HISTORY AND DIVERSITY OF LIFE ACTIVITY 8 EVOLUTION 41 STOP TO THINK 1 a. Using Student Sheet 8.1, “Vertebrate Evolutionary Tree,” find the place in the diagram, also shown in the following diagram, that represents the point in time when birds and crocodiles diverged, or split apart. When did this happen? Add the time in millions of years ago (mya) to the diagram. b. Find the place in the diagram that represents the point in time when mammals diverged from birds and crocodiles. When did this happen? Add the time in millions of years ago (mya) to the diagram. LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 8_3 MyriadPro Reg 9.5/11 5.0 bya 4 bya 3 bya 2 bya 1 bya 4.6 bya 4.28 bya 3.77 bya 900 mya 530 mya present 6 mya Earth forms first evidence of life first fossils of single-celled organisms first multicellular organisms first vertebrates first humans Timeline of Earth LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 8_4 MyriadPro Reg 9.5/11 fish amphibians mammals crocodiles birds common ancestor Vertebrate Evolutionary Tree
ACTIVITY 8 HISTORY AND DIVERSITY OF LIFE 42 EVOLUTION Diversity of Life119120121 You are probably familiar with a variety of mammal species and bird species. Walk out your door, and you might see one or more of the following: pigeons, starlings, squirrels, dogs, or rats. But mammals and birds together make up less than 0.2% of all species in existence today. Most life on Earth is relatively unfamiliar to us because it is small, lives underground, or lives in water. For example, scientists estimate that there are 350,000 species of beetles, a type of insect. That is over 23 times the number of mammal and bird species combined! The following diagram shows how many species exist today in each of the major groups of organisms. The group labeled “chordates” includes all vertebrates: fish, amphibians, reptiles, birds, and mammals. STOP TO THINK 2 a. Use the key to calculate how many total known species there are. b. Calculate the approximate percentage of insects. c. The figure in the previous section shows the evolutionary tree for vertebrates. Which “slice” in this figure would be home to this tree? 119 NGLS4A1 120 NGLS4A2 121 NGCCPA2 LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 8_5 MyriadPro Reg 9.5/11 KEY insects 925,000 non-insect arthropods 123,000 (including spiders and crabs) chordates 43,000 other animals 116,000 (including worms, clams, and snails) protists 30,000 fungi 69,000 (including mushrooms) algae 27,000 vascular plants 248,000 (including trees and ferns) Abundance of Known Species
HISTORY AND DIVERSITY OF LIFE ACTIVITY 8 EVOLUTION 43 Extinction122 Life has existed on Earth for over 4 billion years, and species keep evolving into new and more varieties of life, so why doesn’t Earth have billions of species? You estimated the total number of species that have been documented (although scientists estimate that there may be 10 million species in existence, with most of them yet to be discovered). Recall from the previous activity that separate species are defined as populations whose members don’t interbreed. If every species that has ever existed were still alive, scientists estimate that there would be over 5 billion species on Earth today. This means that over 99% of species that ever evolved have become extinct. Extinction is the complete and permanent disappearance of a species, while speciation is the formation of a new species. Extinction, like speciation, is part of evolution. Through chance, or by being poorly adapted to a changing environment, a species may go extinct through natural selection. Sometimes major groups of organisms go extinct. Trilobites were common until they went extinct 250 mya. Large dinosaurs went extinct 65 mya. At other times, smaller groups became extinct, and sometimes only certain species disappeared. For example, approximately 12,000 years ago, the saber-toothed cat went extinct. As recently as 350 years ago, the dodo bird went extinct. You will learn more about extinction in the activity “The Sixth Extinction?” STOP TO THINK 3 What are some reasons that a species might go extinct? Life Over Time123124 Scientists know that life evolved in the ocean. For the first 3.75 billion years of life on Earth, life existed only in water. Life on land is relatively recent, beginning around 550 million years ago at what is the start of an event that scientists have named the Cambrian explosion. This “explosion” was not an actual explosion. The Cambrian explosion was the rapid evolution of a wide variety of life forms and groups of organisms over a relatively short period of time. Scientists think that the Cambrian explosion lasted around 20 million years. That may seem like a long period of time to most people, but it is a short period of time to scientists studying evolution! During this time, most of the major groups of animals that still exist today in some form began to evolve. 122 NGCCPA1 123 NGLS4A1 124 NGLS4A2
ACTIVITY 8 HISTORY AND DIVERSITY OF LIFE 44 EVOLUTION The following diagram shows when different life forms evolved, beginning with the Cambrian explosion and continuing through today. Note that some life forms went extinct. LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 8_7 MyriadPro Reg 9.5/11 Present first humans first primates 550 mya 400 mya 300 mya 200 mya 100 mya 500 mya Cambrian explosion first ffowering plants first birds first ammonites last scale trees last trilobites last dinosaurs first molluscs first trilobites first vertebrates (fishes) first vascular land plants first ffightless insects first amphibians first ffying insects first reptiles first mammals first scale trees first dinosaurs last ammonites First appearance Last appearance Life Over Time
HISTORY AND DIVERSITY OF LIFE ACTIVITY 8 EVOLUTION 45 STOP TO THINK 4 a. Which life forms no longer exist today? b. When did flowering plants first evolve? c. When did the first humans, Homo sapiens, evolve? ANALYSIS 1. Why do scientists who study evolution think of 20 million years as a short time?125 2. What does the statement “extinction and speciation are both parts of evolution” mean? 3. How did there get to be such a great variety of life?126 4. Write a detailed caption for the following diagram.127128 5. Revisit the issue: How do you think people affected or are affected by the vast diversity of life that has evolved over long periods of time? 125 ELWH682 126 NGSPCE6 127 NGSPOE1 128 SEASCM1 LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 8_6 MyriadPro Reg 9.5/11 most plant-eating dinosaurs all meat-eating and very large crocodiles dinosaurs lizards and snakes birds common ancestor Dinosaur-Bird Evolutionary Tree
EVOLUTION 47 9 Fossil Evidence l a b o r at o r y You learned in the previous activity that many species have become extinct during the history of Earth. How can you know for sure that these creatures ever existed? Some of the evidence is right under your nose—or your feet, to be more precise. Our planet’s thin outer layer, the crust, can be up to 40 kilometers (25 miles) thick. The crust is made up of many layers of rock that have been forming for over 4 billion years—and are still forming today. These rock layers can form when a volcanic eruption covers the land with lava, or when a flood spreads out a layer of mud. Lava, mud, or even sand can eventually harden into solid rock. Any new layer or rock can seal off the layer below it. Sometimes an interesting phenomenon occurs: Organisms become trapped within these sealed-off layers and become part of the rock itself. These trapped organisms can become fossils. Any remains of life preserved in a rock is called a fossil. A fossil can can be an entire organism, a part of an organism, a footprint, a piece of feces, or a piece of shell, bone, or tooth. The scientists who study fossils are paleontologists. In this activity, you will examine real fossils as a paleontologist would.129130 GUIDING QUESTION What kind of evidence do fossils provide about evolution? 129 NGLS4A1 130 NGLS4A2 When and where did this fossil vertebrate live?
ACTIVITY 9 FOSSIL EVIDENCE 48 EVOLUTION MATERIALS For each group of four students 1 set of 4 fossils 1 set of 4 simulated drill cores 1 pair of scissors 1 metric ruler colored pencils For each pair of students 1 hand lens For each student 1 Student Sheet 9.1, “Stratigraphic Columns from Drill Cores” PROCEDURE Part A: Examining Fossils 1. One pair in the group will examine one of the fossils, while the other pair examines another fossil.131132133134 2. Work with your partner to identify the unique features of the fossil. Use the hand lens to help you. 3. In your science notebook, sketch the general shape and unique features of this type of fossil. Record any additional observations that are difficult to show in your sketch, such as color or size. 4. Record anything else you know about the fossil that you may have learned in previous lessons. 5. Repeat Steps 1–4 until you have examined all four fossil types. 6. Read the following section on finding and dating fossils. How Do Scientists Find and Date Fossils? Scientists look for fossils where several layers of rock are visible, one on top of the other. One common place where these layers are visible is in the walls of a deep river canyon. As each rock layer forms, it is deposited on top of an existing layer. In a sequence of rock layers, the top layer, along with any fossils it contains, is younger than any other layer in that sequence, and the bottom layer, along with any fossils it contains, is the oldest layer in that sequence. This is called the law of superposition. 131 NGSPAD1 132 NGSPNS1 133 NGCCNS3 134 NGCCCE2
FOSSIL EVIDENCE ACTIVITY 9 EVOLUTION 49 A diagram representing a series of rock layers, such as the one on the right (below the photo), is called a stratigraphic column. Stratigraphic columns can be made by looking at the sides of cliffs or by looking at drill cores. A drill core is a cylindrical piece of rock removed from Earth by a large drill, similar to the drills that are used to make oil wells. Drill cores can provide samples from many miles beneath Earth’s surface. No single location contains a complete set of all the rock layers or fossils that exist on Earth. To study a particular fossil organism or find out which organisms lived during which geologic era, paleontologists must compare rocks from different places throughout the world. Scientists must also figure out how old the rocks are. With the help of radioactive dating technology, scientists have made good estimates of how old each layer is and how many years each layer took to form. Part B: Figuring Out Fossils How can you determine which fossils are older, which are younger, and which are likely to be from extinct species? You will examine and compare four different drill cores, each representing the rock layers found on different fictional continents.135136137138 7. Each pair will examine one of the drill cores. The top of each drill core is marked with its number. 8. Create a stratigraphic column by sketching in the boundaries of the layers and the fossils found within each layer in the appropriate place on Student Sheet 9.1, “Stratigraphic Columns from Drill Cores.” 9. Based on the evidence within the layers of this drill core, list the fossils in order from youngest to oldest. 10. Repeat Steps 7–9 for one of the remaining two drill cores. (The other pair in your group will examine the other remaining drill core.) 11. Combine all four columns, and look at them side by side. 135 NGES1C1 136 NGSPAD1 137 NGCCPA2 138 ELRS683 Rock layers in the Grand Canyon (top) and a schematic diagram of fossils in rock layers, also known as a stratigraphic column (bottom)
ACTIVITY 9 FOSSIL EVIDENCE 50 EVOLUTION 12. Based on the appearance of the rock layers and the fossils found within each layer, match (or correlate) the layers from each core as best as you can. Make a chart, similar to the following one, that shows your correlation of the rock layers from the four different drill cores. Hint 1: You may want to cut out each column from the Student Sheet so that you can move them around as you try to match up the layers. Hint 2: Layers don’t have to be exactly the same to correlate. 13. Use your correlation chart to list all four fossils in order from youngest to oldest. Hint: If you think a layer found in one drill core is the same as a layer found in another drill core, you can infer that those layers, and the fossils in them, are the same age.139 139 NGSPCE6 Sample Correlation of Stratigraphic Columns Layer 1 Layer 2 Layer 3 Layer 4 Layer 1 Layer 2 Layer 3 Layer 4 Layer 1 Layer 2 Layer 3 Layer 4 Column Y Column Z Column X
FOSSIL EVIDENCE ACTIVITY 9 EVOLUTION 51 ANALYSIS 1. Imagine that scientists have discovered two new fossils. Describe two ways that scientists can figure out the ages of the fossils and whether the two organisms existed at the same time.140 2. Use the evidence from all four drill cores to answer the following questions:141142143144145146 a. Which, if any, of the organisms represented by the fossils may be from species that are now extinct? Explain. b. Which of the fossils that you examined in Part A are from a group of organisms that may have gone extinct, and how long ago did that happen? c. Which fossil species could have lived at the same time? Explain. 140 NGSPCE6 141 142 NGCCPA2 143 SEASAD1 144 NGSPAD1 145 NGLS4A1 146 NGLS4A2
EVOLUTION 53 10 Fossilized Footprints i n v e s t i g at i o n I n the “Fossil Evidence” activity, you examined some fossils that were the preserved remains of whole or partial organisms. But in some cases, the only evidence left by an organism is its tracks. Footprints and other types of animal tracks can be fossilized in the same way as actual body parts. These are called trace fossils. But what can you find out from just footprints? Like detectives, paleontologists can use the information from fossil footprints to determine how an organism moved, how fast it traveled, what type of environment it lived in, and what it might have been doing when its footprint was formed. They might also be able to draw some conclusions about the social behavior of the organism. 147148149150 GUIDING QUESTION What other kinds of information can we get from fossils? 147 NGLS4A1 148 NGLS4A2 149 NGCCNS3 150 NGSPNS1 Few fossil remains are as complete as this 10-million-year-old rhinoceros in Nebraska.
ACTIVITY 10 FOSSILIZED FOOTPRINTS 54 EVOLUTION MATERIALS For each group of four students 1 set of 3 Fossil Footprint Cards 1 metric ruler PROCEDURE151152 Part A: Initial Data 1. Examine Fossil Footprint Card 1, which shows what the team has uncovered so far.153154 2. In your group, discuss what you think was happening while these footprints were being created. You do not have to agree with your other group members, but: • If you disagree with others in your group about what happened, explain to the rest of the group why you disagree. • Listen to and consider other people’s explanations and ideas. 3. Record your ideas in your science notebook. Separate your ideas into observations and inferences. Note: Even though some of your inferences may conflict with other inferences, consider as many ideas as possible. 4. Time passes and more footprints are uncovered. Obtain Fossil Footprint Card 2. 151 NGSPAD1 152 NGCCPA2 153 ELRS683 154 NGCCPA1 Evidence Comes in Steps A fossil footprint site has just been discovered! You take a helicopter to the location, with the hope that your expertise will be useful. The rest of the team is slowly brushing away layers of sediment to carefully uncover the footprints. Your task is to make observations of fossil footprints and use them to draw inferences about what happened in the past. An observation is any description or measurement gathered by the senses or by instruments. An inference is a conclusion based on observations or what is already known. Based on your inferences, you will develop a hypothesis—a tentative explanation—about what happened to form the footprints. As the footprints are uncovered, there will be more evidence to examine. Remain open to new possibilities as the investigation continues.
FOSSILIZED FOOTPRINTS ACTIVITY 10 EVOLUTION 55 5. Repeat Step 2. Record your additional observations and inferences in your science notebook. However, do not change what you wrote for Card 1! 6. Time passes and a third section of footprints is uncovered. Obtain Fossil Footprint Card 3. 7. Repeat Step 2. Record your additional observations and inferences in your science notebook. Remember, do not change what you wrote for Cards 1 and 2! 8. Look back at all your observations and inferences. Try to think of the best possible explanation for how the footprints were formed. Record your strongest hypothesis, or tentative explanation, in your science notebook. If you have two or more explanations in mind, record them all, but rank them from most likely to least likely. 9. Answer Analysis items 1 and 2. Part B: Additional Data 10. Hypotheses change as scientists gather new data. The information in the following table has just come in from the fossil site! Average Depth of Footprints (Scenario 1) CORE CARD 1 CARD 2 CARD 3 Larger footprints 6.0 cm 6.2 cm 8.3 cm Smaller footprints 2.5 cm 2.6 cm ___ a. What hypothesis would the data in the table above support? b. Develop an argument for which hypothesis is strongest. Fossil footprints
ACTIVITY 10 FOSSILIZED FOOTPRINTS 56 EVOLUTION 11. Instead of the data in Scenario 1, imagine that you just received the data in Scenario 2, in the following table. 155156157 Average Depth of Footprints (Scenario 2) CORE CARD 1 CARD 2 CARD 3 Larger footprints 6.0 cm 6.2 cm 6.1 cm Smaller footprints 2.5 cm 2.6 cm ___ a. What hypothesis would the data in Scenario 2 support? b. Develop an argument for which hypothesis is strongest. c. What factor (or combination of factors) might explain the difference in the depth of the footprints in the different scenarios? ANALYSIS 1. What kind of information might be obtained from trace fossils that cannot be obtained from the fossilized remains of the organism itself?158 2. Why is it important for scientists—and people in general—to distinguish between observations and inferences when they develop hypotheses?159160 3. Imagine that the team uncovered a fourth section of footprints. Draw what you predict this fourth section might look like. Explain how it would provide more support for the argument you favor.161 4. How does studying all kinds of fossils help us better understand evolution?162163 5. Revisit the issue: How might studying fossils help people understand how people affect evolution? 155 NGSPEA2 156 NGCCCE2 157 SEASAR1 158 NGCCPA2 159 NGSPEA2 160 NGCCCE2 161 NGSPAD1 162 NGLS4A1 163 NGLS4A2
EVOLUTION 57 11 Family Histories i n v e s t i g at i o n Sasha walked into the kitchen and saw a drawing that she had made in kindergarten on the refrigerator door. She exclaimed, “Where did you find that? It is so old! I made that a million years ago!” Sasha’s dad replied, “I was going through a box of keepsakes and found it. It was always one of my favorites. But it was only seven year ago! If you’d made it a million years ago, there would have been saber-toothed cats around.” “Well, it seems like a million year ago,” said Sasha. “But how do we know that there were saber-toothed cats back then?” “That’s a great question, Sasha!” declared her dad. Fossils have been found in rocks that are 3.77 billion years old! But most fossils have been found in rocks that are all less than 550 million years old, after the Cambrian explosion you learned about in the “History and Diversity of Life” activity. The types of organisms found in different rocks can provide important information about the history of life on Earth. The term fossil record refers to all the fossils that have been found on Earth.164165166167 The fossil record has been used to classify fossils into different groups of organisms, depending on how similar they are. One kind of group is a family. For example, among mammals, dogs are in the family Canidae, which also contains foxes, jackals, coyotes, and wolves. Lions are also related to dogs, but not as closely. They are more related to mammals in the family Felidae, which includes leopards, tigers, cheetahs, house cats, and extinct species such as the saber-toothed cat. You will investigate how the numbers of families of fish, mammals, and reptiles have changed over geological time. GUIDING QUESTION What can you learn about evolution by comparing the fossil records of fish, mammals, and reptiles? 164 NGLS4A1 165 NGLS4A2 166 NGCCNS3 167 NGSPNS1
ACTIVITY 11 FAMILY HISTORIES 58 EVOLUTION Classifying Carnivores CLASSIFICATION LEVEL DOGS LIONS Kingdom Animalia Animalia Phylum Chordata Chordata Class Mammalia Mammalia Order Carnivora Carnivora Family Canidae Felidae Genus Canis Panthera Species familiaris leo MATERIALS For each student colored pencils 1 Student Sheet 11.1, “Graphs of Fossil Families” PROCEDURE 1. The following “History of Fossil Fish Families” table provides the history of all the families of fish currently known from the fossil record. When a fossil is found that does not belong to any family found in earlier geologic time periods, we call it a first appearance. It is the first appearance of that family in the fossil record. When a fossil is found that does not belong to any family found in later geologic time periods, we call it a last appearance. It is the last appearance of that family in the fossil record. Look at the table and discuss the following questions with your partner:168169170 • Between which years did the greatest number of fish families appear in the fossil record? • Between which years did the greatest number of fish families disappear from the fossil record? History of Fossil Fish Families 168 NGCCPA2 169 NGSPAD1 170TIME (MYA) NGSPCE6 > 545 485 425 365 305 245 185 125 65 0 Number of first appearances 0 25 43 162 67 13 52 33 84 299 Number of last appearances 0 9 31 158 49 48 36 20 44 34
FAMILY HISTORIES ACTIVITY 11 EVOLUTION 59 2. The following double bar graph is based on the data shown in the “History of Fossil Fish Families” table. Look at the graph, and discuss with your partner in what ways the graph makes the data easier to interpret. 3. Use the information in the “History of Fossil Reptile Families” table to make a double bar graph for families of reptiles, similar to the one for fish shown above. Since you will compare the graphs, be sure to use the same scale on the y-axis. 171 171 MASP6B5 0 50 100 150 200 299 first appearances last appearances > 545 485 425 365 305 245 185 125 65 0 Millions of years ago Number of Families LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 11_3 MyriadPro Reg 9.5/11 Graph of Fossil Fish Families Over Time A familiar example of a fossilized reptile TIME (MYA) > 545 485 425 365 305 245 185 125 65 0 Number of first appearances 0 0 0 0 3 67 95 68 97 35 Number of last appearances 0 0 0 0 1 57 93 46 84 26 History of Fossil Reptile Families
ACTIVITY 11 FAMILY HISTORIES 60 EVOLUTION 4. Use the information in the “History of Fossil Mammal Families” table to make a double bar graph for families of mammals. Since you will compare the graphs, be sure to use the same scale on the y-axis.172 ANALYSIS 1. Describe the patterns in the three graphs and what these patterns tell you about the evolution of the three groups of organisms. Be sure to include how they are similar and how they are different.173174175176177178 2. Scientists sometimes label periods of time by what organisms were common at the time. a. The period of time from 65 mya until today is often referred to as the age of mammals. Use evidence from this activity to explain why. b. Based on evidence from this activity, what would you call the period of time from 305 mya to 65 mya? Explain your reasoning. 172 ELRS687 173 NGSPAD1 174 SEASAD1 175 NGSPPA2 176 NGLS4A1 177 NGLS4A2 178 NGPEL41 TIME (MYA) > 545 485 425 365 305 245 185 125 65 0 Number of first appearances 0 0 0 0 0 0 6 14 33 404 Number of last appearances 0 0 0 0 0 0 2 8 33 262 History of Fossil Mammal Families
FAMILY HISTORIES ACTIVITY 11 EVOLUTION 61 3. Look at your answer for Analysis item 1 and the following table and graph. Where do you think scientists have placed the amphibian family? Explain your answer. 4. Does the evidence in this activity support the figure of the vertebrate evolutionary tree in the “History and Diversity of Life” activity? 179 179 NGSPCE6 TIME (MYA) > 545 485 425 365 305 245 185 125 65 0 Number of first appearances 0 0 0 3 35 33 19 11 5 15 Number of last appearances 0 0 0 3 16 53 18 5 1 5 History of Fossil Amphibian Families 0 50 100 150 200 first appearances last appearances > 545 485 425 365 305 245 185 125 65 0 Millions of years ago Number of Families LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 11_8 MyriadPro Reg 9.5/11 Graph of Fossil Amphibian Families Over Time
EVOLUTION 63 12 A Whale of a Tale i n v e s t i g at i o n Whales, dolphins, and porpoises are mammals that live in the sea. Like all mammals, they are warm-blooded animals that give birth to live young and need air to breathe. DNA evidence shows that whales are closely related to hoofed land mammals such as hippopotamuses, pigs, cows, and sheep. Genes are made of DNA, which is the molecule that codes for traits in all organisms. All of these mammals are thought to have descended from a single species that lived millions of years ago and is now extinct. But how did this take place? Did sea mammals appear first and then evolve into land mammals, or did it happen the other way around? GUIDING QUESTION How did whales evolve?
ACTIVITY 12 A WHALE OF A TALE 64 EVOLUTION MATERIALS For each pair of students 1 set of 5 Whale Skeleton Cards For each student 1 Student Sheet 12.1, “Whale Fossil Chart” PROCEDURE180181182183184 1. Compare the five Skeleton Cards. Based on the similarities you observe, group the skeletons into two sets, each containing two or three cards. The set of skeletons containing Skeleton A should be called “Group 1.” The other set of skeletons will be “Group 2.” 2. Create a Venn diagram like the following one shown in your science notebook.185186187188 180 NGCCPA2 181 NGLS4A1 182 NGLS4A2 183 NGES1C1 184 NGCCNS3 185 NGSPAD4 186 NGCCPA2 187 MASP6B5 188 NGCCPA1 The Fossil Exhibit You’ve just been hired as the assistant curator of the fossil collection of a museum. On your first day, you discover that the skeletons in the exhibit on the evolution of whales have all been moved to a new room and need to be arranged. Unfortunately, you are not a whale expert, and the skeletons are not clearly labeled. A local middle school has scheduled a field trip to the museum. It is very important that you arrange the skeletons properly before the students arrive. You decide to examine them to see if you can figure out how they should be arranged. Skeleton of a modern blue whale group 1 group 2
A WHALE OF A TALE ACTIVITY 12 EVOLUTION 65 3. Compare the Group 1 skeletons with those of Group 2. In the Venn diagram, describe and record as many similarities and differences as you can. 4. It’s time to figure out how to arrange the exhibit! Use similarities and differences in the skeletons to arrange the cards in order. (While all five skeletons can be in a single line, they don’t have to be.) Record the order in which you have arranged the skeletons. Hint: Place the two least similar skeletons on either side of your desk. Then arrange the other three skeletons between them. 5. Your colleague reminds you that the museum has a collection of modern whale embryos, and you wonder if you could use them in the exhibit. When you inspect the embryos, which are organisms in the early stages of development before they are born or hatched, you discover two phenomena that surprise you:189 • The embryos have some hair on their bodies. Adult whales don’t have any hair. • The embryos have small hind limb buds. Limb buds are the structures that develop into limbs in animals. Adult whales have no external hind limbs. 6. You wonder if these findings might be important for your exhibit. Look at the order of your Skelton Cards, and make any changes that you think are necessary. 190 7. You’re in luck! You discover a chart with information about the relative ages of the five skeletons. Obtain Student Sheet 12.1, “Whale Fossil Chart,” from your teacher. 8. Compare the age data from Student Sheet 12.1 with the order in which you placed the skeletons in Step 6. If necessary, rearrange your Skeleton Cards. Record your final reconstruction of the museum exhibit in your science notebook.191 189 NGLS4A3 190 NGSPEA2 191 NGSPCE6
ACTIVITY 12 A WHALE OF A TALE 66 EVOLUTION ANALYSIS 1. During the evolution of whales ... a. What kinds of skeletal changes have occurred? b. What changes in habitat might have occurred at the same time? 192 2. Use natural selection to explain how these changes (or one of these changes) could have occurred.193 3. How does the observation that whale embryos have hair and hind limb buds help you understand whale evolution?194 4. Answer the question in the introduction: Did sea mammals appear first and then evolve into land mammals, or did it happen the other way around? Explain, using evidence and scientific reasoning.195196197198199 5. In this activity, you examined extinct and modern whale skeletons. How does the study of these skeletons provide evidence about how species are related? 6. Look again at Skeleton A. This is known as an ambulocetid (am-byoo-low-SEE-tid). The word ambulocetid means “walking whale.” Where do you think the ambulocetids lived? Describe how you think they lived.200 EXTENSION Visit the SEPUP Third Edition Evolution page of the SEPUP website at www.sepuplhs.org/middle/third-edition and go to the whale DNA link to learn how DNA evidence is used to investigate the evolutionary history of whales. How does this evidence compare to the evidence in this activity? 192 NGCCPA1 193 NGSPCE6 194 NGLS4A3 195 NGPEL42 196 NGLS4A1 197 NGLS4A2 198 NGLS4A3 199 SEASEX1 200 NGSPEA2
EVOLUTION 67 13 Embryology i n v e s t i g at i o n I n “A Whale of a Tale,” you compared skeletons of five whale species to determine their evolutionary relationships. These skeletons came from mature, or fully grown, animals. Although it is possible to identify similarities and differences between mature animals, scientists also look at developing animals for evidence of evolutionary relationships. In this activity, you will look at developing animal embryos and try to identify evolutionary relationships that might not be obvious in the mature animals.201202 GUIDING QUESTION How can embryos provide evidence about evolutionary relationships? 201 NGLS4A3 202 NGCCPA2 This is an embryonic pig transitioning between early and middle stage development. It is approximately 16 millimeters in length. Structures, such as the eye, nose, mouth, forelimb, hindlimb, and ribs, can be identified.
ACTIVITY 13 EMBRYOLOGY 68 EVOLUTION MATERIALS For each group of four students colored pencils For each pair of students 1 set of 12 Embryonic Limb Cards 1 set of 20 Whole Embryo Cards For each student 1 Student Sheet 13.1, “Comparison of Vertebrate Forelimbs” PROCEDURE Part A: Skeletal Forelimb Comparison 1. With your partner, examine the forelimb skeletons of six animals on Student Sheet 13.1, “Comparison of Vertebrate Forelimbs.” 203204 2. Compare the bones in the skeletons and identify the following: • Hand and/or foot • Wrist • Forearm • Upper arm 3. Use colored pencils to color bones that you think might function the same way. (Use the same color for any structure that performs the same function.) 4. Compare your Student Sheet with the scientifically accepted color-coded diagram provided by your teacher. 5. With your partner, discuss how the structures and functions of the forelimbs you labeled provide evidence about evolutionary relationships between the six animals.205 Part B: Embryonic Limb Comparison 6. Obtain a set of 12 Embryonic Limb Cards from your teacher. The cards show three different limbs—a bat forelimb, a bat hindlimb, and a mouse hindlimb—at three different stages of development: early, middle, and late. 7. For each limb, compare the images on the cards. Sort the cards into three groups: early, middle, and late development. See if you can identify which card corresponds to each limb. 203 NGCCPA2 204 NGLS4A2 205 NGCCSF1
EMBRYOLOGY ACTIVITY 13 EVOLUTION 69 8. As you sort, be sure to discuss what information you are using to decide the order. Record this information in your science notebook. Each card has an identifying letter in the corner. Use these letters to record your sorting in your notebook. 9. Based on your observations and final sorting, discuss with your partner whether you see evidence that these animals are related. Be sure to record your ideas in your science notebook.206 Part C: Whole Embryo Comparison 10. Now you will look at the embryological development of five whole animals: a human, a snake, a bat, a chicken, and a salmon. You will examine the five embryos one stage at a time. a. Obtain the early-stage Whole Embryo Cards from your teacher. There are five embryos, along with the names of the five animals. b. Make and record your observations in your science notebook. c. Identify which animal you think matches each embryo. Each Whole Embryo Card has an identifying letter in the corner. Use these letters to record your decision and rationale in your science notebook. 11. Repeat Step 10 with the middle-stage Whole Embryo Cards. This set of cards shows the next stage of development for the same five animals. a. Identify which early-stage embryo you think matches each middle-stage embryo. Adjust the cards as your thinking changes. b. Record your new sorting and reasoning in your science notebook. 12. Repeat Step 10 with the late-stage Whole Embryo Cards. This third set of cards shows the last stage of development for the same five animals. 13. Share your sorting and reasoning with another pair of students. As a small group, revise your sorting based on your discussion, if necessary. 206 NGLS4A3
ACTIVITY 13 EMBRYOLOGY 70 EVOLUTION 14. Record your final sorting and reasoning in your science notebook.207 15. Based on your observations and final sorting, discuss with your partner what embryological evidence suggests about whether and how the five animals are related. Be sure to record your ideas in your science notebook. EXTENSION Visit the SEPUP Third Edition Evolution page of the SEPUP website at www.sepuplhs.org/middle/third-edition, and go the link for the video of chick embryo development. As you watch, try to identify the points in the video that correspond to images on the cards in Part C. ANALYSIS 1. Was it easy to identify the type of animal when looking at embryological images? Why or why not? 2. Review the observations you recorded in your science notebook.208 a. What patterns did you observe? Hint: A pattern is something that happens in a repeated and predictable way. b. What structures appeared and when? c. What structures disappeared and when? 3. What relationships across different animal species can you see in embryological data that you cannot observe by comparing mature animals? Use data from your investigation to support your answer.209210211212213214 4. Revisit the issue: How do you think analyzing and interpreting many kinds of evidence helps people understand how they affect and are affecting evolution? 207 NGSPAD4 208 NGSPPA2 209 NGPEL43 210 NGSPAD4 211 NGCCPA2 212 NGLS4A3 213 ELRS687 214 SEASAD1
EVOLUTION 71 14 The Sixth Extinction? ta l k i n g i t o v e r I f every species that has ever existed were still alive, scientists estimate that there would be over 5 billion species on Earth today. But most scientists agree that there are approximately 10 million species in existence today. This means that over 99% of species that ever evolved eventually went extinct. Scientists have concluded from the fossil evidence that extinctions have not occurred evenly over time but have come in waves. In this activity, you will see if you can identify patterns in the rates of extinction in the past. You will also compare extinction rates in the distant past to extinction rates since the year 1500.215 GUIDING QUESTION Is the current rate of extinction typical? MATERIALS For each group of four students 1 set of 5 Extinction Event Cards 1 Student Sheet 14.1, “Past Extinction Rates” 215 NGLS4a1 The dodo was a flightless bird that went extinct in the 1600s.
ACTIVITY 14 THE SIXTH EXTINCTION? 72 EVOLUTION PROCEDURE Part A: Rates of Extinction in the Past 1. Look at the following graph of “Past Extinction Rates” (the same graph as Student Sheet 14.1). What does it show? Do you detect any patterns? Explain.216217218219 Past Extinction Rates 2. Get a set of Extinction Event Cards from your teacher, and spread them out so that all group members can read them. Each card represents a wave of extinction identified by scientists in the graph above. 3. Work with your group to match each Extinction Event Card to a peak in the graph above. 4. With your group, identify any similarities and differences among the possible causes for the five mass extinction events identified by scientists, and be prepared to share your ideas with the class.220221 5. The green area on the graph represents the background rate of extinction. This is the average rate of extinction over time. How many times greater than the background rate was the second mass extinction? the fifth mass extinction? 216 NGCCPA1 217 NGCCPA2 218 NGSPAD1 219 NGCCNS3 220 NGCCCE1 221 NGCCCE2 LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 14_2 MyriadPro Reg 9.5/11 Extinction rate (families per million years) 20 5 10 15 0 600 400 200 0 Millions of years ago 500 300 100
THE SIXTH EXTINCTION? ACTIVITY 14 EVOLUTION 73 Part B: Rates of Extinction, 1500–Present222223224225 6. Read the following text about extinction in modern vertebrates. 7. With your partner, discuss the pattern you see in the above graph and what might be causing this pattern. 222 ELRS687 223 ELWH689 224 NGES3C1 225 NGES3C2 Recent Extinctions While the extinctions you have investigated so far all happened a very long time ago—tens or hundreds of millions of years ago—extinction happens all the time. Some scientists study fossils and their living relatives to understand extinctions that are taking place today, or have taken place in the recent past— from hundreds to a few thousand years ago. This allows scientists to determine the current rate of extinction and to compare it with the background rate. A group of scientists examined the number of vertebrate species that have gone extinct since the year 1500. Their results appear below. LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 14_3 MyriadPro Reg 9.5/11 Cumulative percentage of extinctions 2.50 1500–1600 1600–1700 1700–1800 1800–1900 1900–2010 Background Other vertebrates Birds Mammals Time period 2.00 1.50 1.00 0.50 0.00 Extinction in Modern Vertebrates Scientists calculate that the observed extinctions in just the past 114 years would have taken approximately 5,000 years at the background rate of extinction.
ACTIVITY 14 THE SIXTH EXTINCTION? 74 EVOLUTION Part C: Evidence About Causes of Extinction, 1500–Present 8. With your partner, read the following text about extinction in birds. LabAids SEPUP IAPS Evolution 3e Figure: Evo3e SB 14_4 MyriadPro Reg 9.5/11 1500–1525 1775–1800 1875–1900 1975–2000 KEY invasive species hunting agriculture logging residential and commercial development climate change and severe weather other: energy production, mining, transportation, harvesting plants and aquatic resources, water use, pollution, etc. Causes of Bird Extinctions Since 1500 Birds have long fascinated both scientists and the average person, and the distribution and abundance of birds over recent time have been better documented than those of other organisms. Because birds are vertebrates, with hard bones, they form fossils relatively frequently. Scientists studying both fossil birds and their modern relatives have been able to determine the actual or probable causes of extinction for species that have gone extinct since 1500. Their findings are outlined below. Bird Extinctions
THE SIXTH EXTINCTION? ACTIVITY 14 EVOLUTION 75 226227 9. Discuss with your partner what patterns you see in the data about bird extinctions and what conclusions you might make from these patterns. 10. What similarities and differences do you notice when comparing extinctions from long ago with more recent extinctions? 11. Discuss this question with your group: Do you think we are experiencing a sixth mass extinction? Be prepared to share your ideas with the class and to cite evidence to support your answer.228 ANALYSIS 1. In the activity “Fossil Evidence,” you examined fossils of organisms that have gone extinct. Did any of them go extinct during the five extinction events? If so, during which events did the extinctions occur? If not, when on the timeline did they go extinct? 2. Are we experiencing a sixth extinction? Be sure to use scientific evidence from this activity (and elsewhere in the unit) to support your claim.229230 3. Revisit the issue: How might people be affected if the current rate of extinction continues into the future? 4. Should we do anything to reduce the current rate of extinction? Support your answer with evidence, and include the trade-offs of your decision.231232 EXTENSION Visit the SEPUP Third Edition Evolution website at www.sepuplhs. org/middle/third-edition, and go to the link for this activity to learn more about how humans might be affected by extinction of species and loss of biodiversity. Reflect on some of the ways that you benefit from biodiversity. 226 NGLS4B2 227 NGLS4D1 228 NGSPEA2 229 230 SEASAR1 231 NGCCNS4 232 SEASET1
EVOLUTION 77 15 Bacteria and Bugs: Evolution of Resistance r e a d i n g I n “The Sixth Extinction?” activity, you explored how humans affect evolution by causing an increase in the rate of extinction. In this activity, you will revisit a phenomenon you explored in “The Full Course”: the resistance of bacteria to antibiotics. By now, you know more about evolution by natural selection and how the presence of a change in the environment can affect evolution. In the case of bacteria, the presence of antibiotics in the environment can lead to selection for traits that increase resistance. But does resistance to chemicals evolve in other situations? In this activity, you will explore some similar situations to decide if the evolution of resistance happens elsewhere.233234235 GUIDING QUESTION What is the evidence that resistance to chemical control methods is evolving in other types of organisms? 233 NGLS4B2 234 NGLS4C1 235 NGLS4B1 Crop duster spraying chemical weed killer on crops
ACTIVITY 15 BACTERIA AND BUGS: EVOLUTION OF RESISTANCE 78 EVOLUTION MATERIALS For each student 1 Student Sheet 15.1, “Evolution of Resistance” READING 1. Read the following text.236237 2. After reading about each type of organism, complete the corresponding section of Student Sheet 15.1, “Evolution of Resistance.” 238239 House Mice House mice, Mus musculus, can pose problems for people because they eat and contaminate human food. They also occasionally spread certain diseases. For centuries, people have used traps and poisons to get rid of house mice. In 1948, people started using warfarin, a chemical that at high doses causes deadly internal bleeding. At first, warfarin was very effective in controlling mouse populations. But by the 1960s, warfarin began to be less effective. Today, some house mice are no longer affected by warfarin. A new chemical, nicknamed “super warfarin,” was developed. This newer chemical was also highly effective—at first. But now, some populations of house mice are resistant to that chemical as well. Weeds Weeds are any plants that grow where people do not want them. While rarely harmful to people, they can interfere with human activity. For example, weeds are a problem for farmers because they compete with crop plants for space and nutrients. Scientists estimate that weeds caused an average of 50% reduction in corn production in the United States and Canada over the period from 2007 to 2013. This loss equates to 148 million metric tons (more than 326 billion pounds) of corn. The cost of this loss was over $26.7 billion each year. Much effort and money has been put into developing methods for weed control. These methods include the use of chemicals. One commonly used chemical, glyphosate, was shown to kill weeds in 1970. A common brand is Roundup©, which works against a wide variety of plants. For the first 20 years of its use, it remained highly 236 ELRS681 237 ELWH689 238 NGSPOE1 239 SELTDA1
BACTERIA AND BUGS: EVOLUTION OF RESISTANCE ACTIVITY 15 EVOLUTION 79 effective at killing weeds. But in the 1990s, some weeds began to develop resistance to glyphosate. To date, over 30 important weed species are no longer affected by glyphosate. Mosquitoes Mosquitoes spread serious diseases like the Zika virus, West Nile virus, Dengue fever, and malaria. Because of the risk to humans, public health efforts have gone toward reducing adult mosquito populations and reducing mosquito breeding grounds. There are several classes of chemicals commonly used as insecticides. The World Health Organization has reported that since 2010, a total of 60 countries have observed mosquito resistance to at least one class of insecticide, with the first resistance to DDT (one of these insecticides) documented in 1959 in India. In 49 of those countries, mosquitoes are resistant to two or more classes of insecticides. Experts are concerned that this will cause an increase in the number of cases of mosquito-borne diseases and the deaths they cause. Plasmodium As you learned earlier in the unit, mosquitoes spread malaria. A mosquito transmits the parasite (Plasmodium) to a human when taking its blood meal. Malaria is one of the deadliest mosquito-borne diseases in the world. More than 250 million cases are reported each year, leading to more than 1 million deaths. Globally, it costs about $12 billion a year to treat people with malaria. Another $12 billion a year is lost because of decreased productivity due to work absences. In countries where malaria is common, the disease accounts for 60% of visits to health clinics. The original treatment for malaria was the drug chloroquine, which was developed in 1934. This drug, which requires taking pills once a week, worked well for about 30 years. In the 1960s, resistance of Plasmodium to chloroquine was detected in Southeast Asia. Since This Colorado potato beetle, an agricultural pest, is resistant to insecticides.
ACTIVITY 15 BACTERIA AND BUGS: EVOLUTION OF RESISTANCE 80 EVOLUTION then, resistant populations of Plasmodium have spread to nearly all parts of the world where malaria exists. Newer drugs were developed to treat the strains resistant to chloroquine. But eventually, Plasmodium developed resistance to the new drugs too. Currently, only one class of drugs remains effective against all malarial strains. ANALYSIS 1. When you compare all four examples, what similarities and differences do you notice?240241242 2. In the “The Full Course,” you explored why it is important to finish an entire course of antibiotics even if you start to feel better. Use information from this activity and what you now know about evolution to provide a scientific explanation for why this is true.243244245 3. Revisit the issue: Can the evolution of resistance by pests or harmful organisms be avoided, or is it inevitable? Explain. 240 NGCCPA1 241 NGCCCE1 242 NGLS4C1 243 SEASEX1 244 NGSPCE1 245 NGLS4B2
EVOLUTION 81 16 Manipulating Genes i n v e s t i g at i o n I n “The Sixth Extinction?” and “Bacteria and Bugs: Evolution of Resistance,” you learned about ways that people have unintentionally affected evolution. You have also learned that these effects can, in turn, affect people. In this activity, you will gather information about two ways that people are intentionally affecting evolution to produce plants and animals with traits that are desirable to us. One method, called selective breeding, involves selecting and breeding parent organisms with desirable traits. This method has been used for thousands of years to improve the animals and plants used for food. The second method, genetic modification, involves modern biotechnology techniques to change one or a small number of genes in an organism. Genetic modification has been developed and improved over the past 20–30 years.246247 GUIDING QUESTION 246 NGLS4B2 247 NGLS4D1 These Holstein cows are bred for milk production.
ACTIVITY 16 MANIPULATING GENES 82 EVOLUTION How have humans manipulated genes in other organisms? MATERIALS For each pair of students 1 computer with Internet access 1 Student Sheet 16.1, “Manipulating Genes Research” PROCEDURE 1. Listen as your teacher explains how you and the other members in your group of four will select the topic for your group’s research. 2. Following your teacher’s instructions, each pair in the group should search for at least one reference about your topic. 3. Use Student Sheet 16.1, “Manipulating Genes Research,” to record the information you obtain from your reference.248249250251 4. When both pairs have completed Steps 2 and 3, share the results of your research with each other, being sure to compare and contrast your findings. 5. Work as a group to prepare a summary of your research, as directed by your teacher, to share with the rest of the class. 248 ELWH682 249 ELWH688 250 NGSPOE2 251 NGSPOE1 This “golden” rice has been genetically engineered to produce beta carotene, a precursor to Vitamin A.
MANIPULATING GENES ACTIVITY 16 EVOLUTION 83 ANALYSIS 1. How has understanding the cause-and-effect relationship between genes and traits led to advanced methods of changing traits in organisms?252253 2. Compare and contrast the use of selective breeding and genetic modification for manipulating the traits of other organisms researched by the class. Be sure to include the following:254255256 a. How they are similar b. How they are different c. The advantages of each method 3. Revisit the issue: Any method that changes the heritable traits in a population has an effect on the evolution of that population. Do you think that people should intentionally manipulate genes and evolution in this way?257 258 EXTENSION Medical researchers are exploring ways to manipulate human genes to treat certain health conditions, such as some cancers and genetic conditions. This is called gene therapy. Visit the SEPUP Third Edition Evolution website at www.sepuplhs.org/middle/third-edition, and go to the gene therapy links to learn more about recent developments in this field. Develop a brief presentation to share what you learn. 252 NGCCCE1 253 NGCCCE2 254 NGPEL45 255 NGCCCO1 256 SEASCM1 257 NGCCNS4 258 SEASET1
EVOLUTION 85 17 Evolution and Us p r o j e c t As you have learned, the evolution of new species and the extinction of species continue today as they have in the past. You have also learned that people can cause intentional or unintentional changes that affect evolution and that these changes can, in turn, affect people. In this activity, you will synthesize and communicate some of the ideas you have learned in this unit.259260261262263 GUIDING QUESTION How are humans affecting and affected by evolution? 259 NGCCCE1 260 NGCCCE2 261 NGLS4C1 262 ELWH682 263 NGLS4B1 Mosquitoes and the diseases they carry have evolved in response to our attempts to eliminate them. This tropical rainforest is home to many species that are yet to be discovered by people. The passenger pigeon went extinct in 1914, due to hunting by people.
ACTIVITY 17 EVOLUTION AND US 86 EVOLUTION MATERIALS For each group of four students supplies for creating presentations or visual displays PROCEDURE 1. You are working with a scientist who wants to communicate ideas about evolution to the public. Design and produce a presentation to communicate two important ideas to a nonscientific audience: • Evolution by natural selection has a direct impact on people’s lives. • Understanding evolution can help people make choices about whether and how they try to change other organisms. 2. Your presentation should do the following:264265266 • Communicate your ideas clearly and convincingly. • Include both illustrations and any text needed to convey your key points. • Describe at least one way that evolution affects people’s day-to-day lives. • Describe at least one choice you recommend, based on your understanding of evolution. 3. Follow your teacher’s instructions about the formats you may use for your presentation 4. Be prepared to share your presentation with the class according to your teacher’s instructions.267 ANALYSIS 1. Revisit the issue: How have your ideas about evolution and its effects on people changed during this unit? 264 NGSPOE2 265 NGSPOE1 266 SEASCM1 267
EVOLUTION 87 Evolution u n i t s u m m a r y Evolution by Natural Selection Within a population, individual organisms typically exhibit a variety of traits. Many of these variations are caused by variations in genes. Gene variations arise through the process of mutation, which causes changes to the structure and function of the organism. Under certain conditions, some variations may enhance the survival of individuals. For example, a short, strong beak in a Galapagos finch is a physical trait that enhances the bird’s survival when large seeds are the primary available food. When there is a change in the environment, such as a change in food source or a new predator, some individuals in a population may be better able to survive because of their traits. These organisms are better adapted to the new environment, and the traits that help them survive are called adaptations. Over time, individuals with traits that allow them to survive and produce more offspring will transmit their traits to their offspring and generations that follow. The trait will become relatively more common in the population. This process of selection of traits through environmental change is called natural selection. Speciation and Extinction Natural selection leads to changes in populations, and in some cases, to the evolution of new species. For example, in the Galapagos Islands, 13 species of finch have evolved from one original finch population that came to the islands many generations ago. The many species of finch have varying beak shapes and sizes adapted to eating different foods. The millions of species of plants, animals, and microorganisms that live on Earth today evolved over many generations and are related by descent from common ancestors. Evolutionary trees, diagrams that represent hypotheses for evolutionary ancestry, can show these relationships. While there are millions of species on Earth today, billions of species have existed in the past but are now extinct. Sometimes species go extinct because no individuals have traits that allow them to survive and successfully reproduce, whereas other times extinction is caused by major geological events. Some extinctions are likely to have multiple or complex causes.
UNIT SUMMARY 88 EVOLUTION Evidence for Evolution The geologic timeline reflects the vast time scale since Earth originated approximately 4.5 billion years ago. This timeline reveals patterns of appearances and disappearances in the fossil record. The oldest fossil of a single-celled organism dates back to 3.7 billion years ago. Multicellular life originated relatively recently in geologic time, approximately 640 million years ago. Most key events in the evolution of life occurred in very recent geologic time with the start of the Cambrian “explosion” 550 million years ago. Based on evidence from fossils, scientists have developed and tested hypotheses for how major groups of organisms, such as vertebrates, evolved. Scientists use stratigraphy and radiometric dating to determine the age of rock layers and the fossils within them. Stratigraphy is based on the idea that older rock layers are found beneath layers that were formed more recently. Fossils in lower layers are generally older than fossils in upper layers, allowing scientists to reconstruct the evolutionary history of the organisms. Another source of evidence for evolutionary relationships comes from embryos, some of which exhibit traits that are lost during development and are not present in the adults. Scientists use patterns in the data from these lines of evidence to develop cause-and-effect explanations about evolution. Humans and Evolution Humans have mutual cause-and-effect evolutionary relationships with other organisms. Most of the extinctions that have occurred in the past 500 years have been caused by human activities, such as hunting, habit loss, or climate change. The current rate of extinction due to human activity may represent a sixth extinction event. With the loss of so many species, humans may be losing some of the benefits that could be derived from these species. Humans also affect how species evolve, including those species that cause problems for us, because we have used chemicals like pesticides that lead to selection for resistance to these chemicals. These species may become even more difficult to control in the long run. People have intentionally affected species, including many domestic animals and crops, through selective breeding and genetic engineering. In selective breeding (artificial selection), humans select individuals with desirable traits and breed them in the hopes of producing offspring with combinations of the desired traits. Artificial selection
UNIT SUMMARY EVOLUTION 89 produces organisms with traits that are desired by the breeder. For example, dogs have been bred to produce a wide variety of appearances and behaviors desired by pet owners. Crops and livestock have been bred or genetically modified for such traits as pest and disease resistance and to increase the amount of food they produce. There is debate among people about whether we should genetically modify organisms, which requires us to evaluate the evidence and trade-offs of this type of modification. Essential Scientific Terms adaptation cause and effect embryos evidence and trade-offs evolution extinction fossils mutation natural selection patterns speciation structure and function traits variation