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2 Evidence for Evolution After you have finished reading this chapter, you should be able to: Explain how homologous structures, fossils, comparative anatomy, embryology, and biochemistry provide evidence for evolution. Describe the differences between relative dating and absolute dating of fossils. Compare and contrast microevolution and macroevolution. Nothing in biology makes sense except in the light of evolution. Theodosius Dobzhansky Introduction In the mid-19th century, Charles Darwin first proposed a theory to explain the ways species change through natural selection. Since that time, a tremendous body of evidence that supports this theory has been developed. Now, in the 21st century, the theory of evolution by natural selection is accepted by biologists in much the same way that other scientists accept the theory of gravitation and the atomic theory as valid explanations for events that occur in the natural world. No single idea explains the enormous diversity and complexity of life on Earth more powerfully than the theory of evolution proposed by Darwin. Evidence that supports this theory includes fossils (see Figure 2-1), the shapes and structures of living organisms, the chemicals all living things are made of, and the distribution of species on Earth today. ■■ CREATING A FAMILY TREE Biologists try to understand the relationships that exist among different species. Often, simple diagrams can be used to represent these relation24 Figure 2-1 Fossilized trilobites. Time ships. (See Figure 2-2.) Suppose that A, B, C, and D represent four living species. The letters E, F, and G represent ancestral forms of these species that are most likely extinct. In this case, organisms B and C are more closely related because they evolved from their common ancestor, E, most recently. B Present A B C D and C are both equally related to A, with a more distant common ancestor, F. Organism D is the least closely related to the others E because it evolved from their common ancestor G the longest time ago. F An example might make this clearer. Suppose a group of biologists is studying the relaG tionships that exist among horses, cats, whales, and humans. The biologists want to Past develop a family tree for these animals. They Figure 2-2 A diagram need to determine which of these animals used to represent the evolved from common ancestors and at what relationships among time in the past this occurred. This taskLIVING different species. BIOLOGY, 2e/fig. 2-2 ENVIRONMENT requires the biologists to study the similarities and differences that exist in these animals. The biologists would have to learn as much as possible about the animals’ ancestors, which are usually, but not always, extinct species. s/s 26 Evolution ■■ HOMOLOGOUS AND ANALOGOUS STRUCTURES On way to determine the relationships among organisms is to find a characteristic they share that they inherited from a common ancestor. Such a feature is called a homologous structure. For example, similarities exist in the bones of the forelimbs of five very different animals. The wing of a bat, the flipper of a whale, the leg of a cat, the arm of a human, and the wing of a bird—although they appear to be quite different—are all made up of the same types of bones. These bones are attached to each other and to other bones in similar ways. The forelimbs of these five animals are homologous structures and indicate that, long ago, these different animals evolved from a common ancestor. (See Figure 2-3.) Human Cat Bat Whale Bird Figure 2-3 The bones in each forelimb are homologous. Using similarities alone to conclude that organisms evolved from a LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-3 s/s common ancestor can sometimes lead to mistakes. For example, the penguin, shark, and sea lion are all excellent swimmers. (See Figure 2-4.) These very different animals have fins or flippers and streamlined bodies that enable them to propel themselves through the water. However, other evidence shows that these animals have very different ancestors. Because, over time, their ancestors became adapted to living in water, the living species appear similar in ways, even though they are not closely related. This process is called convergent evolution. The features shared by organisms through convergent evolution are called analogous structures. Determining if organisms are related requires looking at other evidence besides homologous structures. For example, the first vertebrates (animals with backbones) to live on land had five digits on their fore- Chapter 2 / Evidence for Evolution 27 Penguin Shark Sea lion Figure 2-4 The similarity in the streamlined bodies of the penguin, shark, and sea lion is an example of convergent evolution. LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-4 s/s limbs. Today, terrestrial vertebrates such as humans, crocodiles, and toads also have five digits that they inherited from a common ancestor. However, the feet of pigs, camels, and deer have only two digits. But we know that humans are more closely related to pigs, camels, and deer than to crocodiles or toads. This means that having five digits, like the more distant ancestors of terrestrial vertebrates, is a primitive trait. Over time, vertebrates that had fewer digits evolved, and this modified trait found in pigs, camels, and deer is called an advanced trait. Learning which traits are primitive and which are advanced is important for biologists as they construct family trees that show the evolutionary relationships among organisms. ■■ EVIDENCE FROM FOSSILS Fossils are traces or remains of dead organisms that have been preserved by natural processes. There are many ways fossils can be formed. Although it is usually only the hard parts of organisms—such as bones, shells, or teeth—that become fossils, occasionally the complete remains of an organism are found. In ancient times, organisms were sometimes trapped in the sticky plant sap that oozed from certain kinds of trees. Later, the sap turned into clear yellow-orange amber, a substance that looks like rock. Amber samples collected from around the world contain mosquitoes, spiders, leaves, and even small vertebrates such as lizards that are many millions of years old. Ice has preserved the remains of woolly mammoths, elephantlike animals that have been extinct for thousands of years. (See Figure 2-5 on page 28.) A common way fossils are formed is through the gradual replacement of the organism’s remains by other substances. In this method of fossil formation, the organism is usually buried in sediments, its tissues slowly 28 Evolution Figure 2-5 Woolly mammoth. replaced by minerals dissolved in underground water. In time, these minerals harden to form an exact copy of the original organism. In fact, the LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-5 s/s remains of dinosaurs found by scientists are usually not the dinosaur’s actual bones but instead are copies of them made from minerals in this manner. (See Figure 2-6.) Tree trunks, too, have been turned into petrified wood by this process. Fossils can also be formed if the original plant or animal creates an impression in soft mud or clay. The mud hardens and the remains of the organism disappear, leaving an empty mold. Later, when this mold fills with minerals, it forms a cast. This process shows only the original external shape of the organism and not the internal structure, as do some other methods of fossil formation. A similar method is the creation of fossil imprints of animal footprints in clay or mud. Today, you can find dinosaur tracks in rock that was once the soft mud in which the animals walked. More than 3.5 million years ago, in eastern Africa, a volcano spread a layer of ash over the land. A Figure 2-6 Dinosaur skull —a type of fossil. Chapter 2 / Evidence for Evolution 29 light rain dampened the ash. At least 20 types of animals, including giraffes, a horse, birds, a wild cat, and at least three ancient humans, walked through the ash. The ash dried, baked by the heat of the sun. In time, the ash hardened into stone. The animals left footprints behind that today provide evidence of their presence long ago. These footprints were discovered by accident. A group of scientists working in the area were relaxing by throwing dried elephant dung at each other. When one scientist slipped and fell, he got a close-up view of the strange marks on the ground. After further investigation, the team of scientists made one of the most important discoveries of our time—the fossil footprints of early human ancestors, two adults and a youngster, who walked side by side, 3,600,000 years ago. ■■ THE AGE OF FOSSILS Fossils can be used as evidence for evolution only if the age of the fossils can be determined. For many years, scientists could determine only whether one fossil was older or younger than another—that is, their age relative to one another. Relative age is determined by observing the layers of the rock in which the fossils are found. Remains of plants and animals are normally found in sedimentary rock. This type of rock forms, often at the bottom of lakes or seas, when layers of sediments such as sand, mud, or clay are deposited. Eventually the lower layers are compressed by the layers settling down above them. The compressed lower layers become cemented into rock. (See Figure 2-7.) In a series of undisturbed layers, the oldest are found at the bottom and the most recent at the top. Therefore, the ages of fossils found in the layers can be related to the ages of the layers. Because this method determines only if one fossil is younger or older than another, it is called relative dating. (See Figure 2-8 on page 30.) Stream carrying sediments Ocean Sedimentary rocks Figure 2-7 Formation of sedimentary rocks. 30 Evolution Today, scientists can determine the age of fossils much more accurately than by the position of rock layers. Very precise methods have been developed that determine when the fossil was formed. This method of dating is called absolute dating. Absolute dating techniques usually depend on radioactive isotopes, atoms of elements that release energy in the form of radiation at a uniform rate. As energy is released in a process called radioactive decay, the radioactive isotope changes into a different element. The process of radioactive decay continues. Over time, fewer atoms of the original isotope and more of the new element, called the decay product, are present. By measuring the amount of each element present in a sample, the length of time since the fossil was formed can be calculated. Several methods use radioactive elements to measure the age of rocks and the fossils they conFigure 2-8 Fossils in tain. One method uses the ratio of the radioactive a series of layers of element uranium to its decay product, lead, in a sedimentary rock. sample. Another method calculates the ratio of radioactive potassium to its decay product, argon. These methods can LIVING ENVIRONMENT 2e/fig. 2-8formed s/s date fossilsBIOLOGY, and rocks that many millions of years ago. Another method of absolute dating uses isotopes of carbon. This method measures the ratio of radioactive carbon-14 to carbon-12. When an organism is alive, it takes in fixed amounts of both carbon-12 and carbon-14. When it dies, the carbon-14 in the organism begins to change to its decay product, nitrogen, while the carbon-12 remains unchanged. Thus, the ratio of carbon-14 to carbon-12 changes. How long ago an organism died can be determined by finding the ratio of carbon-14 to carbon-12 in a sample from the organism. Radiocarbon dating can be used to date the remains of living things such as logs and bones that are less than 50,000 years old. ■■ EVIDENCE FROM COMPARATIVE ANATOMY A structure that has little or no function in an organism, but is clearly related to a more fully developed structure in another organism, is known as a vestigial structure. It is thought that such a structure did serve a Chapter 2 / Evidence for Evolution 31 Backbone Rib Python Leg bones Figure 2-9 The skeletons of some snake species have vestigial leg bones. function in an ancestral organism. The appendix, a vestigial structure LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-9 s/ssac (rev.attached 10/13/03) to the place where the small present in humans, is a small and large intestines meet. In appearance, the appendix is a smaller copy of the cecum, which is a large pouch found in plant-eating mammals such as rabbits. In a rabbit, the cecum contains microorganisms that digest the plant materials the animal eats. The fact that a similar organ in one species is still useful is evidence that humans evolved from an ancestor that once had this larger, functional structure. There are many examples of vestigial structures found in organisms alive today. These structures support the idea that species have evolved over time as a result of natural selection. For example, although snakes have no legs, there are tiny leg bones in the skeletons of some snake species. These vestigial structures show that snakes evolved from animals that had legs. (See Figure 2-9.) ■■ EVIDENCE FROM COMPARATIVE EMBRYOLOGY Figure 2-10 illustrates five animals at very early stages in their development. Although all of these embryos resemble one another, you might be Salamander Chicken Gill slits Yolk sac Pig Gill slits Monkey Gill slits Human Gill slits Yolk sac Figure 2-10 The early embryos of these five animals show some similarities. LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-10 s/s (rev. 10/13/03) Gill slits 32 Evolution surprised to learn that these show the embryos of a salamander, a chicken, a pig, a monkey, and a human. These diagrams provide evidence that all vertebrates follow a common plan in their early stages of embryological development. We are now learning that this is due to their having similar sets of genes, the molecular instructions for life; and this similarity comes from having common ancestors. Check Your Understanding How do organisms’ similarities in anatomy and embryology provide evidence for evolution? ■■ EVIDENCE FROM COMPARATIVE BIOCHEMISTRY The chemistry of living things, called biochemistry, provides some of the strongest evidence that organisms evolved from common ancestors long ago. All organisms store the genetic information that is passed from one generation to the next in DNA molecules in a manner that is almost exactly the same. This genetic code shows that all organisms are related in fundamental ways. Biochemistry also shows how different species are related to each other. Proteins, a type of molecule in all living things, are made up of smaller units called amino acids. Cytochrome c is a protein that helps organisms get energy. The 100 amino acids that make up the cytochrome c of humans and chimpanzees are identical. There are six differences in the amino acids that make up the cytochrome c of humans and monkeys. The cytochrome c of humans and kangaroos differs in 14 amino acids. Biologists now recognize that a small number of amino acid differences means that the two species are more closely related in evolutionary terms. On the other hand, a large number of differences means the two species are more distantly related. More evidence for evolution has been discovered by molecular biologists who have uncovered the specific genes that direct the body development of very diverse animals. Called homeotic genes, these genes are like molecular architects. Within them are stretches of DNA, or homeobox sequences, which act like switches to turn the genes on and off. Importantly, the homeobox sequence in a fruit fly’s genes is almost identical to that in a mouse’s genes—further evidence for a common ancestor long ago. These common homeobox genes in diverse organisms help to explain the similarities in early embryonic development, as shown in Figure 2-10. Chapter 2 / Evidence for Evolution 33 1 DNA molecules from Species A, B, and C. 2 DNA molecules are broken apart by heating. Species Species Species A B C 3 Hybrid DNA molecules form after mixing and cooling. Species A & B are more closely related than Species A & C. A+A A+B A+C Figure 2-11 DNA hybridization. DNA molecules store a species’ genetic information in long sequences LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-11 s/s (rev. 10/13/03),(10/20/03) of subunits called nucleotides. For example, the DNA in each human cell that contains all the genetic information for a human has about one billion nucleotides in a specific order. The nucleotides that are present, and the order in which they are found, identify the species. By matching a DNA sequence from one individual with the DNA sequence from another, scientists can determine if the sequences belong to the same, closely related, or distantly related species. Again, the greater the similarity, the closer the species are related. The most common method used to compare DNA sequences is called DNA hybridization. (See Figure 2-11.) DNA molecules from two different individual organisms are mixed together to form “hybrid” DNA. Wherever the molecules from the two individuals are similar, they attach to each other. Heat can be used to separate the hybrid DNA molecules. The greater the similarity, the greater the numbers of attachments and the more difficult it is to once again separate the two molecules. Biologists therefore measure the temperature required to separate the hybrid DNA molecules. The higher the temperature, the greater the number of attachments; hence the more closely related the species are. Many evolutionary puzzles have now been solved using this technique of DNA hybridization. For example, ornithologists, scientists who study birds, disagreed on whether the flamingo was more closely related to storks or to ducks and geese. DNA hybridization studies have shown that flamingos are indeed closely related to storks, both having evolved most recently from a common ancestor. (See Figure 2-12 on page 34.) A more famous controversy involved the relationship of the giant panda to the red panda. (See Figure 2-13 on page 35.) It has been known for a long time that the smaller red panda is a member of the raccoon family. Was the giant panda, whose anatomy and behavior closely resemble 34 Evolution ? ? Flamingo Stork Duck Figure 2-12 DNA hybridization shows that flamingos are closely related to storks. that of the red panda, also a member of the same family and not a bear, LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-12 s/s panda first became as was commonly thought? From 1869, when the giant known to the Western world, until recently, this question remained unanswered. DNA hybridization has now shown that the giant panda is indeed a bear. The two pandas have developed by convergent evolution to resemble one another in some ways. This makes sense, because they are both native to the same mountainous environment in China, and both eat similar plants. ■■ EVIDENCE FOR MICROEVOLUTION Evolution occurs all the time within a species—on a small scale—as inheritable traits in a population change through natural selection from one generation to another. This kind of change is known as microevolution. When it occurs over a short period of time, this process does not produce significant enough change to result in a new species. However, by studying examples of microevolution, biologists can learn about the process of evolution that does lead to significant changes, new species, and eventually entirely new groups of organisms. Change on a large scale over a long enough period of time is known as macroevolution. You may have observed moths fluttering around a bright light outdoors on a summer night. One species of moth, the peppered moth, has been carefully studied in England for more than a century. This study provides one of the best-known examples of microevolution. When the peppered moth was first studied, most were a light color. The moths were well camouflaged when they rested on trees and rocks covered with lightcolored lichens. (See Figure 2-14 on page 36.) In 1845, a dark-colored peppered moth was observed for the first time, in the northern English city of Manchester. At that time, soot and smoke produced by coal-burning Chapter 2 / Evidence for Evolution 35 Figure 2-13 DNA hybridization has shown that the red panda (above) and the giant panda are similar as a result of convergent evolution. factories had begun to pollute the air. The trees and rocks became dark with soot, and the lichens began to die. The light-colored moths were easily seen against the new, darker backgrounds on which they rested. Increasingly, the light-colored moths became the easy prey of insecteating birds. By 1900, most of the peppered moths in Manchester were dark-colored. Why did this happen? The darker moths were better camouflaged against the tree trunks and rocks blackened by the soot from the factories. The process of dark-colored moths replacing light-colored moths is known as industrial melanism. It has been observed with many species 36 Evolution Light-colored peppered moth Soot-covered bark of tree Figure 2-14 The light-colored and dark-colored peppered moth populations vary due to microevolution. Dark-colored peppered moth since then, not only in Manchester but also in similar industrial cities in the United States, such as Pittsburgh, Pennsylvania. It is important to LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-14they s/s were understand that the light-colored moths did not change color; replaced by dark-colored moths through natural selection. The peppered moths with a dark color were the ones least likely to be preyed upon and so most likely to survive to pass on their genetic traits. It has been discovered that the dark color was a mutation that occurred infrequently in the population, due to the genetic variation that occurs naturally within every species. Natural selection, acting on the peppered moth, in the process of microevolution, changed the moth population from the lightcolored to the dark-colored variety. Interestingly, but not surprisingly, as air pollution controls have been implemented over the years, the number of light-colored moths is once again increasing. Another example of microevolution that you can now recognize is the appearance of drugresistant bacteria, as described in Chapter 1. ■■ EVIDENCE FOR MACROEVOLUTION The large-scale changes of macroevolution led to the development of new adaptive features, new species, and new groups of species. Examples of new features that resulted from macroevolution are the legs on amphibians (such as a salamander), the egg of a turtle, and the large brain of primates. An entirely new group of species that arose through macroevolution was the flowering plants. Macroevolution has also led to the disappearance of species, for example, the extinction of dinosaurs. Naturalists living in Europe and England were accustomed to observing relatively small numbers of species. People thought that each species Chapter 2 / Evidence for Evolution 37 always existed exactly as it currently appeared, living in the same place it was currently living. However, as a result of wide-ranging explorations around the globe, such as Darwin’s voyage on the Beagle, it came to be realized that many more species of organisms existed than anyone could have possibly imagined. For example, tropical rain forests revealed a diversity of life previously unimagined. In just one hectare (10,000 square meters) of a tropical rain forest, more than 100 species of trees and up to 40,000 species of insects can be found! The rain forests that surround the Amazon River in South America are home to more than 1600 bird species. As Darwin traveled up the west coast of South America, he was impressed by the gradual changes in the kinds of plants and animals that live there. These changes corresponded to the gradually changing environments. This wide diversity of species strongly supported the idea that over a very long period of time, organisms had evolved, adapting to many different environments in many different ways. It was becoming difficult to argue that each of these many different species had remained unchanged from its original form. Another observation led Darwin to his theory of evolution. Darwin realized that there was a relationship between the types of plants and animals and the regions they live in. The study of the relationship of living things to their distribution in particular areas is called biogeography. Biogeography raises many questions that can logically be answered only by the theory of evolution. For example: ◆ Why are rabbitlike animals distributed throughout the world? (See Figure 2-15.) The European rabbit, the Patagonian hare of South America (related to rats), and the Australian banded hare (related to kangaroos) are similar in appearance and behavior. It is most likely that these animals evolved, from unrelated ancestors, over millions of years in Figure 2-15 Rabbitlike animals are found throughout the world. 38 Evolution Mitochondrial Eve Meets Mungo Man In 1987, a group of scientists thought they could use a better method to study human evolution. Instead of studying bits and pieces of fossil remains, they decided to study the genes contained within the cells of living people. These genes, passed from generation to generation, have stored within them a history of our origins. The molecular biologists decided to examine the DNA located in our cells’ mitochondria. Unlike ordinary DNA—the genetic material in the nuclei of our cells that we get from both parents—mitochondrial DNA (mtDNA) in our cells comes only from our mother. From one generation to the next, mtDNA never gets mixed with the DNA in the genes we get from our father. The researchers collected mtDNA from women living in many parts of the world. By studying the similarities and differences in mtDNA in these women, the researchers were able to look back in time to study the origins of human history. Their startling conclusion was that the molecular evidence indicated that all humans alive today are the descendants of a single female who lived in Africa about 200,000 year ago. Some people began to call this person “Mitochondrial Eve.” Since 1987, scientists have disagreed widely on the results. Some scientists claim that the computer program the researchers used for their analysis was not used correctly. Others think that Mitochondrial Eve lived only 150,000 years ago. Still others point to evidence showing that modern humans evolved much earlier—and in several parts of the world, not only in Africa. In 2001, scientists determined that mtDNA from a human fossil found years earlier in Mungo, Australia, showed no linkage to any humans living today. Therefore, “Mungo Man,” as the fossil is called, could not have descended from Mitochondrial Eve. “Put the gloves on, Mitochondrial Eve, because Mungo Man has stepped into the ring,” began a recent article on the topic, showing how the debate still continues. This kind of open discussion is what science is all about—questions are asked and answered; and then, when more evidence is found, even more questions arise. For now, the answer to this question about our ancestry remains undecided. similar environments but in different areas of Earth. They adapted in some of the same ways to fill similar roles in their respective environments. Hence, today these animals superficially resemble each other. (This is, in fact, another example of convergent evolution.) ◆ Why are the species found in South American rain forests more closely related to those species that live in South American deserts than to species found in Asian or African rain forests? Could it be that all the South American animals evolved from common ancestors, adapting to Chapter 2 / Evidence for Evolution 39 the rain forest or desert environments in their own particular ways, while the African and Asian species evolved independently, adapting to their own regions? ◆ Darwin’s questions about biogeography were most clearly answered when he investigated the Galápagos Islands off the coast of Ecuador. (See Figure 2-16.) Why did each of these 13 islands have its own unique species of birds and reptiles, very similar to those on the other islands and to those on the mainland? If each island were to be given its own unique species in some special process, why weren’t they fundamentally different from each other? It is much more logical to accept that ancestors of these animals arrived from the South American mainland after the islands—which had formed from volcanoes on the ocean’s bottom—appeared above the water’s surface. On each isolated island, the ancestral birds and reptiles evolved independently, in response to the slightly different environments they lived in. These questions, and many others, can be explained only by recognizing that organisms have changed over time in response to particular environmental pressures. One final type of evidence was used by Darwin to support his theory P MEXICO AN AMA VENEZUELA Cocos Island Malpelo Island COLOMBIA Pacific Ocean ECUADOR ´ Galapagos Islands PERU Figure 2-16 On the Galápagos Islands, the ancestral birds and reptiles evolved independently in response to their environments. LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-16 s/s (rev. 10/13/03),(10/20/03) 40 Evolution of evolution. Darwin argued that a particular adaptation that exists not only in its most advanced form but also in a wide range of intermediate forms suggests that evolution has occurred. An example he used in this argument—the eye—is also one that those who object to the theory of evolution sometimes use. They have argued: How could an organ as remarkable and highly developed as the eye be the product of a long series of small evolutionary changes? How could an eye have evolved from something else? In On the Origin of Species, Darwin showed that there are many animals that have organs sensitive to light in a variety of ways. For example, a sea star has small gel-filled depressions that may concentrate light at a particular spot on its body. (See Figure 2-17.) These depressions cannot form an actual picture of a sea star’s surroundings. Yet for the sea star, this type of “eye” is sufficient to aid the animal’s survival and gives it an advantage over animals that do not have even this structure. (Likewise, other invertebrates have a variety of “eyes” that have differing abilities to see or perceive different levels of light.) On the other hand, it is possible that this type of adaptation in an ancestral species was improved on by natural selection. This improvement led to a Figure 2-17 The sea structure in a later species that eventually could star has eyespots at the “see,” as our eye does today. Darwin described tip of each arm. this gradual selection of structures (“transparent layers”) to perceive light. “We must suppose,” Darwin stated, “that there is a power, represented by natural selection or the survival of the fittest, always intently watching each slight alteration in the transparent layers; and carefully preserving each which, under varied circumstances, in any way or in any degree, tends to produce a distinctive image.” LABORATORY INVESTIGATION 2 How Have Animals Evolved Over Time? INTRODUCTION Much evidence to support the theory of evolution is found in Earth’s fossil record. In this investigation, you will make a model that shows how a series of imaginary fossil organisms may have evolved. MATERIALS “Creature Sheet” handout (from the Teacher’s Manual), scissors, glue or tape, unlined paper PROCEDURE 1. Cut out each imaginary creature from the Creature Sheet. 2. Select one creature that seems to represent the oldest fossil, the ancestor of all the others. (Hint: Its form will be simpler than the others.) 3. Determine which creature appears to have evolved next. Continue arranging the drawings to show a pattern of evolution from one form to the next. There are several possible ways to arrange them. You may arrange your creatures in a straight line or in a set of branched lines similar to the branches of a tree. The pattern can branch into two or more directions. You should be able to support the order you place the organisms in with some logical reasons. 4. Glue or tape the creatures to the unlined paper in the order in which you have arranged them. Draw arrows that show the direction of evolutionary change. INTERPRETIVE QUESTIONS 1. Did you choose a linear or branching diagram, or a combination of the two types? Explain your choice. 2. What shared characteristics did you use to determine the main sequences or branches of your pattern? 3. Imagine what kind of environment the organisms at the end of each branch or sequence of the pattern would live in. How might they move? What might they eat? Chapter 2 / Evidence for Evolution 41 42 Evolution ■■ CHAPTER 2 REVIEW Answer these questions on a separate sheet of paper. VOCABULARY The following list contains all of the boldfaced terms in this chapter. Define each of these terms in your own words. absolute dating, advanced trait, analogous structures, biochemistry, biogeography, convergent evolution, decay product, DNA hybridization, embryos, fossils, genes, homeobox sequence, homologous structure, industrial melanism, macroevolution, microevolution, primitive trait, radioactive isotopes, relative dating, sedimentary rock, vestigial structure PART A—MULTIPLE CHOICE Choose the response that best completes the sentence or answers the question. 1. Which organism has the forelimb with the most “primitive” trait? a. bird b. human c. horse d. whale 2. Examples of homologous structures are a. a dog’s tail and a fish’s tail b. a bee’s eye and a fish’s eye c. a porpoise’s flipper and a hawk’s wing d. a crab’s claw and a penguin’s wing. 3. Evidence for evolution is shown by a. homologous structures b. fossils c. biochemistry d. all of these. 4. Scientists who study birds are called a. ornithologists b. ichthyologists c. aviaries d. biogeographers. 5. The streamlined bodies and powerful tails of sharks, dolphins, and ichthyosaurs (ocean-dwelling relatives of the dinosaurs) illustrate a. divergent evolution b. convergent evolution c. retention of ancestral traits d. development of vestigial organs. 6. Which of the following is not compared in order to clarify the evolutionary relationships among related organisms? a. analogous structures b. embryo development c. genes d. DNA hybridization 7. Fossils are usually found in a. radioactive isotopes b. igneous rock c. trap rock d. sedimentary rock. 8. DNA from a giant panda hybridizes more readily with DNA from a brown bear than with DNA from a red panda. This indicates that a giant panda Chapter 2 / Evidence for Evolution 43 9. 10. 11. 12. 13. 14. 15. a. shares numerous analogous structures with a brown bear b. is more closely related to a brown bear than to a red panda c. is more closely related to a red panda than to a brown bear d. lives in a similar environment to a brown bear. Which of the following statements is false? a. In late nineteenthcentury Manchester, natural selection resulted in the peppered moths turning from light gray to dark gray. b. Light-colored moths had dark-colored offspring because the moths needed to blend in with sooty tree bark. c. The technical name for the color change from light to dark as a result of pollution is industrial melanism. d. The peppered moths’ change in color is an example of microevolution. The older a specimen of sedimentary rock, a. the greater the amount of radioactive isotopes in it b. the closer to the surface it is found in undisturbed rock layers c. the greater the amount of decay products present d. the more likely it is to contain moldand-cast fossils. Examples of vestigial structures include a. a seal’s flipper and a penguin’s wing b. a duckbill platypus’s bill and a duck’s bill c. a bat’s foot and a human’s foot d. a human’s appendix and a whale’s hind limbs. In three layers of undisturbed sedimentary rock, a. the middle layer contains the oldest fossils b. the top layer contains the youngest fossils c. the fossils in the bottom layer are younger than the fossils in the middle layer d. the fossils in the bottom layer have usually been destroyed by geologic activity. Homeobox gene sequences provide evidence in support of evolution because a. they evolved only recently b. mammals have these genes but no other animals do c. similar homeobox gene sequences are found in very different organisms d. fruit flies have a very ancient form of these genes. The study of the relationship of plants and animals to their regional distribution is called a. ornithology b. paleontology c. biogeography d. melanism. Sea urchins and jellyfish are both round, have no head or tail, and live in the ocean; but their embryos develop in radically different ways. They probably a. are descended from the same ancestor b. should be placed in the same group c. have numerous vestigial structures d. are not very closely related. 44 Evolution PART B—CONSTRUCTED RESPONSE Use the information in the chapter to respond to these items. 16. Scientists analyzed the protein cytochrome c in different organisms in order to construct this evolutionary diagram. Explain how such biochemical data provide evidence for evolution. Candida (yeast) Saccharomyces (yeast) Neurospora Moth Screwworm Tuna Turtle Penguin Chicken Duck Pigeon Kangaroo Rabbit Pig Donkey Horse Dog Monkey Human 23.4 9.6 17.4 2.1 28.1 9.9 5.7 6.5 15.2 17.2 1.1 4.9 1 .5 1 1.2 3.3 3.1 1.6 1.1 1.3 4.6 2.7 .6 1.4 1.1 .1 2.9 .9 1.4 3.0 6.9 .2 .8 1.7 0 5 5.4 25 10 15 20 Average minimum number of mutations 30 17. Use the diagram to answer the following questions. a. Is a turtle most closely related to a dog, tuna, penguin, horse, or LIVING ENVIRONMENT BIOLOGY, 2e/fig. 2-Q16 s/s silkworm moth? b. What organism is most closely related to a horse? c. Which group of organisms is least closely related to the other groups—fungi, insects, fish, reptiles, birds, or mammals? d. What can you conclude about the turtle’s placement on this “family tree”? 18. Describe three ways in which fossils are formed. 19. Describe and give specific examples of evidence for evolution from fossils, comparative embryology, comparative biochemistry, comparative anatomy, microevolution, and biogeography. 20. Compare and contrast relative dating and absolute dating. Chapter 2 / Evidence for Evolution 45 PART C—READING COMPREHENSION Base your answers to questions 21 through 23 on the information below and on your knowledge of biology. Source: Science News (February 22, 2003): vol. 163, p. 126. Farming Sprouted in Ancient Ecuador People living in the lowlands of what’s now southwestern Ecuador began to grow squash between 10,000 and 9,000 years ago, about the same time that residents of Mexico’s southern highlands domesticated the vegetable (SN: 5/24/97, p. 322), according to a study in the Feb. 14 Science. The comparably ancient roots of plant cultivation in these two regions indicate that “in South America, there was no single center of agricultural origins,” conclude Dolores R. Piperno of the Smithsonian Tropical Research Institute in Balboa, Panama, and Karen E. Stothert of the University of Texas at San Antonio. In the soil of two prehistoric sites in Ecuador, the scientists isolated and studied microscopic crystals from squash rinds that had been uncovered there. These ancient crystals were the same size as those in the squash’s modern domesticated form, but not those in its present-day wild counterpart. Piperno and Stothert were able to date tiny bits of carbon that were trapped inside the ancient crystals as they formed. 21. State what has been learned about farming that went on in Ecuador and Mexico approximately 10,000 years ago. 22. Explain what conclusion can be made about agriculture in South America. 23. Explain how scientists have learned about plants that were grown by people in Ecuador long ago.