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生物学家恩斯特·迈尔将物种定义为“实际上或潜在的杂交种群，在有机会的情况下不会与其他同类种群杂交。”

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.”

许多物种起源的一个关键事件是一个种群的基因库（一个种群在任何时候的所有基因）与同一物种的其他种群的分离，从而防止种群杂交。

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.

随着基因库的隔离，一个独立的种群可以遵循自己的进化过程。

With its gene pool isolated, a separate population can follow its own evolutionary course.

在许多物种的形成过程中，种群的最初隔离似乎一直是一个地理障碍。

In the formation of many species, the initial isolation of a population seems to have been a geographic barrier.

这种进化新物种的模式被称为异地物种形成。

This mode of evolving new species is called allopatric speciation.

许多因素可以在地理上隔离一个群体。

Many factors can isolate a population geographically.

可能会出现山脉，并逐渐分裂只能栖息在低地湖泊中的生物种群。某些鱼类种群可能会以这种方式被隔离。

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.

同样，一个正在蔓延的冰川可能会逐渐分割种群，或者像巴拿马地峡这样的大陆桥可能会形成并分隔两边海洋中的海洋生物。

Similarly, 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.

地理屏障必须有多强大才能将人口分隔开来？

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

这取决于有机体跨越障碍的能力。

It depends on the ability of the organisms to move across barriers.

Birds and coyotes can easily cross mountains and rivers.

被风吹来的树木花粉也不会受到这种障碍的阻碍，许多植物的种子可能会在动物身上来回携带。

The passage of wind-blown tree pollen is also not hindered by such barriers, and the seeds of many plants may be carried back and forth on animals.

相比之下，小型啮齿动物可能会发现深峡谷或宽阔的河流是有效的障碍。

In contrast, small rodents may find a deep canyon or a wide river an effective barrier.

例如，美国西南部的大峡谷将白尾羚羊松鼠的活动范围与近亲哈里斯羚羊松鼠的活动范围分开。

For example, the Grand Canyon, in the southwestern United States, separates the range of the white-tailed antelope squirrel from that of the closely related Harris’ antelope squirrel.

白尾羚羊松鼠体型较小，下面有一条白色的较短的尾巴，生活在加利福尼亚州南部峡谷以北和科罗拉多河以西的沙漠中。

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.

哈里斯羚羊松鼠在大峡谷南部的沙漠中活动范围更有限。

Harris’ antelope squirrel has a more limited range in deserts south of the Grand Canyon.

地理隔离为新物种的发展创造了机会，但这并不一定会导致新物种的出现，因为只有当基因库经历了足够的变化，在被隔离的种群与其亲代种群之间建立生殖障碍时，才会发生物种形成。

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.

当一个种群规模较小且与世隔绝时，异地物种形成的可能性会增加，这使得它比一个大种群更有可能使其基因库发生实质性的变化。

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.

例如，在不到200万年的时间里，来自南美大陆的少量流浪动植物设法移生于加拉帕戈斯群岛，形成了现在居住在这些岛屿上的所有物种。

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.

当海洋岛屿的距离足够远，可以让种群孤立地进化，但又足够近，可以偶尔发生分散时，它们实际上就是进化的户外实验室。

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.

加拉帕戈斯岛链是世界上最伟大的进化展示之一。

The Galapagos island chain is one of the world’s greatest showcases of evolution.

每个岛屿都是从水下火山中诞生的，并逐渐被从其他岛屿和大陆乘着洋流和风流浪而来的有机体所覆盖。

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.

有机体也可以被其他有机体带到岛屿上，比如长距离飞行的海鸟，它们的羽毛上附着着种子。

Organisms can also be carried to islands by other organisms, such as sea birds that travel long distances with seeds clinging to their feathers.

今天加拉帕戈斯群岛上的物种，大多数在其他地方都没有出现过，是从南美大陆漂浮、飞行或被风吹过海洋的有机体的后代。

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.

例如，加拉帕戈斯群岛链总共有13种关系密切的鸟类，称为加拉帕戈斯雀类。

For instance, the Galapagos island chain has a total of thirteen species of closely related birds called Galapagos finches.

这些鸟有许多相似之处，但它们的食性和喙类型不同，这与它们吃的东西有关。

These birds have many similarities but differ in their feeding habits and their beak type, which is correlated with what they eat.

积累的证据表明，所有13种雀类物种都是从定居在其中一个岛屿上的一小群祖先鸟类进化而来的。

Accumulated evidence indicates that all thirteen finch species evolved from a single small population of ancestral birds that colonized one of the islands.

创始种群从大陆迁徙后，在岛上完全与世隔绝，其基因库可能发生了重大变化，成为一个新物种。

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.

后来，这种新物种中的一些个体可能被暴风雨吹到了邻近的岛屿上。

Later, a few individuals of this new species may have been blown by storms to a neighboring island.

在第二个岛上，第二个创始种群与世隔绝，可能进化成第二个新物种，稍后可能会重新殖民其创始种群迁出的岛屿。

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.

今天，加拉帕戈斯岛上的每个岛屿都有多种雀类，有些岛屿上的雀类多达10种。

Today each Galapagos island has multiple species of finches, with as many as ten on some islands.

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

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

According to paragraph 1, allopatric speciation is possible when

How is paragraph 2 related to paragraph 1?

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

Paragraph 3 supports the idea that white-tailed antelope squirrels and Harris’ antelope squirrels have which of the following in common?

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

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.

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

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

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

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

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.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: 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 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. This question is worth 2 points.

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. Similarly, 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 carried 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 States, separates the range of the white-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. Harris’ 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 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.