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Geographic Isolation of Species

Biologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.”A key event in the o rigin of many species is the separation of a population with its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolation of a population seems to have been a geographic barrier This mode of evolving new species is called allopatric speciation.

Many factors can isolate a population geographically.A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain fish populations might become isolated in this way. Similarity, a creeping glacier may gradually divide a population, or a land bridge such as the Isthmus of Panama may form and separate the marine life in the ocean waters on either side.

How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across barriers.Birds and coyotes can easily cross mountains and rivers. The passage of wind-blown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned back and forth on animals In contrast, small rodents may find a deep canyon or a wide river an effective barrier. For example, the Grand Canyon, in the southwestern United Slates, separate the range of the while-tailed antelope squirrel from that of the closely related Harris’ antelope squirre l. Smaller, with a shorter tail that is white underneath, the white-tailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams' antelope squirrel has a more limited range in deserts south of the Grand Canyon.

Geographic isolation creates opportunities for new species to develop, but it does not necessarily lead to new species because speciation occurs only when the gene pool undergoes enough changes to establish reproductive barriers between the

isolated population and its parent population.The likelihood of allopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially. For example,in less than two million years, small populations of stray animals and plants from the South American mainland that managed to colonize the Galapagos Islands gave rise to all the species that now inhabit the islands.

When oceanic islands are far enough apart to permit populations to evolve in isolation, but close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution.The Galapagos island chain is one of the world s greatest showcases of evolution Each island was born from underwater volcanoes and was gradually covered by organisms derived from strays that rode the ocean currents and winds from other islands and continents. Organisms can also be carried to islands by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.

The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches These birds have many similarities but differ in their feeding habits and

their beak type, which is correlated with what they eat Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the https://www.sodocs.net/doc/b46989737.html,pletely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species. Later, a few individuals of this new species may have been blown by storms to a neighboring island. Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated.Today each Galapagos island has multiple species of finches, with as many as ten on some islands.

paragraph 1

Biologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.” A key event in the origin of many species is the separa tion of a population with its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolation of a population seems to have been a geographic barrier This mode of evolving new species is called allopatric speciation.

1.The word “key” in the passage is closest in m eaning to

A.early

B.crucial

C.noticeable

D.frequent

2.The word “initial” in the passage is closest in meaning to

A.best

https://www.sodocs.net/doc/b46989737.html,ual

C.first

D.actual

3.According to paragraph 1. allopatric speciation is possible when

A. a population contains all the different genes present in a species at a particular time

B. a population becomes isolated due to the presence of a geographic barrier

C. genetic mixing begins to occur in previously separate populations of a species

D. a species is successful in crossing a geographic barrier

paragraph 1&2

Biologist Ernst Mayr defined a species as “an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.”A key event in the origin of many species is the separation of a population with its gene pool (all of the genes in a population at any one time) from other populations of the same species, thereby preventing population interbreeding With its gene pool isolated, a separate population can follow its own evolutionary course. In the formation of many species, the initial isolation of a population seems to have been a geographic barrier This mode of evolving new species is called allopatric speciation.

Many factors can isolate a population geographically. A mountain range may emerge and gradually split a population of organisms that can inhabit only lowland lakes, certain fish populations might become isolated in this way. Similarity, a creeping glacier may gradually divide

a population, or a land bridge such as the Isthmus of Panama may form and separate the marine life in the ocean waters on either side.

4.How is paragraph 2 related to paragraph 1?

A. Paragraph 2 points out a number of ways in which the phenomenon of geographic isolation mentioned in paragraph 1 can occur

B. Paragraph 2 identifies discoveries that led to the conclusion presented in paragraph 1 that geographic isolation has played a rote in the origin of many species

C. Paragraph 2 provides evidence supporting the statement in paragraph 1 that a population can follow its own evolutionary course once its gene pool becomes isolated

D. Paragraph 2 explains why the term “allopatric” was adopted to describe the method of speciation described in paragraph 1

paragraph 3

How formidable must a geographic barrier be to keep populations apart? It depends on the ability of the organisms to move across barriers. Birds and coyotes can easily cross mountains and rivers. The passage of wind-blown tree pollen is also not hindered by such barriers, and the seeds of many plants may be earned back and forth on animals In contrast, small rodents may find a deep canyon or a wide river an effective barrier. For example, the Grand Canyon, in the southwestern United Slates, separate the range of the while-tailed antelope squirrel from that of the closely related Harris’ antelope squirrel. Smaller, with a shorter tail that is white underneath, the white-tailed antelope squirrel inhabits deserts north of the canyon and west of the Colorado River in southern California Hams' antelope squirrel has

a more limited range in deserts south of the Grand Canyon.

5.In paragraph 3, the author contrasts a variety of organisms to illustrate which of the following points?

A. Geographic barriers are less likely to keep apart populations of plants than populations of animals.

B. Geographic barriers are more likely to keep apart populations of large organisms than populations of small organisms

C. Some members of a species are able to cross geographic barriers, while other members of the same species are not.

D. The effectiveness of geographic barriers in keeping organisms apart depends on an organism’s ability to move across barriers.

6.Paragraph 3 supports the idea that white-tailed antelope squirrels and Hams' antelope squirrels have which of the following in common?

A. They are the two smallest rodents now found in the southwestern United States.

B. They have white coloring underneath their tails

C. They cannot cross the Grand Canyon

D. They cannot survive in desert conditions

paragraph 4

Geographic isolation creates opportunities for new species to develop, but it does not necessarily lead to new species because speciation occurs only when the gene pool undergoes enough changes to establish reproductive barriers between the isolated population and its parent population. The likelihood of allopatric speciation increases when a population is small as well as isolated, making it more likely than a large population to have its gene pool changed substantially. For example, in less than two million years, small populations of stray animals and plants from the South American mainland that managed to colonize the Galapagos Islands gave rise to all the species that now inhabit the islands.

7.The word “undergoes” in the passage is closest in meaning to

A.experiences

B.allows

C.prevents

D.causes

8.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information

A. Geographic isolation is sometimes but not always the reason for the creation of reproductive barriers between a parent population and the populations descended from it

B. Genetic changes in a geographical isolated population do not necessarily make the population look different enough from its parent population to be considered a new species

C. Geographical isolation allows the separated populations to evolve independently of each other and so can lead to the formation of new species

D. Geographic isolation can lead to new species only if the gene pool of the isolated population changes enough to prevent it from reproducing with the parent population

9.According to paragraph 4, why does the size of a population affect the likelihood of allopatric speciation?

A. Because smaller populations are more likely than larger ones to become geographically isolated

B. Because the gene pool of a small isolated population is more likely to undergo substantial change than is the gene pool of a larger population

C. Because a isolated population can become a new species with substantially less change to its gene pool than would be required by a larger population

D. Because smaller populations are more likely to be made up of stray animals or plants than larger populations are

10.The word “managed” in the passage is closest in meaning to

A.were able

B.were forced

C.arrived

D.expanded

paragraph 5

When oceanic islands are far enough apart to permit populations to evolve in isolation, but

close enough to allow occasional dispersions to occur, they are effectively outdoor laboratories of evolution. The Galapagos island chain is one of the world s greatest showcases of evolution Each island was born from underwater volcanoes and was gradually covered by organisms derived from strays that rode the ocean currents and winds from other islands and continents. Organisms can also be carried to islands

by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.

11.Paragraph 5 supports the idea that the Galapagos island chain was able to become “one of the world's greatest showcases of evolution” primarily because of

A. the richness of the volcanic soil of each of the islands in the chain

B. the distance of the individual islands from each other and from the mainland

C. the relativity long time it took for the islands to become covered by organisms

D. the outdoor laboratories that scientists have built on the islands to study evolution

paragraph 6

The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands. Completely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a new species. Later, a few individuals of this new species may have been blown by storms to a neighboring island. Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated. Today each Galapagos island has multiple species of finches, with as many as ten on some islands.

12.According to paragraph 6. what is true about the thirteen species of Galapagos finches?

A. All thirteen species are now found on most of the Galapagos Islands

B. All thirteen species are descended from the same population of ancestral birds

C. All thirteen species evolved on the island that was originally colonized by finches from the mainland.

D. All thirteen species occur only in small, completely isolated populations.

13.Look at the four squares [■] that indicate where the following sentence could be added to the passage.

This process of speciation and colonization could have been repeated over and over again, gradually involving all the islands in the chain.

The species on the Galapagos Islands today, most of which occur nowhere else, descended from organisms that floated, flew, or were blown over the sea from the South American mainland For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the https://www.sodocs.net/doc/b46989737.html,pletely isolated on the island after migrating from the mainland, the founder population may have undergone significant changes in its gene pool and become a n ew species. [■] Later, a few individuals of this new species may have been blown by storms to a neighboring island. [■]Isolated on this second island, the second founder population could have evolved into a second new species, which could later recolonize the island from which its founding population emigrated.[■]Today each Galapagos island has multiple species of finches, with as many as ten on some islands.[■]

14.Directions：An introductory sentence for a brief summary of the passage is provided https://www.sodocs.net/doc/b46989737.html,plete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.

The geographic isolation of a population can result in the rise of a new species.

Answer Choices

A.Isolation can result when a geographic barrier forms and splits a population or when a few organisms somehow get carried across an existing geographic barrier and form a new population

B.Speciation is more likely when an isolated population is small because significant genetic changes are more likely to occur in a small population than in a large one

C.Because of the geographic isolation of the Galapagos Islands, the species that now inhabit them have gene pools that have not changed very much since the islands were first populated.

D.Fish populations are more easily isolated by geographic barriers than are populations of most other organisms because fish cannot move across areas where there is no water.

E.The Galapagos Islands are well situated for speciation because they provide opportunities for population isolation while also making occasional dispersions between islands possible.

F.Evidence indicates that the first organisms to reach the Galapagos Islands were probably a small population of finches that,in less than two million years of isolation,evolved into thirteen species.

Explaining Dinosaur Extinction

Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K-T event, because it is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous, derived from the German name Kreidezeit). Many explanations have been proposed for why dinosaurs became extinct. For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants

first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out. In fact, several scientists have suggested that the duckbill dinosaurs and homed dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resource of rapidly growing flowering plants Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork.

This focus on explaining dinosaur extinction misses an important point the extinction at the end of the Cretaceous was a global event that killed off organisms up and down the food chain. It wiped out many kinds of plankton in the ocean and many marine organisms that lived on the plankton at the base of the food chain. These included a variety of clams and snails, and especially the ammonites, a group of shelled squidlike creatures that dominated the Mesozoic seas and had survived many previous mass extinctions. The K-T event marked the end of the marine reptiles, such as the mosasaurs and the plesiosaurs, which were the largest creatures that had ever lived in the seas and which ruled the seas long before whales evolved. On land, there was also a crisis among the land plants, in addition to the disappearance of dinosaurs. So any event that can explain the destruction of the base of the food chain (plankton in the ocean, plants on land) can better explain what happened to organisms at the top of the food chain, such as the dinosaurs. By contrast, any explanation that focuses strictly on the dinosaurs completely misses the point The Cretaceous extinctions were a global phenomenon, and dinosaurs were just a part of a bigger picture.

According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth. Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20-70 kilometers per second, or 45,000-156,000 miles per hour.

Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy. When the bolide struck this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames. The bolide struck an area of the Yucatan Peninsula of Mexico known as Chicxulub, excavating a crater 15-20 kilometers deep and at least 170 kilometers in diameter. The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean. Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an altitude of 100 kilometers. Most of it fell back immediately, but some of it remained as dust in the atmosphere for months. This material, along with the smoke from the fires,

shrouded Earth, creating a form of nuclear winter. According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis halted, and most plants on land and in the sea died. With the bottom of the food chain destroyed, dinosaurs could not survive.

paragraph 1

Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K-T event, because it is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous, derived from the German name Kreidezeit). Many explanations have been proposed for why dinosaurs became extinct. For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out. In fact, several scientists have suggested that the duckbill dinosaurs and homed dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resource of rapidly growing flowering plants Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly

acquired a taste for dinosaur eggs after 120 million years of coexistence Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork.

1. In paragraph 1, why does the author include a discussion of when flowering plants evolved?

A. To help explain why some scientists believe that the development of flowering plants led to dinosaur extinction

B. To cast doubt on the theory that the development of flowering plants caused dinosaurs to become extinct

C. To suggest that dinosaurs were able to survive for as long as they did because of the availability of flowering plants

D. To emphasize that duckbill dinosaurs and horned dinosaurs were the first dinosaurs to become extinct

2. The word “allegedly” in the passage is closest in meaning to

A. inevitably

B. gradually

C. Supposedly

D. Increasingly

3. According to paragraph 1 the extinction of the dinosaurs is unlikely to have been the result of competition from mammals because

A. mammals would not have been capable of eating dinosaur eggs

B. mammals did not appear in any significant numbers until after the Late Triassic

C. mammals and dinosaurs did not, in fact, compete for any of the same resources

D. mammals and dinosaurs lived together for roughly 120 million years before the extinction

paragraph 2

This focus on explaining dinosaur extinction misses an important point the extinction at the end of the Cretaceous was a global event that killed off organisms

up and down the food chain. It wiped out many kinds of plankton in the ocean and many marine organisms that lived on the plankton at the base of the food chain. These included a variety of clams and snails, and especially the ammonites, a group of shelled squidlike creatures that dominated the Mesozoic seas and had survived many previous mass extinctions. The K-T event marked the end of the marine reptiles, such as the mosasaurs and the plesiosaurs, which were the largest creatures that had ever lived in the seas and which ruled the seas long before whales evolved. On land, there was also a crisis among the land plants, in addition to the disappearance of dinosaurs. So any event that can explain the destruction of the base of the food chain (plankton in the ocean, plants on land) can better explain what happened to organisms at the top of the food chain, such as the dinosaurs. By contrast, any explanation that focuses strictly on the dinosaurs completely misses the point The Cretaceous extinctions were a global phenomenon, and dinosaurs were just a part of a bigger picture.

paragraph 3

According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth. Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20-70 kilometers per second, or 45,000-156,000 miles per hour. Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy. When the bolide struck this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames. The bolide struck an area of the Yucatan Peninsula of Mexico known as Chicxulub, excavating a crater 15-20 kilometers deep and at least 170 kilometers in diameter. The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean. Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an altitude of 100 kilometers. Most of it fell back immediately, but some of it remained as dust in the atmosphere for months. This material, along with the smoke from the fires, shrouded Earth, creating a form of nuclear winter. According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis

halted, and most plants on land and in the sea died. With the bottom of the food chain destroyed, dinosaurs could not survive.

4. According to paragraph 2, what is problematic about some scientists' focus on dinosaur extinction?

A. Dinosaurs became extinct so long ago that no theory about their disappearance can be proven scientifically.

B. Dinosaurs were not the only organisms that went extinct at the end of the Cretaceous period.

C. More marine organisms went extinct during the Cretaceous than did dinosaur species.

D. It is more important to understand how plankton and other marine organisms came to thrive during the Cretaceous period.

5. According to paragraph 2, each of the following became extinct during the K-T event EXCEPT

A. early species of whales

B. marine reptiles

C. various species of clams

D. many species of land plants

6. What makes the extinction of “the ammonites” especially significant?

A. They were among the largest creatures that ever lived.

B. They existed at the lowest level of the food chain.

C. They had been able to survive in the Mesozoic seas.

D. They had survived many previous mass extinctions.

7. The word “halted” in the passage is closest in meaning to

A. slowed

B. stopped

C. contracted

D. declined

8. The word “strictly" in the passage is closest in meaning to

A. exclusively

B. mainly

C. initially

D. Wrongly

9. The word “crisis” in the passage is closest in meaning to

A. collapse

B. disturbance

C. critical situation

D. loss

10. How does paragraph 3 relate to paragraph 2?

A. Paragraph 3 provides an alternative explanation to the one provided in paragraph 2.

B. Paragraph 3 provides an explanation that satisfies the conditions set forth in paragraph 2.

C. Paragraph 3 provides the facts to support the theory presented in paragraph 2.

D. Paragraph 3 presents a theory that calls into question the position described in paragraph 2.

paragraph 3

According to one theory, the Age of Dinosaurs ended suddenly 65 million years ago when a giant rock from space plummeted to Earth. Estimated to be ten to fifteen kilometers in diameter, this bolide (either a comet or an asteroid) was traveling at cosmic speeds of 20-70 kilometers per second, or 45,000-156,000 miles per hour. Such a huge mass traveling at such tremendous speeds carries an enormous amount of energy. When the bolide struck this energy was released and generated a huge shock wave that leveled everything for thousands of kilometers around the impact and caused most of the landscape to burst into flames. The bolide struck an area of the Yucatan Peninsula of Mexico known as Chicxulub, excavating a crater 15-20 kilometers deep and at least 170 kilometers in diameter. The impact displaced huge volumes of seawater, causing much flood damage in the Caribbean. Meanwhile, the bolide itself excavated 100 cubic kilometers of rock and debris from the site, which rose to an

altitude of 100 kilometers. Most of it fell back immediately, but some of it

remained as dust in the atmosphere for months. This material, along with the smoke from the fires, shrouded Earth, creating a form of nuclear winter. According to computerized climate models, global temperatures fell to near the freezing point, photosynthesis halted, and most plants on land and in the sea died. With the bottom of the food chain destroyed, dinosaurs could not survive.

11. Paragraph 3 answers all of the following questions EXCEPT:

A. Why did the bolide fall to Earth?

B. How fast was the bolide traveling?

C. How was the bolide capable of generating a shock wave?

D. How did the bolide cause flood damage to the Caribbean?

12. Paragraph 3 strongly suggests that if the bolide impact theory is correct, the majority of the extinctions associated with the K-T event resulted from

A. sunlight being blocked for months by dust and smoke in Earth's atmosphere

B. widespread flooding that followed the displacement of huge volumes of seawater

C. the leveling of the landscape by the shock wave that was generated when the bolide struck Earth

D. the rise in global temperatures caused by the fires that burned much of the landscape

13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.

Some explanations seem plausible until the facts are considered.

Where woul d the sentence best fit? Click on a square [■] to add the sentence to the passage.

Dinosaurs rapidly became extinct about 65 million years ago as part of a mass extinction known as the K-T event, because it is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world (K is the traditional abbreviation for the Cretaceous,

derived from the German name Kreidezeit). [■] Many explanations have been proposed for why d inosaurs became extinct. [■] For example, some have blamed dinosaur extinction on the development of flowering plants, which were supposedly more difficult to digest and could have caused constipation or indigestion—except that flowering plants first evolved in the Early Cretaceous, about 60 million years before the dinosaurs died out. [■]In fact, several scientists have suggested that the duckbill dinosaurs and homed dinosaurs, with their complex battery of grinding teeth, evolved to exploit this new resou rce of rapidly growing flowering plants. [■]Others have blamed extinction on competition from the mammals, which allegedly ate all the dinosaur eggs—except that mammals and dinosaurs appeared at the same time in the Late Triassic, about 190 million years ago, and there is no reason to believe that mammals suddenly acquired a taste for dinosaur eggs after 120 million years of coexistence Some explanations (such as the one stating that dinosaurs all died of diseases) fail because there is no way to scientifically test them, and they cannot move beyond the realm of speculation and guesswork.

14. Direction: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it.

To review the passage, click VIEW TEXT.

Over the years, scientists have proposed a number of theories as to why dinosaurs suddenly became extinct about 65 million years ago.

Answer Choices

A. Many explanations for dinosaur extinction have been proposed, but most of them are either called into question by known facts or are merely unsupported hypotheses.

B. Focusing on dinosaurs misses the point that the extinction, at about the same time, of the shelled squidlike creatures that dominated the Mesozoic seas was far more scientifically significant.

C. Computerized climate models of global temperature fluctuations support the theory that a huge rock from space hit the Yucatan Peninsula in Mexico about 65 million years ago.

D. Although mammals and dinosaurs appeared at about the same time in the Late Triassic, the K-T event, which marked the end of the dinosaurs, apparently had relatively little impact on mammals.

E. Any satisfactory explanation of the mass extinction of dinosaurs must take into account the fact that the disappearance of the dinosaurs was part of a global mass extinction.

F. A huge bolide striking Earth would have created conditions in which most plants would have died, thus explaining the mass extinction of organisms—including dinosaurs—further up the food chain.

Callisto and Ganymede

From 1996 to 1999, the Galileo spacecraft passed through the Jovian system, providing much information about Jupiter's satellites. Callisto, the outermost of Jupiter's four largest satellites, orbits the planet in seventeen days at a distance from Jupiter of two million kilometers. Like our own Moon. Callisto rotates in the same period as it revolves, so it always keeps the same face toward Jupiter. Its noontime surface temperature is only about -140°C, so water ice is stable on its surface year-round. Callisto has a diameter of 4.820 kilometers, almost the same as that of Mercury. Its mass is only one-third as great, which means its density must be only one-third as great as well. This tells us that Callisto has far less of the rocky metallic materials found in the inner planets and must instead be an icy body through much of its interior.

Callisto has not fully differentiated, meaning separated into layers of different density materials. Astronomers can tell that it lacks a dense core from the details of its gravitational pull on the Galileo spacecraft during several very close flybys.

This fact surprised scientists, who expected that all the big icy moons would be differentiated. It is much easier for an icy body to differentiate than for a rocky one, since the melting temperature of ice is so low. Only a little heating will soften the ice and get the process started, allowing the rock and metal to sink to the center and the slushy ice to float to the surface. Yet Callisto seems to have frozen solid before the process of differentiation was complete.

Like our Moon's highlands, the surface of Callisto is covered with impact craters. The survival of these craters tells us that an icy object can form and retain impact craters m its surface. In thinking of ice so far from the Sun, it is important not to judge its behavior from that of the much warmer ice we know on Earth; at the temperatures of the outer solar system, ice on the surface is nearly as hard as rock, and behaves similarly. Ice on Callisto does not deform or flow like ice in glaciers on Earth. Callisto is unique among the planet-sized objects of the solar system in its absence of interior forces to drive geological evolution. The satellite was born dead and has remained geologically dead for more than four billion years.

Ganymede, another of Jupiter's satellites and the largest in our solar system, is also cratered, but less so than Callisto. About one-quarter of its surface seems to be as old and heavily cratered; the rest formed more recently, as we can tell by the sparse covering of impact craters as well as the relative freshness of the craters. Ganymede is a differentiated world, like the terrestrial planets. Measurements of its gravity field tell us that the rock and metal sank to form a core about the size of our Moon, with a mantle and crust of ice floating above it. In addition, the Galileo spacecraft discovered that Ganymede has a magnetic field, the signature of a partially molten interior. Ganymede is not a dead world, but rather a place of continuing geological activity powered by an internal heat source. Much of its surface may be as young as half a billion years.

The younger terrain is the result of tectonic and volcanic forces Some features formed when the crust cracked, flooding many of the craters with water from the interior. Extensive mountain ranges were formed from compression of the crust, forming long ridges with parallel valleys spaced one to two kilometers apart. In

some places older impact craters were split and pulled apart. There are even indications of large-scale crustal movements that are similar to the plate tectonics of Earth.

Why is Ganymede different from Callisto? Possibly the small difference in size and internal heating between the two led to this divergence in their evolution. But more likely the gravity of Jupiter is to blame for Ganymede's continuing geological activity. Ganymede is close enough to Jupiter that tidal forces from the giant planet may have episodically heated its interior and triggered major convulsions on its crust.

paragraph 1

From 1996 to 1999, the Galileo spacecraft passed through the Jovian system, providing much information about Jupiter's satellites. Callisto, the outermost of Jupiter's four largest satellites, orbits the planet in seventeen days at a distance from Jupiter of two million kilometers. Like our own Moon. Callisto rotates in the same period as it revolves, so it always keeps the same face toward Jupiter. Its noontime surface temperature is only about -140°C, so water ice is stable on its surface year-round. Callisto has a diameter of 4.820 kilometers, almost the same as that of Mercury. Its mass is only one-third as great, which means its density must be only one-third as great as well. This tells us that Callisto has far less of the rocky metallic materials found in the inner planets and must instead be an icy body through much of its interior.

1. According to paragraph 1, which of the following statements about Callisto is true?

A. It is the satellite closest to Jupiter's surface

B. Its surface temperature is constant at all times of the day.

C. It has the same mass and diameter as the planet Mercury

D. It completes one rotation every seventeen days.

2. According to paragraph 1, how do scientists know that Callisto is made up largely of ice?

A. A sample of its interior was taken by the Galileo spacecraft in the late 1990s

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