Factors such as natural selection and chance events can

Section 16 3 1 FOCUS Objectives 16.3.1 Identify the condition necessary for a new species to evolve. 16.3.2 Describe the process of speciation in the alápagos finches. Vocabulary Preview Introduce students to the Vocabulary terms by explaining that speciation, or the formation of new species, comes about because of one or more types of reproductive isolation: behavioral, geographic, or temporal isolation. Reading Strategy Suggest that students make two graphic organizers as they read to summarize the information in the two parts of the section. For example, they might make a concept map to show the types of isolating mechanisms that lead to speciation and a flowchart to show how speciation of Darwin s finches occurred. 2 INSTRUCT Isolating Mechanisms ddress Misconceptions Students may think that being able to mate more widely rather than just within a species would be an evolutionary advantage, not a disadvantage. Explain that attempting to mate with a member of another species almost always results in reproductive failure and therefore wastes time and energy that might have been spent mating effectively with a conspecific. lso, even if mating does take place and offspring are produced, they are likely to be less well adapted to the niche of either of the parent species and, thus, selected against. 16 3 The Process of Speciation Key Concepts What factors are involved in the formation of new species? Describe the process of speciation in the alápagos finches. Vocabulary speciation reproductive isolation behavioral isolation geographic isolation temporal isolation Reading Strategy: Using Visuals efore you read, preview Figure 16 16. s you read about speciation of Darwin s finches, notice what happens at each step in the diagram. SECTION RESOURCES Print: Teaching Resources, Section Review 16 3, Enrichment Reading and Study Workbook, Section 16 3 dapted Reading and Study Workbook, Section 16 3 Issues and Decision Making, Issues and Decisions 16 Lesson Plans, Section 16 3 Factors such as natural selection and chance events can change the relative frequencies of alleles in a population. ut how do these changes lead to the formation of new species, or speciation? Recall that biologists define a species as a group of organisms that breed with one another and produce fertile offspring. This means that individuals in the same species share a common gene pool. ecause a population of individuals has a shared gene pool, a genetic change that occurs in one individual can spread through the population as that individual and its offspring reproduce. If a genetic change increases fitness, that allele will eventually be found in many individuals of that population. Isolating Mechanisms iven this genetic definition of species, what must happen for a species to evolve into two new species? The gene pools of two populations must become separated for them to become new species. s new species evolve, populations become reproductively isolated from each other. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred. t that point, the populations have separate gene pools. They respond to natural selection or genetic drift as separate units. Reproductive isolation can develop in a variety of ways, including behavioral isolation, geographic isolation, and temporal isolation. ehavioral Isolation One type of isolating mechanism, behavioral isolation, occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior. For example, the eastern and western meadowlarks shown in Figure 16 11 are very similar birds whose habitats overlap in the center of the United States. Members of the two species will not mate with each other, however, partly because they use different songs to attract mates. Eastern meadowlarks will not respond to western meadowlark songs, and vice versa. Figure 16 11 The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs. pplying Concepts What type of reproductive isolation does this situation illustrate? Technology: itext, Section 16 3 Transparencies Plus, Section 16 3 iodetectives Videotapes, The alápagos Islands: limpse Into the Past 404 Chapter 16

bert squirrel eographic Isolation With geographic isolation, two populations are separated by geographic barriers such as rivers, mountains, or bodies of water. The bert squirrel in Figure 16 12, for example, lives in the Southwest. bout 10,000 years ago, the Colorado River split the species into two separate populations. Two separate gene pools formed. enetic changes that appeared in one group were not passed to the other. Natural selection worked separately on each group and led to the formation of a distinct subspecies, the Kaibab squirrel. The bert and Kaibab squirrels have very similar anatomical and physiological characteristics, indicating that they are closely related. However, the Kaibab squirrel differs from the bert squirrel in significant ways, such as fur coloring. eographic barriers do not guarantee the formation of new species, however. Separate lakes may be linked for a time during a flood, or a land bridge may temporarily form between islands, enabling separated populations to mix. If two formerly separated populations can still interbreed, they remain a single species. lso, any potential geographic barrier may separate certain types of organisms but not others. large river will keep squirrels and other small rodents apart, but it does not necessarily isolate bird populations. Temporal Isolation third isolating mechanism is temporal isolation, in which two or more species reproduce at different times. For example, three similar species of orchid all live in the same rain forest. Each species releases pollen only on a single day. ecause the three species release pollen on different days, they cannot pollinate one another. How can temporal isolation lead to speciation? Kaibab squirrel rand Canyon UTH Lake Mead RIZON ila River Colorado Lake Powell River Range of Kaibab squirrel 0 125 250 Miles COLORDO NEW MEXICO Rio rande Figure 16 12 When two populations of a species become reproductively isolated, new species can develop. The Kaibab squirrel evolved from the bert squirrel. The Kaibab squirrels were isolated from the main population by the Colorado River. 0 125 250 Kilometers Range of bert squirrel pplying Concepts Divide the class into several groups, and challenge each group to brainstorm a scenario in which a small population of a species becomes geographically isolated from the remainder of the species long enough to evolve into a separate species. Urge groups to consider both natural events and human activities when they brainstorm ways that geographic isolation could come about. Have each group elect a spokesperson to describe their scenario to the class. In each case, ask: Why did the geographically isolated population evolve into a different species? (nswers will vary depending on scenarios. Students might say, for example, that the isolated population was genetically different to begin with because of founder effect and that it became even more different through time due to different selective pressures.) Figure 16 12 Have students use the key to locate the range of each type of squirrel. Point out how the Colorado River effectively isolates the two types of squirrels geographically, despite the closeness of their ranges. Inferring Have students infer the reproductive characteristics of species most likely to be affected by temporal isolation. sk: What must be true about the reproductive behavior of species that are isolated by temporal isolation? (Their reproductive behavior must be limited to a certain time of day or a certain season.) Inclusion/Special Needs Create a flowchart showing in a simple way the steps that occurred in the speciation of Darwin s finches. Include the following steps: founders arriving; founders remaining geographically isolated; directional selection occurring in different environments; reproductive isolation developing; and interspecific competition leading to improved adaptation to the species niches. dvanced Learners Invite students who need extra challenges to research the process of speciation in Hawaiian birds called honeycreepers. Like Darwin s finches, honeycreepers underwent adaptive radiation after colonizing islands with vacant niches. sk students to share their findings with the class and to point out similarities and differences between the Hawaiian and alápagos cases. nswers to... If two populations reproduce at different times, they are unlikely to reproduce with each other. Eventually they may become separate species. Figure 16 11 ehavioral isolation Evolution of Populations 405

16 3 (continued) Testing Natural Selection in Nature Figure 16 13 Point out that the woodpecker finch uses its beak to hold a cactus spine, which it pokes into holes in trees in order to spear insects. sk: What tool does its beak resemble? (Students might say pliers or needle-nosed pliers.) If another species of fruit-eating finch was discovered, what type of beak do you think it would have? (Students are likely to infer that it would have a beak like the vegetarian tree finch, which also eats fruit.) pplying Concepts Point out that the rants used the scientific method in their research on the alápagos Islands. Challenge students to recall the steps typically involved in the scientific method. ssign a student to record the steps on the board. (sk a question, gather information and form hypothesis, experiment, record and analyze data, draw conclusion) Then, have students describe each step of the scientific method as it applies to the rants research. (For example, the rants problem was to demonstrate natural selection in action. Their hypotheses were that there was enough inheritable variation in beak size and shape to provide raw material for natural selection and that variation in beak size and shape produced differences in fitness.) NST Shape of Head and eak Common Name of Finch Species Vegetarian tree finch Large insectivorous tree finch Woodpecker finch Cactus ground finch Sharp-beaked ground finch Main Food Fruits Insects Insects Cacti Seeds Seeds Feeding daptation Parrotlike beak rasping beak alápagos Islands Finches Uses cactus spines Large crushing beak Pointed crushing beak Large ground finch Testing Natural Selection in Nature Large crushing beak Habitat Trees Trees Trees round round round Figure 16 13 Detailed genetic studies have shown that these finches evolved from a species with a more-or-less general-purpose beak. Formulating Hypotheses Suggest how one of these beaks could have resulted from natural selection. NST For: Links on speciation Visit: www.scilinks.org Web Code: cbn-5163 Now that you know the basic mechanisms of evolutionary change, you might wonder if these processes can be observed in nature. The answer is yes. In fact, some of the most important studies showing natural selection in action involve descendants of the finches that Darwin observed in the alápagos Islands. Those finch species looked so different from one another that when Darwin first saw them, he did not realize they were all finches. He thought they were blackbirds, warblers, and other kinds of birds. The species he examined differed greatly in the sizes and shapes of their beaks and in their feeding habits, as shown in Figure 16 13. Some species fed on small seeds, while others ate large seeds with thick shells. One species used cactus spines to pry insects from dead wood. One species, not shown here, even pecked at the tails of large sea birds and drank their blood! Once Darwin discovered that these birds were all finches, he hypothesized that they had descended from a common ancestor. Over time, he proposed, natural selection shaped the beaks of different bird populations as they adapted to eat different foods. That was a reasonable hypothesis. ut was there any way to test it? No one thought so, until the work of Peter and Rosemary rant from Princeton University proved otherwise. For more than twenty years, the rants, shown in Figure 16 14, have been collaborating to band and measure finches on the alápagos Islands. They realized that Darwin s hypothesis relied on two testable assumptions. First, in order for beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection. Second, differences in beak size and shape must produce differences in fitness that cause natural selection to occur. Download a worksheet on speciation for students to complete, and find additional teacher support from NST SciLinks. FCTS ND FIURES Fruit fly speciation The North merican fruit fly Rhagoletis pomonella appears to be in the process of speciation. efore the 1800s, R. pomonella infested only hawthorn trees. Then, when apple trees were introduced to North merica, the fruit fly began infesting them as well. Today, the species exists in separate populations on each type of fruit tree. The different populations do not interbreed and have some genetic differences. lthough they are still one species, they appear to be on their way to becoming separate species. 406 Chapter 16

The rants tested these hypotheses on the medium ground finch on Daphne Major, one of the alápagos islands. This island is large enough to support good-sized finch populations, yet small enough to enable the rants to catch and identify nearly every bird belonging to the species under study. Variation The rants first identified and measured as many individual birds as possible on the island. They recorded which birds were still living and which had died, which had succeeded in breeding and which had not. For each individual, they also recorded anatomical characteristics such as wing length, leg length, beak length, beak depth, beak color, feather colors, and total mass. Many of these characteristics appeared in bell-shaped distributions typical of polygenic traits. These data indicate that there is great variation of heritable traits among the alápagos finches. Natural Selection Other researchers who had visited the alápagos did not see the different finches competing or eating different foods. During the rainy season, when these researchers visited, there is plenty of food. Under these conditions, finches often eat the most available type of food. During dry-season drought, however, some foods become scarce, and others disappear altogether. t that time, differences in beak size can mean the difference between life and death. To survive, birds become feeding specialists. Each species selects the type of food its beak handles best. irds with big, heavy beaks, for example, select big, thick seeds that no other species can crack open. The rants most interesting discovery was that individual birds with different-sized beaks had different chances of survival during a drought. When food for the finches was scarce, individuals with the largest beaks were more likely to survive, as shown in Figure 16 15. eak size also plays a role in mating behavior, because big-beaked birds tend to mate with other big-beaked birds. The rants observed that average beak size in that finch population increased dramatically over time. This change in beak size is an example of directional selection operating on an anatomical trait. y documenting natural selection in the wild, the rants provided evidence of the process of evolution: The next generation of finches had larger beaks than did the generation before selection had occurred. n important result of this work was their finding that natural selection takes place frequently and sometimes very rapidly. Changes in the food supply on the alápagos caused measurable fluctuations in the finch populations over a period of only decades. This is markedly different from the slow, gradual evolution that Darwin envisioned. What type of natural selection did the rants observe in the alápagos? FCTS ND FIURES irds and flies in Hawaii In the process of adaptive radiation on the alápagos Islands, one species of finches gave rise to many species of finches. Similarly, five million years of adaptive radiation in Hawaiian birds known as honeycreepers resulted in a wide array of beak shapes and as many as 43 species. However, human actions have resulted in the extinction of most of these species. Percentage Surviving 50 40 30 20 10 0 6 Figure 16 14 Peter and Rosemary rant have demonstrated that natural selection is still a force in the evolution of the alápagos finches. pplying Concepts How does their research demonstrate natural selection? Figure 16 15 This graph shows the survival rate of one species of ground-feeding finches, the medium ground finch, eospiza fortis. Using Tables and raphs What trend does this graph show? ird Survival ased on eak Size 7 8 9 10 11 12 13 eak Size (mm) Diverging physical traits do not cause all speciation. In Hawaii, a variety of courtship songs has separated the native Drosophila fruit fly into more than 500 species! Some Hawaiian Drosophila sound more like cicadas than flies. Others make a cricketlike noise. Still others make a sound like that of a North merican fruit fly, but they create the sound by vibrating their abdomen instead of their wings. Inferring Challenge students to assume the role of an evolutionary biologist. First, have them choose a population that they will study. Then, ask them to list the kinds of observations they would need to make to determine if the population were undergoing natural selection. (You would need to observe evidence of inheritable variation in the population and evidence that different phenotypes vary in fitness.) sk: What might you observe if speciation were occurring? (You might observe that various populations of the species had become separated so they no longer shared the same gene pool. You also might observe genetic differences between the populations.) Figure 16 15 Challenge students to predict how the graph would be different if birds with medium-sized beaks were more likely to survive. (It would resemble a normal curve.) sk: What type of natural selection would this represent? (Stabilizing selection) nswers to... Directional selection Figure 16 13 Sample answer: For the large ground finch, natural selection favored birds with large, heavy beaks that could crush tough seeds. Figure 16 14 Their research shows that birds with larger beaks have a better chance of surviving during a drought and that average beak size has increased dramatically over time as a result. Figure 16 15 The larger a bird s beak, the greater its chances of survival. Evolution of Populations 407

16 3 (continued) Speciation in Darwin s Finches Encourage students to view the segment The alápagos Islands: limpse Into the Past on Videotape Two. pplying Concepts Challenge students to apply the concept of founder effect to the example of Darwin s finches. sk: How did the first finches to arrive on the alápagos compare genetically with finches on the mainland? (They had only a tiny fraction of the total genetic variability of the mainland finches and, through chance, may not have been genetically representative of mainland finches.) To find out more about ongoing research on the alápagos, view the segment The alápagos Islands: limpse Into the Past, on Videotape Two. Speciation in Darwin s Finches The rants work demonstrates that finch beak size can be changed by natural selection. If we combine this information with other evolutionary concepts you have learned in this chapter, we can show how natural selection can lead to speciation. We can devise a hypothetical scenario for the evolution of all alápagos finches from a single group of founding birds. Speciation in the alápagos finches occurred by founding of a new population, geographic isolation, changes in the new population s gene pool, reproductive isolation, and ecological competition. Founders rrive Many years ago, a few finches from the South merican mainland species flew or were blown to one of the alápagos Islands, as shown in Figure 16 16. Finches are small birds that do not usually fly far over open water. These birds may have gotten lost, or they may have been blown off course by a storm. Once they arrived on one of the islands, they managed to survive and reproduce. eographic Isolation Later on, some birds from species crossed to another island in the alápagos group. ecause these birds do not usually fly over open water, they rarely move from island to island. Thus, finch populations on the two islands were essentially isolated from each other and no longer shared a common gene pool. How did finches arrive in the alápagos Islands? Hypothesis Hypothesis Explain to students that the arrows in the diagrams imply evolutionary relationships. For example, in hypothesis, both and are presumed to have evolved from. nswers 1. Hypothesis suggests that Lake 1 and Lake 2 brown fishes are unrelated, and the gold fishes from the two lakes are unrelated. This differs from hypothesis, which suggests that Lake 1 gold fishes evolved from Lake 2 gold fishes and that Lake 2 brown fishes evolved from Lake 1 brown fishes. 2. In hypothesis, the brown () and gold () fish populations each evolved independently. In hypothesis, the brown () fish populations in both lakes evolved from fish type in Lake 1, and the gold () fish populations in both lakes evolved from fish type in Lake 2. 3. This evidence supports hypothesis. 4. Scientists would need to ask whether fishes from the two lakes can interbreed and produce fertile offspring. If not, then they are members of different species. How re These Fish Related? research team studied two lakes in an area that sometimes experiences flooding. Each lake contained two types of similar fish: a dull brown form and an iridescent gold form. The team wondered how all the fish were related, and they considered the two hypotheses diagrammed on the right. 1. Interpreting raphics Study the two diagrams. What does hypothesis indicate about the ancestry of the fish in Lake 1 and Lake 2? What does hypothesis indicate? 2. Comparing and Contrasting ccording to the two hypotheses, what is the key difference in the way the brown and gold fish populations might have formed? FCTS ND FIURES Not just finches mong birds on the alápagos Islands, Darwin s finches are better known than the mockingbirds, but it was the four species of mockingbirds that first drew Darwin s attention to the diversity of species on the islands. Darwin discovered that three of the mockingbird Lake 1 Lake 2 = Possible ancestor = Contemporary brown form = Contemporary gold form Lake 1 Lake 2 Shows possible line of descent 3. Drawing Conclusions DN analysis showed that the brown and gold fish from Lake 1 are the most closely related. Which hypothesis does this evidence support? 4. sking Questions To help determine whether the brown and gold fish are members of separate species, what question might scientists ask? species were each confined to a single island, and it surprised him to learn that the islands were all similar and within sight of one another. Like Darwin s finches, the mockingbird species had evolved differences primarily in the size and shape of their beaks that reflected the different food sources they relied upon. 408 Chapter 16

Figure 16 16 Speciation in the alápagos finches occurred by founding of new populations, geographic isolation, gene pool changes, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species. Changes in the ene Pool Over time, populations on each island became adapted to their local environments. The plants growing on the first island may have produced small thin-shelled seeds, whereas the plants on the second island may have produced larger thick-shelled seeds. On the second island, directional selection would favor individuals with larger, heavier beaks. These birds could crack open and eat the large seeds more easily. Thus, birds with large beaks would be better able to survive on the second island. Over time, natural selection would have caused that population to evolve larger beaks, forming a separate population,. Reproductive Isolation Now, imagine that a few birds from the second island cross back to the first island. Will the population- birds breed with the population- birds? Probably not. These finches choose their mates carefully. s part of their courtship behavior, they inspect a potential partner s beak very closely. Finches prefer to mate with birds that have the samesized beak as they do. In other words, big-beaked birds prefer to mate with other big-beaked birds, and smaller-beaked birds prefer to mate with other smaller-beaked birds. ecause the birds on the two islands have different-sized beaks, it is likely that they would not choose to mate with each other. Thus, differences in beak size, combined with mating behavior, could lead to reproductive isolation. The gene pools of the two bird populations remain isolated from each other even when individuals live together in the same place. The two populations have now become separate species. Ecological Competition s these two new species live together in the same environment (the first island), they compete with each other for available seeds. During the dry season, individuals that are most different from each other have the highest fitness. The more specialized birds have less competition for certain kinds of seeds and other foods, and the competition among individual finches is also reduced. Over time, species evolve in a way that increases the differences between them. The species- birds on the first island may evolve into a new species, C. Continued Evolution This process of isolation on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire alápagos island chain. Over many generations, it produced the 13 different finch species found there today. Use the steps in this illustration to explain how other Darwin finches, such as the vegetarian tree finch that feeds on fruit, might have evolved. alapagos Islands SOUTH MERIC Founders rrive few finches travel from South merica to one of the islands. There, they survive and reproduce. eographic Isolation Some birds from species cross to a second island. The two populations no longer share a gene pool. Changes in the ene Pool Seed sizes on the second island favor birds with larger beaks. The population on the second island evolves into a population,, with larger beaks. Eventually, populations and evolve into separate species. Make Connections Environmental Science Introduce students to the concept of adaptive radiation, which is the evolution of many diversely adapted species from a common ancestor. Explain that Darwin s finches could not have evolved into so many different species if the niche resources they found had already been put to use by other birds. Point out that the alápagos are volcanic islands that at one time had many available niche resources for land birds. sk: esides the formation of islands by volcanoes, how might an area come to support so many niches established by invading species through adaptive radiation? (One possible answer is the occurrence of a natural disaster, such as a forest fire or flood, that destroys habitats. s the original habitats gradually return, the species reestablish niches.) Inferring Point out the role of ecological competition in the speciation of Darwin s finches. sk: How does competition increase the differences between species? (Individuals that are more specialized and less like the other species have less competition and higher fitness. This leads to an increase in the differences between the species.) Figure 16 16 Challenge students to create new captions for the sequence of drawings to explain how another species of Darwin s finches, such as the vegetarian tree finch, might have evolved. sk volunteers to share their captions with the class. They flew, or were blown, from South merica. Evolution of Populations 409

16 3 (continued) Studying Evolution Since Darwin pplying Concepts Challenge students to explain in two or three sentences the process of speciation, using concepts of Mendelian inheritance. sk volunteers to share their statements with the class. 3 SSESS Evaluate Understanding Call on students at random to name and give examples of each of the ways reproductive isolation comes about. Reteach Have students write on a note card a brief summary of each step in the speciation of Darwin s finches. Then, have students shuffle the cards and try to put them back in the correct order. ssess students paragraphs on the basis of their completeness, accuracy, and organization. The paragraphs should begin with a topic sentence, such as the rants hypothesis, followed by sentences providing details related to the topic sentence. Paragraphs should also include the conclusions the rants reached based on their research. If your class subscribes to the itext, use it to review the Key Concepts in Section 16 3. Figure 16 17 Paleontologists study fossils to find clues about previous life forms. 16 3 Section ssessment 1. Key Concept How is reproductive isolation related to the formation of new species? 2. Key Concept What type of isolating mechanism was important in the formation of alápagos finch species? 3. Explain how behavior can play a role in the evolution of species. 16 3 Section ssessment Studying Evolution Since Darwin It is useful to review and critique the strengths and weaknesses of evolutionary theory. Darwin made bold assumptions about heritable variation, the age of Earth, and relationships among organisms. New data from genetics, physics, and biochemistry could have proved him wrong on many counts. They didn t. Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the ancient past. Limitations of Research The rants research clearly shows the effects of directional selection in nature. The rants data also show how competition and climate change affect natural selection. The work does have limitations. For example, while the rants observed changes in the size of the finches beaks, they did not observe the formation of a new species. Scientists predict that as new fossils are found, they will continue to expand our understanding of how species evolved. Unanswered Questions The studies of the rants fit into an enormous body of scientific work supporting the theory of evolution. Millions of fossils show that life has existed on Earth for more than 3 billion years and that organisms have changed dramatically over this time. These fossils form just a part of the evidence supporting the conclusion that life has evolved. Remember that a scientific theory is defined as a well-tested explanation that accounts for a broad range of observations. Evolutionary theory fits this definition. To be sure, many new discoveries have led to new hypotheses that refine and expand Darwin s original ideas. No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain. Why is understanding evolution important? ecause evolution continues today, driving changes in the living world such as drug resistance in bacteria and viruses, and pesticide resistance in insects. Evolutionary theory helps us understand and respond to these changes in ways that improve human life. 4. What recent research findings support Darwin s theory of evolution? 5. Critical Thinking Inferring Suppose that a drought on an island eliminates all but plants that produce large, tough seeds. ll the finches on the island have very small beaks. How might this environmental change impact the survival of this finch population? Summarizing Write a paragraph that summarizes the rants research with alápagos finches. Your summary should include the main points of the research. Hint: The first sentence in your summary might state the rants hypothesis. 1. For new species to evolve, populations must be reproductively isolated from each other. 2. eographic isolation 3. ehavioral isolation can occur when two populations that could otherwise interbreed do not because of differences in behavior. This can lead to the evolution of different species. 4. Peter and Rosemary rant have provided evidence of the process of evolution through their studies of changes of beak size and shape in alápagos finches. 5. Only those finches that can find another food source or those with beaks large enough to crack the seeds will survive. 410 Chapter 16