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RC Passage
Direction for the questions 6 to 9: The passage below is accompanied by a set of four questions. Choose the best answer to each question.
Grove snails as a whole are distributed all over Europe, but a specific variety of the snail, with a distinctive white-lipped shell, is found exclusively in Ireland and in the Pyrenees mountains that lie on the border between France and Spain. The researchers sampled a total of 423 snail specimens from 36 sites distributed across Europe, with an emphasis on gathering large numbers of the white-lipped variety. When they sequenced genes from the mitochondrial DNA of each of these snails and used algorithms to analyze the genetic diversity between them, they found that a distinct lineage (the snails with the white-lipped shells) was indeed endemic to the two very specific and distant places in question.
Explaining this is tricky. Previously, some had speculated that the strange distributions of creatures such as the white-lipped grove snails could be explained by convergent evolutionâin which two populations evolve the same trait by coincidenceâbut the underlying genetic similarities between the two groups rules that out. Alternately, some scientists had suggested that the white-lipped variety had simply spread over the whole continent, then been wiped out everywhere besides Ireland and the Pyrenees, but the researchers say their sampling and subsequent DNA analysis eliminate that possibility too. âIf the snails naturally colonized Ireland, you would expect to find some of the same genetic type in other areas of Europe, especially Britain. We just donât find them,â Davidson, the lead author, said in a press statement.
Moreover, if theyâd gradually spread across the continent, there would be some genetic variation within the white-lipped type, because evolution would introduce variety over the thousands of years it would have taken them to spread from the Pyrenees to Ireland. That variation doesnât exist, at least in the genes sampled. This means that rather than the organism gradually expanding its range, large populations instead were somehow moved en mass to the other location within the space of a few dozen generations, ensuring a lack of genetic variety.
âThere is a very clear pattern, which is difficult to explain except by involving humans,â Davidson said. Humans, after all, colonized Ireland roughly 9,000 years ago, and the oldest fossil evidence of grove snails in Ireland dates to roughly the same era. Additionally, there is archaeological evidence of early sea trade between the ancient peoples of Spain and Ireland via the Atlantic and even evidence that humans routinely ate these types of snails before the advent of agriculture, as their burnt shells have been found in Stone Age trash heaps.
The simplest explanation, then? Boats. These snails may have inadvertently traveled on the floor of the small, coast-hugging skiffs these early humans used for travel, or they may have been intentionally carried to Ireland by the seafarers as a food source. âThe highways of the past were rivers and the oceanâas the river that flanks the Pyrenees was an ancient trade route to the Atlantic, what weâre actually seeing might be the long lasting legacy of snails that hitched a rideâŠas humans travelled from the South of France to Ireland 8,000 years ago,â Davidson said.
Full RC Video Analysis
RC Line-wise Explanation
Paragraph 1
"Grove snails as a whole are distributed all over Europe, but a specific variety of the snail, with a distinctive white-lipped shell, is found exclusively in Ireland and in the Pyrenees mountains that lie on the border between France and Spain."
Explanation: While grove snails are found throughout Europe, a specific type with white-lipped shells is only found in Ireland and the Pyrenees mountains, located between France and Spain.
"The researchers sampled a total of 423 snail specimens from 36 sites distributed across Europe, with an emphasis on gathering large numbers of the white-lipped variety."
Explanation: Researchers collected 423 snails from 36 different European locations, focusing on gathering as many of the white-lipped snails as possible.
"When they sequenced genes from the mitochondrial DNA of each of these snails and used algorithms to analyze the genetic diversity between them, they found that a distinct lineage (the snails with the white-lipped shells) was indeed endemic to the two very specific and distant places in question."
Explanation: The researchers sequenced the mitochondrial DNA of these snails and analyzed the genetic diversity, confirming that the white-lipped snails form a distinct lineage exclusive to Ireland and the Pyrenees.
Paragraph 2
"Explaining this is tricky."
Explanation: Understanding this phenomenon is challenging.
"Previously, some had speculated that the strange distributions of creatures such as the white-lipped grove snails could be explained by convergent evolutionâin which two populations evolve the same trait by coincidenceâbut the underlying genetic similarities between the two groups rules that out."
Explanation: Some scientists had suggested that the strange distribution of the white-lipped snails could be due to convergent evolution, where unrelated species evolve similar traits by coincidence. However, the genetic similarities between the two groups rule this explanation out.
"Alternately, some scientists had suggested that the white-lipped variety had simply spread over the whole continent, then been wiped out everywhere besides Ireland and the Pyrenees, but the researchers say their sampling and subsequent DNA analysis eliminate that possibility too."
Explanation: Other scientists proposed that the white-lipped snails spread across Europe and were later wiped out except in Ireland and the Pyrenees, but the researchers' DNA analysis disproves this theory as well.
"If the snails naturally colonized Ireland, you would expect to find some of the same genetic type in other areas of Europe, especially Britain. We just donât find them,â Davidson, the lead author, said in a press statement."
Explanation: Lead researcher Davidson explained that if the snails had naturally colonized Ireland, some of their genetic type would be found in other parts of Europe, especially Britain, but no such evidence was found.
Paragraph 3
"Moreover, if theyâd gradually spread across the continent, there would be some genetic variation within the white-lipped type, because evolution would introduce variety over the thousands of years it would have taken them to spread from the Pyrenees to Ireland."
Explanation: If the snails had spread slowly across Europe, there would be genetic variation among the white-lipped snails, because evolution would cause changes over the thousands of years it would have taken to reach Ireland from the Pyrenees.
"That variation doesnât exist, at least in the genes sampled."
Explanation: However, no such variation exists in the samples of genes collected by the researchers.
"This means that rather than the organism gradually expanding its range, large populations instead were somehow moved en mass to the other location within the space of a few dozen generations, ensuring a lack of genetic variety."
Explanation: This suggests that rather than gradually spreading, large populations of snails were somehow moved together to the new location within a few dozen generations, preventing genetic diversity from developing.
Paragraph 4
"There is a very clear pattern, which is difficult to explain except by involving humans,â Davidson said."
Explanation: Davidson pointed out that the pattern of distribution is clear and hard to explain without involving humans.
"Humans, after all, colonized Ireland roughly 9,000 years ago, and the oldest fossil evidence of grove snails in Ireland dates to roughly the same era."
Explanation: Humans colonized Ireland around 9,000 years ago, and the oldest grove snail fossils in Ireland also date from around that time.
"Additionally, there is archaeological evidence of early sea trade between the ancient peoples of Spain and Ireland via the Atlantic and even evidence that humans routinely ate these types of snails before the advent of agriculture, as their burnt shells have been found in Stone Age trash heaps."
Explanation: Archaeological evidence shows that early sea trade existed between Spain and Ireland via the Atlantic, and that humans ate these snails before agriculture, with their burnt shells found in Stone Age refuse piles.
Paragraph 5
"The simplest explanation, then? Boats."
Explanation: The simplest explanation for how the snails traveled between Ireland and the Pyrenees is that they were transported by boats.
"These snails may have inadvertently traveled on the floor of the small, coast-hugging skiffs these early humans used for travel, or they may have been intentionally carried to Ireland by the seafarers as a food source."
Explanation: The snails could have unintentionally traveled on the small boats that early humans used for travel, or they might have been deliberately brought to Ireland as a food source.
"The highways of the past were rivers and the oceanâas the river that flanks the Pyrenees was an ancient trade route to the Atlantic, what weâre actually seeing might be the long lasting legacy of snails that hitched a rideâŠas humans travelled from the South of France to Ireland 8,000 years ago,â Davidson said."
Explanation: Davidson suggested that rivers and oceans were the main routes for travel in ancient times. The snails may have traveled with humans as they moved from the South of France to Ireland 8,000 years ago.
RC Paragraph Explanation
Paragraph 1 Summary
A specific variety of grove snail with a white-lipped shell is found only in Ireland and the Pyrenees. Researchers analyzed 423 snails and confirmed that the white-lipped variety is genetically distinct and endemic to these two locations.
Paragraph 2 Summary
Various theories, such as convergent evolution or the spread and subsequent disappearance of the white-lipped snails, were tested and ruled out. The researchers found no evidence supporting these ideas.
Paragraph 3 Summary
The absence of genetic variation in the white-lipped snails suggests that rather than spreading gradually, large populations of the snails were moved en masse to Ireland and the Pyrenees in a short time span.
Paragraph 4 Summary
Davidson suggests that the pattern of snail distribution can only be explained by human involvement, pointing to evidence that humans colonized Ireland around the same time the snails appeared and that these snails were likely carried by early humans.
Paragraph 5 Summary
The most plausible explanation is that the snails were transported by boats, either unintentionally on small boats used for travel or deliberately as a food source by ancient seafarers traveling from France to Ireland.
RC Quick Table Summary
| Paragraph Number | Main Idea |
|---|---|
| Paragraph 1 | A distinct white-lipped variety of grove snail is found only in Ireland and the Pyrenees, confirmed by genetic analysis. |
| Paragraph 2 | Theories like convergent evolution or the spread and disappearance of snails were tested and rejected by the researchers. |
| Paragraph 3 | The lack of genetic variation suggests that the snails were moved quickly over a short time, rather than gradually spreading. |
| Paragraph 4 | Davidson argues that the distribution pattern can only be explained by human involvement, with humans colonizing Ireland at the same time as the snails appeared. |
| Paragraph 5 | The snails were likely transported by boats, either unintentionally or intentionally, by early humans traveling between France and Ireland. |
RC Questions
Ques 6. All of the following evidence supports the passageâs explanation of sea travel/trade EXCEPT:
Ques 7. : In paragraph 4, the evidence that âhumans routinely ate these types of snails before the advent of agricultureâ can be used to conclude that:
Ques 8. Which one of the following makes the author eliminate convergent evolution as a probable explanation for why white-lipped grove snails are found in Ireland and the Pyrenees?
Ques 9. The passage outlines several hypotheses and evidence related to white-lipped grove snails to arrive at the most convincing explanation for:
Option B makes another mistake of stating these snails migrated from Pyrenees to Ireland, whereas these snails are present in both the locations. This is a factual inaccuracy. Hence, Option C is the correct answer.
