The Pan-American land bridge, or isthmus, connecting North and South

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.

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.   1 
 D 
 1. Which of the following statements finds the LEAST support in the passage?

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

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

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

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

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

sahilchaudhary · Answer

8 minutes and got only 1 correct (3rd question) out of 3.

The 2nd question was very much in deep in the second paragraph.

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 .

Since 3 animals (which migrated from south to north) survive in north today, the animals that migrated from north to south would have been more than 3 (because North American creatures proved better colonizers).

Can anyone help me explain the first question as I am still unable to understand it?

Juz2play · Answer

First of all, we need just paragraph 1 to solve question 1.
(A) can be supported by "Yet the two oceans had already begun to form their distinctive personalities long before the is thmus was fully formed."
(B) can be supported by "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." 
(C) can be supported by "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."
(E) can be supported by "The isthmus cleaved populations of marine organisms, creating sister species. These twin species, called “geminates,” then evolved independently."
 So, (D) IS NOT SUPPORTED BY THE PASSAGE.

Carcass · Answer

when you see THE least means, in other words, that all choices but one must be true Do you see D and its elements mentioned in the passage somowhere ?? NO. So D is out and therefore is the correct answer

PUNEETSCHDV · Answer

I have doubts with first and third questions

For 1st I marked E and for 3rd I marked C

Q.11. Which of the following statements finds the LEAST
support in the passage?
A. Population divergences resulting from the formation of the
Pan-American isthmus were more a process than an 
event.
B. The divergence in ocean temperature during the formation 
of the Pan-American isthmus resulted in a divergence in 
the ocean’s nutrient value.
C. Genetic differences among pistol shrimp have grown to the 
point that there are now at least two distinct species of 
these shrimp.
D. The part of ocean which is now the Pacific grew deeper 
due to the geologic forces that created the Pan-American 
isthmus.

here is support for D 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.

Atlantic grew shallower and pacific grew deeper

E. Not until the Pan-American isthmus was fully formed did 
geminate marine organisms begin to develop in that area
of the ocean.

and opposite for E

Yet the two oceans had 
already begun to form their distinctive personalities long before 
the isthmus was fully formed.

So Geminate Organishms begin developing before isthmus was formed by process explained after this line .

for Q.3 . The author mentions the mating habits of pistol shrimp
in order to show that
A. Some species of marine organism inhabiting the Pacific Ocean
are now entirely distinct from those in the Atlantic Ocean.
B. Twin species of marine organisms can each survive even though
one species can no longer mate with the other.
C. Since the formation of the Pan-American isthmus, some marine 
germinates no longer mate with their sister species.
D. Germinate species that do not mate with one another are 
considered separate species.
E. The evolutionary impact of the Pan-American isthmus was
greater for marine organisms than for land animals.

The isthmus cleaved populations of 
marine organisms, creating sister species. These twin species, 
called “geminates,” then evolved independently.

The 2 species are not entirely different they are sister species , almost similar but do not mate.

C is what the authors intends to show.

gmatter0913 · Answer

For the first question, the option E has support in the passage. Please refer to the below sentences from the Paragraph 1.

The isthmus cleaved populations of 
marine organisms, creating sister species. These twin species, 
called “geminates,” then evolved independently.

Option E is specific to 'geminates', and from the above sentences 'geminates' evolved only after the formation of the isthmus.

For the third question, please read the below sentences from Paragraph 1.

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.

Mating of Pistol shrimp is mentioned as an example in the passage. What is this example for? This is an example for the aforementioned behavior that twin species evolved independently (without mating).

Option C is true, but it is not the point why the author mentions mating of the pistol shrimp

gmatter0913 · Answer

I thought the answer is B for the first question because I didn't see any form of causality in the below sentences from Paragraph 1. The answer option mentions 'resulted' indicating causation. But, when I re-read the sentences carefully, I got a doubt if I am missing something here.

Can you guys help me interpret the below sentence?

In the above sentence, what is the role of 'their up-swelling currents carrying rich nutrients' ?

In my view, it is a modifier modifying the clause 'Pacific waters grew cooler'. 
So, it makes a meaning that it is the up-swelling currents carrying rich nutrients that made the Pacific waters cooler.

But, this meaning is still the opposite of B.

Can somebody help me understand how you found support for option B?

b2bt · Answer

1. Which of the following statements finds the LEAST
support in the passage?
A. Population divergences resulting from the formation of the
Pan-American isthmus were more a process than an 
event. --- The first para is talking compltely about this. As we can see, there were changes in nutrient, temperature and everything mentioned in para. Note that isthmus didn't form by that time. Everything after the line - Yet.... are useful to evaluate this. 
 Eliminate A

B. The divergence in ocean temperature during the formation 
of the Pan-American isthmus resulted in a divergence in 
the ocean’s nutrient value. --- As the  seabed rose , Pacific 
waters grew cooler, their up-swelling currents carrying rich 
nutrients,
 Eliminate B

C. Genetic differences among pistol shrimp have grown to the 
point that there are now at least two distinct species of 
these shrimp. --- The isthmus cleaved populations of 
marine organisms, creating sister species. These twin species,....
...that Pacific pistol shrimp no longer mate 
with those from the Atlantic Ocean...
 Eliminate C

D. The part of ocean which is now the Pacific grew deeper 
due to the geologic forces that created the Pan-American 
isthmus. --- wait a minute.. As  the seabed rose , Pacific 
waters grew cooler, their up-swelling currents

E. Not until the Pan-American isthmus was fully formed did 
geminate marine organisms begin to develop in that area
of the ocean. --- 
The isthmus cleaved populations of 
marine organisms, creating sister species

In fact, it was these new conditions, and not so 
much the fully-formed isthmus, that spawned changes in the 
shrimp population
 Eliminate E

daboo343 · Answer

Could any expert please explain how the answer of third question is A

neetis5 · Answer

Hey Daboo, I have quoted below (and underlined relevant portions) a suitable response from the previous replies (gmatter0913). Please do go through it (once more if you have already), and let me know if you are not clear with respect to any particular portion: Cheers!

thinkvision · Answer

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

A. Species of marine organisms in the Atlantic Ocean number fewer today than before the formation of the Pan-American isthmus.

We can’t tell about the overall number of species as we don’t know how many species each region had before the migration.

Also, we don’t know about the extinction of any old / birth of any new species since the exchange, this answer can’t be correct.

B. The number of terrestrial animal species in South America today exceeds the number prior to the formation of the Pan-American isthmus.

We can’t tell about the overall number of species as we don’t know how many species each region had before the migration.

Also, we don’t know about the extinction of any old / birth of any new species since the exchange, this answer can’t be correct.

C. Of the indigenous North American species that migrated south across the Pan-American isthmus, more than three survive to this day.

The correct answer is C.

The second paragraph provides ample support for this inference. The author states that the terrestrial species migrating south were “better colonizers” than the ones migrating north, that more than half of those in the south today came from the north, and that only three animal species migrating north across the isthmus survive today. It is readily inferable, then, that more than three species that migrated south across the isthmus survive today.

D. Since the formation of the Pan-American isthmus, fewer terrestrial animals have traveled north across the isthmus than south.

We have the information about the number of animals that survived but not the number of animals that travelled. It may be that one million animals travelled and only three survided.

E. As the Pan-American isthmus began to form, most (avoid superlative … surely incorrect) pistol shrimp migrated west to what is now the Pacific Ocean.

Rickooreoisb · Answer

For Question 1,

A. Population divergences were more a process than an event
Supported P1 stresses gradual environmental change before full isthmus.

B. Ocean temperature divergence led to nutrient divergence
I am not sure of this, the passage states the qualities of pacific and altantic, no where is it mentioned that temperature divergence "LED TO" nutrient divergence

C. Genetic differences among pistol shrimp now define at least two species
Supported : shrimp no longer mate their sister species.

D. Pacific grew deeper due to geologic forces
Supported : there is a comparison given where one is hot, another is cold, etc. so when atlantic is shallow, pacific is deeper. See the comparitive form of adjective used, "deepER", given atlantic is shallow, pacific is definitely deeper, otherwise what is the point of stating atlantic is shallow in comparison with pacific.

Not until the isthmus was fully formed did geminates begin to develop
Contradicted : passage says changes occurred before full formation.

I choose E.

WiziusCareers1 · Answer

Question 1: Which statement finds the LEAST support?

Correct Answer: D
 The Logic:  This is a contradiction based on physical geography described in the text.
 The Evidence:  The passage states, "As the seabed rose, Pacific waters grew cooler." If the seabed is rising, the ocean depth above it is decreasing (becoming shallower).
 Why (D) fails:  Statement (D) claims the Pacific grew deeper. There is no evidence in the text to support an increase in depth; in fact, the mention of the rising seabed implies the opposite.

Question 2: Which statement is most readily inferable?

Correct Answer: C
 The Logic:  This is a quantitative inference. You must compare the migration numbers for the North and South.
 The Evidence: 
The text says "more than half of South America’s mammals" trace their lineage to the North American colonizers. This implies a huge number of successful species survived in the South.
In contrast, the text says "Only three animals" (armadillo, opossum, and hedgehog) from the South survive as transplants in the North.
 Why (C) is correct:  Since "more than half of South America's mammals" originated in the North and still survive there today, that number is clearly greater than three. Therefore, it is highly inferable that more than three indigenous North American species survive today in their new southern habitat.

Question 3: Why does the author mention the mating habits of pistol shrimp?

Correct Answer: A
 The Logic:  This is an Author’s Purpose question. The shrimp are an example used to illustrate the result of "gemination."
 The Evidence:  The author defines "geminates" as twin species that "evolved independently." To prove they have truly become different species, the author points out that they "no longer mate with those from the Atlantic Ocean."
 Why (A) is correct:  The inability to interbreed is the biological definition of becoming entirely distinct species. The author mentions the mating habits specifically to show that the evolutionary process triggered by the rising seabed has reached completion, resulting in two separate species.

kartickdey · Answer

Bunuel ,  Sajjad1994 , 
Q3: In the option A, I think, the word "entirely" is too strong. That is why I choose option D. Please clarify.

kartickdey · Answer

Bunuel ,  Sajjad1994 
Q1: both the option D and E appear correct to me. Please clarify which one is better.

Sajjad1994 · Answer

Question 1 and 3 both have debatable answers and i suggest not to bother for it.

Question is archived.

The Pan-American land bridge, or isthmus, connecting North and South

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