular Mostly Mostly Multicellular Multicellular CELLS unicellular multicellular FOOD SOURCE Autotroph or Autotroph or Autotroph or Heterotroph Autotroph Heterotroph heterotroph heterotroph heterotroph REPRODUCTION Asexual Asexual Most asexual Asexual Asexual and Most sexual, and sexual sexual some asexual 1. What are the two parts of the scientific name of an organism? ____________________________________ 2. What is the broadest taxonomic classification of living organisms? _____________________ 3. What is the primary characteristic that divides the domain Eukarya from the other two domains? ________ _______________________________________________________________________________________ 4. How are plants and animals alike? ___________________________________________________________ 5. List four characteristics that are used to classify organisms into kingdoms. ___________________________ _______________________________________________________________________________________ 6. How are bacteria and archaea similar? How are they different? ____________________________________ _______________________________________________________________________________________ 7. Identify the mode of reproduction of organisms that are classified in the following kingdoms or domains: Bacteria - _____________ Protists - ______________ Plants - _______________ Archaea - _____________ Fungi - ________________ Animals - ______________
8. Complete the chart to identify which kingdoms contain prokaryotic organisms and which contain eukaryotic organisms. Prokaryotes Eukaryotes
9. List the levels of classification of living things, starting with the broadest taxonomic classification. _______________________________________________________________ ________ 10. Complete the Venn diagram to identify which kingdoms contain autotrophic and heterotrophic organisms.
11. Identify which kingdom or kingdoms include unicellular organisms. Which kingdom or kingdoms include multicellular organisms? Unicellular kingdoms: ____________________________________________________________________ Multicellular kingdoms: ___________________________________________________________________ 12. Your friend finds a new organism and lists its basic characteristics. Use your friend’s list to classify the organism in a currently recognized kingdom. _______________________________ It has nuclei in its cells. It has more than one cell. It cannot make its own food. It can reproduce both asexually and sexually. It absorbs nutrients from dead organisms. 13. Many biologists think that protists do not really form a kingdom of their own, but rather represent members of several different groups. Why might biologists think this? Do you agree or disagree? Why? __________________________________________________________________________ __________________________________________________________________________