Chapter 5 Classification Lesson 30

Chapter 5 • Lesson 30 Classification Objectives: 3.5,1,3.5.2 Key Words • taxonomy • species • domain • kingdom • prokaryote • binomial nomenclature • genus • phylogeny • phylogenetic tree eukaryote • autotroph • heterotroph dichotomous key Getting the Idea Earth is home to millions of different kinds of organisms. Each of them shares some features with all other organisms. Yet, there is great diversity in their forms and structures. To aid in organizing and understanding information about these diverse life forms, scientists have devised a system for classifying them. This system changes as scientists learn more about evolutionary relationships. Classification The field of biology that deals with classifying organisms is taxonomy. Early taxonomists classified organisms based solely on similarities that were easy to observe. Today's taxonomists classify organisms based on analysis and comparison of their physical structures, embryological development, chemical processes, and DNA. These data are used to group organisms in ways that reflect evolutionary relationships. In modern taxonomy, there are eight major levels of classification: domain, kingdom, phylum, class, order, family, genus, and species. As you move from the top to the bottom of this hierarchy, each classification is more specific, so that groups at each level are smaller than groups at the level above it. That is, organisms within each category are more closely related as you move from the domain toward the species level. The bottom level in the hierarchy is the species. Recall that a species is a group of organisms that share most characteristics and can interbreed to produce fertile offspring. A species contains only one kind of organism. For example, the coyote belongs to the species Canis latrans, while the wolf—a close relative of the coyote—belongs to the species Canis lupus. Domains and Kingdoms Domains are the largest groups in the modern classification system. The three domains are Bacteria, Archaea, and Eukarya. The next level down is the kingdom. As shown below, each domain includes one or more kingdoms of organisms. Bacteria and Archaea The domain Bacteria is made up of one kingdom, Eubacteria, which means "true bacteria." All members of this kingdom are the organisms called bacteria. Recall that bacteria are single-celled prokaryotes, organisms that lack a membrane-bound nucleus and organelles. Bacteria have cell walls that contain a substance called peptidoglycan. The domain Archaea also contains one kingdom, Archaebacteria. Like bacteria, archaea are one-celled prokaryotes. However, the cell walls of archaea do not contain peptidoglycan. Also, the plasma membranes of archaea are made up of different lipids than the plasma membranes of bacteria and eukaryotes. Many archaea live in extreme environments, such as thermal vents and salt lakes. Bacteria and archaea were once classified in the same kingdom, but increased knowledge about archaea led scientists to develop the domain level of classification. Scientists now recognize that bacteria and archaea are as different from each other as they are from eukaryotes. In fact, most scientists no longer use the terms "eubacteria" and "archaebacteria." Many scientists think that archaea evolved before bacteria, but there is disagreement on this point. Eukarya The domain Eukarya contains four kingdoms: Protista (protists), Fungi, Plantae (plants), and Animalia (animals). All members of these kingdoms are eukaryotes, organisms whose cells contain a nucleus and other membrane-bound organelles. Protists and fungi may be unicellular or multicellular. All plants and animals are multicellular. The table summarizes the major characteristics of the four kingdoms in this domain. As you review the table, remember that an autotroph is an organism that makes its own food, while a heterotroph feeds on other organisms. The fungus, plant, and animal kingdoms are thought to have evolved from the fourth eukaryotic kingdom—Protista. Protists can be unicellular, multicellular, or colonial (organisms of the same species functioning together). This varied group of organisms contains all types of algae, amoebas, paramecia, and euglenas. The kingdom Fungi includes organisms that have cell walls but lack vascular tissue and chloroplasts. Most fungi are multicellular. Fungi are heterotrophic, or unable to make their own food. These organisms live in all environments and include important decomposers and parasites. Yeasts, molds, mildews, rusts, and mushrooms are found in this kingdom. Organisms in the kingdom Plantae are multicellular and autotrophic, meaning they make their own food. All plants have rigid cell walls made of cellulose. Plants are a major group of living things, containing more than 400,000 identified species, including trees, grasses, herbs, ferns, and mosses. Animals, or organisms in the kingdom Animalia, are multicellular heterotrophs whose cells lack cell walls. Most animals have interior digestive tracts, body symmetry, and nerve tissue. Animalia contains more species than any other kingdom in the domain Eukarya. Kingdoms are divided into phyla (singular phylum). The organisms in each phylum have more in common and are more closely related to one another than are organisms that share only a kingdom. Phyla are subdivided into classes, then orders, family, genera, and species. Binomial Nomenclature In the 1700s, a Swedish botanist named Carolus Linnaeus devised a system for naming organisms that is still in use today. This system is called binomial nomenclature because it gives each kind of organism a unique two-part name, such as Canis lupus. The first part of an organism's name identifies its genus. A genus (plural: genera) is a classification group below family that includes related species. Organisms in the same genus are very closely related. The second part of the scientific name identifies an organism's species. Identifying Evolutionary Relationships When classifying organisms, scientists attempt to identify evolutionary links among them. That is, they try to figure out how organisms are related and how they have diversified and changed over time. Phylogeny is the evolutionary history of a group of organisms. Scientists show the development of groups of organisms from a common ancestor in branching diagrams called phylogenetic trees. A branch in a phylogenetic tree indicates the separation of a species or larger group from an ancestral species or group. In some trees, the lengths of the branches indicate the amount of time since a group of organisms evolved from a common ancestor. Lines may also be labeled to indicate the traits that distinguish one group of organisms from another. Some phylogenetic trees are complex and show many branches. The simple example below shows the relationships and distinguishing traits of the main groups, or taxa, of plants. Phylogenetic trees can be used to determine how closely related organisms are. As you can see from the pattern of branching, angiosperms are more closely related to gymnosperms than to bryophytes. The branches show that the evolutionary separation, or divergence, of angiosperms and gymnosperms occurred after the divergence of gymnosperms from seedless vascular plants. The order in which other plant groups diverged is also shown. For example, you can see that bryophytes diverged from the common ancestor of plants earlier than any of the other groups shown. Ultimately, all organisms have a common ancestor. If a phylogenetic tree were drawn for all life that has ever existed on Earth, its branches would lead back to a single stem. Evidence of this common ancestor comes from both fossils and living organisms. Recall from the previous lesson that fossils are the remains or traces of past organisms. Through structural changes over time, the fossil record documents how past organisms are related to one another and to organisms living today. The fossil record suggests that all organisms on Earth evolved from a single ancestral species that lived about 3.5 billion years ago. The Changing Nature of Taxonomy Scientific knowledge tends to be strong and long-lasting because it is based on evidence that is supported by observation, testing, and experience. However, scientific knowledge can change because of new discoveries and evidence. Taxonomy has experienced many changes over time. The earliest classification system can be traced back to the Greek philosopher Aristotle (384-322 BCE). Aristotle classified organisms as either plants or animals. He grouped animals by whether they swam, flew, or walked. Centuries later, Linnaeus developed a hierarchical system of classification. Linnaeus based his classification on body shapes and traits such as fur and feathers. For example, he grouped bats with mammals, not with birds. The largest and most general group in Linnaeus's system was the kingdom. Although scientists still use a hierarchical classification, it has been changed many times. For example, modern taxonomists recognize the domain as the largest taxonomic grouping. Many changes in classification have resulted from advances in technology. For example, microorganisms were discovered only after the invention of microscopes. At first, these organisms were classified as either plants or animals. In the nineteenth century, scientists realized that a new kingdom was needed. The kingdom Protista was introduced, including all microscopic organisms. As scientists learned more about biology, they separated prokaryotes from eukaryotes. This meant putting prokaryotes into a separate kingdom from protists. Most recently, prokaryotes have been divided into two kingdoms, Eubacteria and Archaebacteria. That change was based on more knowledge about the chemicals that make up these organisms. Studies of both mushrooms and green plants, including those organisms' chemistry, led to another major change-the establishment of the kingdom Fungi. Fungi such as mushrooms had been classified as plants. Scientists now know that although mushrooms look like plants, the two groups are only distantly related. Other changes in taxonomy have resulted from the acceptance of the theory of evolution and an understanding of DNA. Knowledge from these two areas has shifted the focus of classification away from anatomy and physiology. Taxonomists now try to group organisms based on evolutionary relationships and shared ancestry. However, anatomy and other forms of evidence are still used to identify those relationships. Dichotomous Keys When a scientist observes an organism but does not know its classification, the scientist may use a classification key. Dichotomous keys are one kind of classification key that scientists use to identify organisms. The word dichotomous means "divided into two parts." A dichotomous key is a tool that can be used to determine the identity of something by matching observations to a series of paired statements or questions. Based on observations about an organism, the user chooses one of the statements or answers to a question in the first pair. Each statement or answer then leads either to the identity of an organism or to another pair of choices. By working through an entire series of statements or questions, a taxonomist can often identify an organism. The dichotomous key below is a simple example To identify the wild cat in the illustration, use its observed traits to choose between the first pair of statements. Because the cat shown does not have a solid coat, you will choose statement 1 b. This statement directs you to choose between the pair of descriptions labeled 3. Because the cat shown is spotted, the correct choice is statement 3b. Based on this information, it is possible to identify the cat as Acinonyx jubatus, which is the scientific name for a cheetah. Focus on Inquiry Classification is based on both observations and inferences. Observations may include the shape of a leg bone or the chemical makeup of a cell wall. Taxonomists use those observations, and what they already know, to infer how organisms are related. Many organisms have been moved from one taxonomic group to another. Until recently, taxonomists classified wolves and domestic dogs as two separate species, Cants lupus and Canis familiaris. Now domestic dogs are classified with wolves, as the subspecies Canis lupus familiaris. Similarly, fungi were once classified as plants but were then moved into their own separate kingdom. Review the information presented in this lesson and recall what you already know about dogs and fungi. What inferences can you make to explain the proposed change in the classification of wolves and dogs? Why does it make sense that fungi were removed from the plant kingdom? Are the reasons for changing the classifications of dogs and fungi the same? Why or why not?