The Tree of Life Classification Based on Evolutionary Relationships Modern classification is based on evolutionary relationships.

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1 CHAPTER 17 The Tree of Life GETTING READY TO LEARN Preview Key Concepts 17.1 The Linnaean System of Classification Organisms can be classified based on physical similarities Classification Based on Evolutionary Relationships Modern classification is based on evolutionary relationships Molecular Clocks Molecular clocks provide clues to evolutionary history Domains and Kingdoms The current tree of life has three domains. Review Academic Vocabulary Write the correct word for each definition. convergent evolution eukaryotic prokaryotic species 1. : cell containing a nucleus and membrane-bound organelles 2. : cell not containing a nucleus or membranebound organelles 3. : process in which two unrelated species evolve similar traits while adapting to similar environments 4. : group of organisms that can breed and produce fertile offspring Preview Biology Vocabulary Try to guess the meaning of each boldfaced word from its context. PHRASE 1. The scientific name for humans, written in binomial nomenclature, is Homo sapiens. 2. Scientists can use genetic and fossil evidence to study the phylogeny for a group of species. 3. Each taxon in the Linnaean classification system gets more and more specific, from kingdom to species. 4. The evolutionary relationships among certain species can be shown visually in a cladogram. MY GUESS 283

2 SECTION 17.1 The Linnaean System of Classification KEY CONCEPT Organisms can be classified based on physical similarities. Student text pages Linnaeus developed the scientific naming system still used today. Do you know what an Irish daisy is? What about a lion s tooth flower? A dandelion? Irish daisy, lion s tooth, and dandelion are all common names for the same flower. You can imagine how confusing it could be for scientists to try to talk about a particular organism if everyone has a different name for it. In the 1750s, the Swedish botanist Carolus Linnaeus developed a system for classifying* and naming living things. Linnaeus system gave naturalists a common language, which made it easier for them to talk about their findings with one another. Taxonomy Taxonomy is the science of naming and classifying organisms. Linnaean taxonomy classifies organisms based on their physical similarities. A group of organisms in a classification system is called a taxon (plural, taxa). The species is the basic taxon of the Linnaean system. A species is a group of organisms that can breed and produce offspring that can reproduce. With a few changes, the Linnaean system of naming is still used today. Scientific Names Binomial nomenclature (by-noh-mee-uhl NOH-muhn- KLAY-chuhr) is a system that gives each species a two-part scientific name using Latin words. The first part of the name is the genus. The second part describes the species. Genus A genus (plural, genera) includes one or more species that are thought to be closely related. For example, the genus Quercus includes more than 500 species of oak trees. Genus names always begin with a capital letter. They are written in italics or underlined. Cardinal The cardinal is a species of bird. A species is the basic taxon of the Linnean system. VISUAL VOCAB Binomial nomenclature is a standard naming system that gives each species a two-part name using Latin words. two name naming system bi nomial (1) Genus (2) species nomen clature * ACADEMIC VOCABULARY classifying organizing into groups 284 McDougal Littell Biology

3 Species descriptor The second part of the name describes the species. The species descriptor is also written in italics or underlined. It is written in lowercase letters. Quercus alba is the scientific name for white oak trees. The species descriptor is never written alone because the same descriptor may be used for many different genera. For example, Quercus alba is the scientific name for white oak trees and Tyto alba is the scientific name for barn owls. The descriptor alba means white. Alone, alba does not refer to any particular species. There can be many different common names for one particular species. Irish daisy, lion s tooth, and dandelion are all common names. But there is only one scientific name for this plant: Taraxacum officinale. Not only can there be many names for an organism in one language, but there are many more names for an organism in different languages. Scientific names help scientists around the world talk about particular organisms without confusion. Find the scientific name for dandelion in the paragraph above. Circle the genus. Draw a box around the species descriptor. Linnaeus classification system has seven levels. The Linnaean system of classification has seven levels: kingdom, phylum (plural, phyla), class, order, family, genus, and species. The most general level is kingdom. A kingdom contains one or more phyla. A phylum contains one or more classes, and so on. Underline the seven levels of Linnaeus classification system. kingdom phylum class order family genus species The Linnaean classification system has limitations. Linnaeus system is based on physical similarities between organisms. However, organisms that are physically similar are not always closely related. Recall that unrelated species can evolve similar traits through convergent evolution, as they adapt to similar environments. Each level in the Linnaean system of classification is nested, or included, in the level above it. Interactive Reader 285

4 Today, scientists use molecular and genetic research to help classify living things. Genetic similarities give strong evidence that organisms are related. Genetic information has resulted in some organisms getting reclassified into different groups. What is one limitation of Linnaeus classification system? Visual Connection See classification system in student text, pg Vocabulary Check taxonomy taxon binomial nomenclature genus Fill in the blanks with the correct term from the list above. Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. Binomial nomenclature has two parts: the and the species descriptor. 2. is the science of naming and classifying organisms. 3. A is a group of organisms in a classification system, such as a species or a genus The Big Picture 4. What is one reason that scientific names, instead of common names, help scientists to communicate about organisms? 5. What are the seven levels of classification, from biggest to smallest? 6. Linnaeus classification system is based on physical similarities. What type of information has been used more recently to determine relationships between organisms? 286 McDougal Littell Biology

5 SECTION 17.2 Classification Based on Evolutionary Relationships KEY CONCEPT Modern classification is based on evolutionary relationships. B.8.2 Cladistics is classification based on common ancestry. Today, scientists agree that organisms should be classified based on evolutionary relationships and not just physical similarities. Sometimes, physical similarities can give evidence for evolutionary relationships. Other times, physical similarities do not mean that organisms are related. For example, both birds and bats have wings, but they are not closely related. Phylogeny The evolutionary history for a group of species is called a phylogeny (fy-lahj-uh-nee). A phylogeny is made using more than just physical similarities. It is made using evidence from living species, the fossil record, and molecular data, such as DNA and protein sequences. Phylogenies can be shown as branching tree diagrams. Each branch represents a species or another taxon. An evolutionary tree shows how different groups of species are related to each other. Cladistics The most common method used to make evolutionary trees is called cladistics. Cladistics (kluh-dihs-tihks) is classification based on evolutionary relationships, or common ancestry. A cladogram is an evolutionary tree that suggests how species may be related. Over evolutionary time, certain traits in a group of species, or a clade, stay the same. Each species also has traits that have changed over time. Derived characters are traits that are shared by some species but not by others. These traits can be used to figure out evolutionary relationships. The more closely related species are, the more derived characters they will share. What does What a cladogram does a cladogram show? show? VOCABULARY At the root of the words cladistics and cladogram is the word clade. A clade is a group of species that shares a common ancestor. Interactive Reader 287

6 Interpreting a Cladogram The main features of a cladogram are shown in the cladogram for tetrapods below. Tetrapods are vertebrates that share a common ancestor with four limbs. Some tetrapods, such as snakes, no longer have the four limbs that their known ancestors have. Derived characters Derived characters are shown as hash marks. All species above a hash mark share the derived character it represents. There are six derived characters labeled on the tetrapod cladogram. Nodes Each place where a branch splits is called a node. Nodes repesent the most recent common ancestor shared by a clade. There are five nodes on the tetrapod cladogram. Identifying clades You can identify a clade by using the snip rule. Whenever you snip a branch under a node, a clade falls off. There are five clades in the tetrapod cladogram. CLADOGRAM FOR TETRAPODS A cladogram shows the likely evolutionary relationships among a group of species based on common ancestry and derived characters. CLADE Tetrapoda clade The diapsida clade includes lizards, Amniota clade snakes, crocodiles, alligators, and birds. Reptilia clade Diapsida clade Archosauria clade amphibians mammals turtles/tortoises lizards/snakes crocodiles/alligators birds SKULL OPENINGS IN FRONT OF THE EYE AND IN THE JAW OPENINGS IN THE SIDE OF THE SKULL SKULL OPENINGS BEHIND THE EYE FEATHERS AND TOOTHLESS BEAKS NODE This node represents the most recent common ancestor of all the species above this point amphibians, mammals, reptiles, and birds. FOUR LIMBS WITH DIGITS EMBRYO PROTECTED BY AMNIOTIC FLUID DERIVED CHARACTER This hash mark represents a derived character. All species above this mark have embryos that are protected by a liquid called amniotic fluid. 288 McDougal Littell Biology

7 Molecular evidence reveals species relatedness. An evolutionary tree is always a work in progress. With new evidence, trees can be changed to show how species are likely related. Today, molecular evidence, such as DNA sequences, is used to help learn about species relatedness. In some cases, molecular data agree with classification based on physical similarities. In other cases, molecular data lead scientists to classify species in a different way. What can lead scientists to change an evolutionary tree? 17.2 Vocabulary Check phylogeny cladistics cladogram derived character Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. Use the correct term from the list above to answer questions about the diagram below. A B C 1. What type of diagram is shown above? 2. The for a group of species is represented by this type of diagram. 3. Make a mark on the diagram above to represent a derived character shared by species A, B, and C. 4. What method is used to make this type of branching tree shown above? 17.2 The Big Picture 5. What types of information are used to make a cladogram? Interactive Reader 289

8 SECTION 17.3 Molecular Clocks KEY CONCEPT Molecular clocks provide clues to evolutionary history. B.8.3 Molecular clocks use mutations to estimate evolutionary time. Recall from Chapter 8 that a mutation is a change in an organism s DNA. Mutations happen regularly over evolutionary time. With more time, more mutations build up. Therefore, the more time that has passed since two species branched from a common ancestor, the more molecular differences they will have. Species that are closely related will have fewer differences. A molecular clock is a model that uses mutation rates to measure evolutionary time. MOLECULAR EVOLUTION Mutations add up at a fairly constant rate in the DNA of species that evolved from a common ancestor. Ten million years later one mutation in each lineage G T A C G T A T T C Another ten million years later one more mutation in each lineage G T A A G T A T T C Species A G A A C G T A T T DNA sequence from a common ancestor C The DNA sequences from two descendant species show mutations that have accumulated (black). G A A C G T A T G C The mutation rate of this sequence equals one mutation per ten million years. G A A C C T A T G C Species B To estimate mutation rates, scientists must find links between molecular data and real time. For example, geologic events and fossil evidence can help scientists to figure out how long ago two species diverged from a common ancestor. Look at the figure above. How much time has passed since species A and B branched from a common ancestor? Mitochondrial DNA and ribosomal RNA provide two types of molecular clocks. DNA and RNA are found in several parts of cells. The DNA in the nucleus of cells is called nuclear DNA. Mitochondria, the energy factories of cells, contain mitochondrial DNA. Ribosomes, the organelles that make proteins, contain ribosomal RNA. 290 McDougal Littell Biology

9 Different molecules have different mutation rates. Scientists can choose which molecule to use as a molecular clock depending on what they are measuring. Molecules with faster mutation rates can be used as molecular clocks for recent evolutionary time. Molecules with slower mutation rates can be used as molecular clocks when studying longer periods of evolutionary time. Mitochondrial DNA (mtdna) has a mutation rate that is faster than nuclear DNA. It is therefore useful for studying closely-related species that have diverged from a common ancestor recently in evolutionary time. Ribosomal RNA (rrna) has regions that have a very slow mutation rate. This makes it useful for studying distantly-related species, such as those in different kingdoms, which have diverged from a common ancestor much longer ago. Animal Cell ribosome Similarly, you use different clocks depending on how much time you are measuring. You probably use hours to measure your day, but you wouldn t use hours to measure your age you would use years. Which molecule Which molecule is more useful is more for useful studying for distant studying evolutionary distant time mitochondrial evolutionary time mitochondrial DNA or ribosomal DNA RNA? or ribosomal RNA? nucleus mitochondrion Mitochondrial DNA and ribosomal RNA are both used as molecular clocks Vocabulary Check molecular clock mitochondrial DNA ribosomal RNA Match the description below with the correct term from the list above. Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. slow mutation rate 2. fast mutation rate 17.3 The Big Picture 3. What does a molecular clock measure? 4. Why might a scientist choose mitochondrial DNA instead of ribosomal RNA as a molecular clock? Interactive Reader 291

10 SECTION 17.4 Domains and Kingdoms KEY CONCEPT The current tree of life has three domains. B.8.2 Classification is always a work in progress. The tree of life is a model that shows the most current understanding of how all living things are related. Over time, the organization of the tree of life has changed. With new findings and new discoveries, scientists changed how organisms are classified. Until the mid-1800s, all living things were divided into two kingdoms: Plantae (plants) and Animalia (animals). Then, a third kingdom, called Protista, was added for single-celled organisms. A fourth kingdom, called Monera, was added in the early 1900s to give prokaryotes a kingdom of their own. In the mid-1900s, kingdom Fungi was added for organisms such as mushrooms and molds. In 1977, kingdom Monera was divided into two groups of prokaryotes Bacteria and Archaea. Each of these changes was based on new scientific findings. HISTORY OF THE KINGDOM SYSTEM 1753 Two kingdoms 1866 Three kingdoms 1938 Four kingdoms 1959 Five kingdoms 1977 Six kingdoms Animalia Plantae Animalia Plantae Protista Animalia Plantae Protista Animalia Plantae Connecting Protista Animalia CONCEPTS Classification Recall from Section 17.1 that the kingdom is the largest category in the Linnaean classification system. Plantae Protista Monera Fungi Monera Fungi Bacteria Archaea Genetic studies show that the two groups of prokaryotes, bacteria and archaea, are very different from each other. In terms of genes, they are more different from each other than animals are from plants. In which of the six kingdoms are humans categorized? Prokaryote The three domains in the tree of life are Bacteria, Archaea, and Eukarya. The six kingdoms can be divided into three domains. A domain is a grouping larger than a kingdom. Prokaryotes are divided into two domains: Bacteria and Archaea. All eukaryotes are placed into a third domain, called Eukarya. Eukaryote A prokaryotic cell does not have a membrane-bound nucleus or organelles. A eukaryotic cell has a membrane-bound nucleus and organelles. 292 McDougal Littell Biology

11 TREE OF LIFE The most recent classification system divides life into three domains, which include six kingdoms. Domain: Bacteria Domain: Archaea Domain: Eukarya Kingdom: Protista Kingdom: Plantae Kingdom: Fungi Kingdom: Bacteria Kingdom: Archaea Kingdom: Animalia Scientists constructed this evolutionary tree by comparing rrna sequences from species in each of the six kingdoms. The distances between branches represent the number of differences in rrna sequences among these species. Source: C. Woese, PNAS 97:15. Bacteria The domain Bacteria includes the single-celled prokaryotes in the kingdom Bacteria. This domain is one of the largest groups of organisms on Earth. In fact, there are more bacteria in your mouth than there are people that have ever lived! Bacteria are often classified by their shape or whether they cause disease. Archaea The domain Archaea (ahr-kee-uh) contains single-celled prokaryotes in the kingdom Archaea. Archaea and bacteria are both single-celled prokaryotes, but the two groups of organisms have many differences. For example, they have very different cell wall chemistry. Archaea are often classified for the extreme environments they can live in, such as deep sea vents and hot geysers. Eukarya The domain Eukarya (yoo-kar-ee-uh) is made up of all organisms with eukaryotic cells. Some eukaryotes are single-celled, like most protists. Other eukaryotes are multicellular, like you. The domain Eukarya includes the kingdoms Protista, Plantae, Fungi, and Animalia. Interactive Reader 293

12 Notice that protists, plants, fungi, and animals are all grouped in the same domain. But bacteria and archaea are in different domains. Scientists have agreed on this grouping because it clearly represents the diversity of prokaryotes. This grouping reflects the fact that protists, plants, fungi, and animals are more closely related to each other than are bacteria and archaea. What are What the three are the domains three of domains life? of life? 17.4 Vocabulary Check Bacteria Eukarya Archaea Mark It Up Go back and highlight each sentence that has a vocabulary word in bold. 1. List the kingdom(s) that belong to each domain in the chart below. DOMAIN BACTERIA ARCHAEA EUKARYA Kingdom(s) 17.4 The Big Picture 2. Why has the classification of different groups of living things changed over the years? 3. How is a kingdom different from a domain? 294 McDougal Littell Biology

13 Chapter 17 Review 1. The scientific name for the grey wolf is Canis lupis. Draw a circle around the genus. Draw a square around the species descriptor. B Based on the information in the cladogram, which of the following pairs of organisms is most closely related? B.8.2 a. 1 & 4 b. 1 & 5 c. 2 & 5 d. 4 & What information does a cladogram give? B.8.2 a. the physical similarities among species b the evolutionary relationships among species c. the number of years ago that species evolved d. the taxonomy of species 4. What key information does modern classification rely upon that was B.8.2 not available in Linnaeus time? 5. Arrange the following terms in order from largest grouping of organisms to smallest grouping of organisms: family, class, domain, phylum, species, kingdom, genus, order B Name two types of molecules that are commonly used as molecular clocks. 7. Which kingdoms are included in each of the three domains? B.8.3 B Do you think that the current organization in the tree of life will ever change? Defend your answer. B.8.2 Interactive Reader 295