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2021年1月30日发(作者：mutuality)

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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

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.

How formidable must a geographic barrier be to keep populations apart? It

depends on the ability of the organisms to move across 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.

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 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 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 tely 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 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 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.

word “key” in the passage is closest in meaning to

l able nt

word “initial” in the passage is closest in meaning to

ing 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.

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.

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.

aph 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.

word “undergoes” in the passage is closest in meaning to

ences ts

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

ing 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

word “managed” in the passage is closest in meaning to

able forced d ed

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.

aph 5 supports the idea that the Galapagos island chain was able to

become “one of the world's greatest showcases of evolution” primarily because

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.

ing 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.

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 tely 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.[

■

]

ions

：

An introductory sentence for a brief summary of the passage is

provided te 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

ion 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

tion 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

e 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.

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.

Galapagos Islands are well situated for speciation because they provide

opportunities for population isolation while also making occasional dispersions

between islands possible.

ce 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

indig estion

—

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.