The Pan-American Land Bridge, or Isthmus, Connecting North and South America

Reading Passage Question

The Pan-American land bridge, or isthmus, connecting North and South merica was formed volcanically long after dinosaurs became extinct. The isthmus cleaved populations of marine organisms, creating sister species. These twin species, called “geminates,” then evolved independently. Scientists observe, for example, that Pacific pistol shrimp no longer mate with those from the Atlantic Ocean. Yet the two oceans had already begun to form their distinctive personalities long before the is thmus was fully formed. As the seabed rose, Pacific waters grew cooler, their up-swelling currents carrying rich nutrients, while the Atlantic side grew shallower, warmer, and nutrient poor. In fact, it was these new conditions, and not so much the fully-formed isthmus, that spawned changes in the shrimp population.

For terrestrial life, the impact of the isthmus was more immediate. Animals traversed the newly formed bridge in both directions, although North American creatures proved better colonizers—more than half of South America’s mammals trace direct lineage to this so-called Great American Biotic Exchange. Only three animals—the armadillo, opossum, and hedgehog—survive as transplants in the north today.

“The Pan-American land bridge, or isthmus, connecting North and South America”- is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 3 comprehension questions. The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analyzing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

Question1
Which of the following statements finds the LEAST support in the passage?

1. Population divergences resulting from the formation of the Pan-American isthmus were more a process than an event.
2. The divergence in ocean temperature during the formation of the Pan-American isthmus resulted in a divergence in the ocean’s nutrient value.
3. Genetic differences among pistol shrimp have grown to the point that there are now at least two distinct species of these shrimp.
4. The part of ocean which is now the Pacific grew deeper due to the geologic forces that created the Pan-American isthmus.
5. Not until the Pan-American isthmus was fully formed did geminate marine organisms begin to develop in that areaof the ocean.

Question 2
Which of the following statements is most readily inferable from the information in the passage?

1. Species of marine organisms in the Atlantic Ocean number fewer today than before the formation of the Pan-American isthmus.
2. The number of terrestrial animal species in South America today exceeds the number prior to the formation of thePan-American isthmus.
3. Of the indigenous North American species that migrated south across the Pan-American isthmus, more than three survive to this day.
4. Since the formation of the Pan-American isthmus, fewer terrestrial animals have traveled north across the isthmus than south.
5. As the Pan-American isthmus began to form, most pistol shrimp migrated west to what is now the Pacific Ocean.

Question 3
The author mentions the mating habits of pistol shrimp in order to show that

1. Some species of marine organism inhabiting the Pacific Ocean are now entirely distinct from those in the Atlantic Ocean.
2. Twin species of marine organisms can each survive even though one species can no longer mate with the other.
3. Since the formation of the Pan-American isthmus, some marine germinates no longer mate with their sister species.
4. Germinate species that do not mate with one another are considered separate species.
5. The evolutionary impact of the Pan-American isthmus was greater for marine organisms than for land animals.

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