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RC Passage
Direction for the questions 19 to 21: The passage below is accompanied by a set of three questions. Choose the best answer to each question.
Scientists have long recognised the incredible diversity within a species. But they thought it reflected evolutionary changes that unfolded imperceptibly, over millions of years. That divergence between populations within a species was enforced, according to Ernst Mayr, the great evolutionary biologist of the 1940s, when a population was separated from the rest of the species by a mountain range or a desert, preventing breeding across the divide over geologic scales of time. Without the separation, gene flow was relentless. But as the separation persisted, the isolated population grew apart and speciation occurred.
In the mid-1960s, the biologist Paul Ehrlich - author of The Population Bomb (1968) - and his Stanford University colleague Peter Raven challenged Mayr's ideas about speciation. They had studied checkerspot butterflies living in the Jasper Ridge Biological Preserve in California, and it soon became clear that they were not examining a single population. Through years of capturing, marking and then recapturing the butterflies, they were able to prove that within the population, spread over just 50 acres of suitable checkerspot habitat, there were three groups that rarely interacted despite their very close proximity.
Among other ideas, Ehrlich and Raven argued in a now classic paper from 1969 that gene flow was not as predictable and ubiquitous as Mayr and his cohort maintained, and thus evolutionary divergence between neighbouring groups in a population was probably common. They also asserted that isolation and gene flow were less important to evolutionary divergence than natural selection (when factors such as mate choice, weather, disease or predation cause better-adapted individuals to survive and pass on their successful genetic traits). For example, Ehrlich and Raven suggested that, without the force of natural selection, an isolated population would remain unchanged and that, in other scenarios, natural selection could be strong enough to overpower gene flow.
Full RC Video Analysis
RC Line-wise Explanation
Paragraph 1
"Scientists have long recognised the incredible diversity within a species."
Explanation: Scientists have known for a long time that members of the same species can be very different from each other.
"But they thought it reflected evolutionary changes that unfolded imperceptibly, over millions of years."
Explanation: They believed that this diversity was the result of very slow evolutionary changes happening over millions of years.
"That divergence between populations within a species was enforced, according to Ernst Mayr, the great evolutionary biologist of the 1940s, when a population was separated from the rest of the species by a mountain range or a desert, preventing breeding across the divide over geologic scales of time."
Explanation: According to Ernst Mayr, a leading biologist in the 1940s, such variation arose when a group of organisms was cut off from the rest of their species by physical barriers like mountains or deserts, stopping them from interbreeding for a very long time.
"Without the separation, gene flow was relentless."
Explanation: If there were no barriers, then genes would continue to mix freely among all groups in the species.
"But as the separation persisted, the isolated population grew apart and speciation occurred."
Explanation: However, over time, the separated group would develop distinct characteristics, eventually becoming a new species.
Paragraph 2
"In the mid-1960s, the biologist Paul Ehrlich - author of The Population Bomb (1968) - and his Stanford University colleague Peter Raven challenged Mayr's ideas about speciation."
Explanation: In the 1960s, scientists Paul Ehrlich and Peter Raven questioned Ernst Mayrâs views on how new species form.
"They had studied checkerspot butterflies living in the Jasper Ridge Biological Preserve in California, and it soon became clear that they were not examining a single population."
Explanation: They were observing checkerspot butterflies in a preserve in California and realized these butterflies didnât belong to just one single, unified population.
"Through years of capturing, marking and then recapturing the butterflies, they were able to prove that within the population, spread over just 50 acres of suitable checkerspot habitat, there were three groups that rarely interacted despite their very close proximity."
Explanation: By studying the butterflies over many yearsâtagging and tracking themâthey discovered that even in a small area of 50 acres, there were three distinct groups that almost never mixed with one another.
Paragraph 3
"Among other ideas, Ehrlich and Raven argued in a now classic paper from 1969 that gene flow was not as predictable and ubiquitous as Mayr and his cohort maintained, and thus evolutionary divergence between neighbouring groups in a population was probably common."
Explanation: Ehrlich and Raven wrote a famous paper in 1969 where they argued that gene flow (genetic mixing) wasnât always happening predictably or everywhere, as Mayr believed, and so it was likely that nearby groups in the same population could still evolve differently.
"They also asserted that isolation and gene flow were less important to evolutionary divergence than natural selection (when factors such as mate choice, weather, disease or predation cause better-adapted individuals to survive and pass on their successful genetic traits)."
Explanation: They also claimed that natural selectionâsurvival of the fittest due to environment and other pressuresâwas more crucial for species evolution than just being isolated or having gene flow.
"For example, Ehrlich and Raven suggested that, without the force of natural selection, an isolated population would remain unchanged and that, in other scenarios, natural selection could be strong enough to overpower gene flow."
Explanation: They gave examples to show that if natural selection wasnât at work, even isolated groups wouldnât evolve much. And in some cases, natural selection could be so powerful that it could cause changes even when gene flow was happening.
RC Paragraph Explanation
Paragraph 1 Summary
This paragraph introduces the traditional belief in evolutionary biology that diversity within a species arises very slowly over millions of years, especially when populations are physically separated by barriers. Ernst Mayrâs theory emphasized that such separation was essential for new species to form.
Paragraph 2 Summary
This paragraph describes how biologists Ehrlich and Raven discovered, through a detailed study of checkerspot butterflies, that even populations living in close proximity (within 50 acres) could remain genetically separate, challenging the idea that physical separation was always necessary for speciation.
Paragraph 3 Summary
Ehrlich and Raven proposed that natural selection plays a more significant role in evolutionary divergence than gene flow or isolation. They argued that gene flow is not always consistent and that natural selection alone could drive the formation of new traits or species even without geographic separation.
RC Quick Table Summary
| Paragraph Number | Main Idea |
|---|---|
| Paragraph 1 | Traditional view: species diversity needs geographic separation and time. |
| Paragraph 2 | Ehrlich and Raven found isolated butterfly groups in close proximity. |
| Paragraph 3 | Natural selection may drive divergence more than gene flow or isolation. |
RC Questions
Ques 19. Which of the following best sums up Ehrlich and Ravenâs argument in their classic 1969 paper?
Ques 20. All of the following statements are true according to the passage EXCEPT
Ques 21. The author discusses Mayr, Ehrlich and Raven to demonstrate that
