Comprehension
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 neighboring 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...
Question: 1

Which of the following best sums up Ehrlich and Raven's argument in their classic 1969 paper?

Updated On: Jul 31, 2025
  • Ernst Mayr was wrong in identifying physical separation as the cause of species diversity
  • Checkerspot butterflies in the 50-acre Jasper Ridge Preserve formed three groups that rarely interacted with each other
  • While a factor, isolation was not as important to speciation as natural selection
  • Gene flow is less common and more erratic than Mayr and his colleagues claimed.
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The Correct Option is C

Solution and Explanation

The argument presented by Ehrlich and Raven in their classic 1969 paper is comprehensively summarized by focusing on the relative importance of different evolutionary mechanisms. The prevailing thought, led by Ernst Mayr, attributed considerable importance to physical separation (isolation) as a crucial driver of speciation, suggesting that this separation reduces gene flow and allows speciation to occur over time. However, Ehrlich and Raven challenged this view based on their study of checkerspot butterflies. They observed that even within a limited geographic area, distinct groups existed that rarely interacted, indicating that separation wasn't the only mechanism in play.
According to their findings, while isolation contributes to evolutionary divergence, it is not as significant as natural selection in driving speciation. Factors such as mate choice, weather, disease, or predation, which influence natural selection, were argued to be more critical. Therefore, the correct answer is:
While a factor, isolation was not as important to speciation as natural selection.
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Question: 2

All of the following statements are true according to the passage EXCEPT

Updated On: Jul 31, 2025
  • Gene flow contributes to evolutionary divergence
  • The Population Bomb questioned dominant ideas about species diversity
  • Evolutionary changes unfold imperceptibly over time.
  • Checkerspot butterflies are known to exhibit speciation while living in close proximity
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The Correct Option is B

Solution and Explanation

To determine which statement is NOT true according to the passage, we need to evaluate each option against the information provided.
  • Gene flow contributes to evolutionary divergence: The passage states that Ehrlich and Raven argued gene flow was not as predictable and ubiquitous as previously thought, implying gene flow does play a role in divergence. This statement is true.
  • The Population Bomb questioned dominant ideas about species diversity: The passage describes that Ehrlich and Raven challenged Mayr's ideas about speciation in The Population Bomb, related to gene flow and evolutionary mechanisms. Thus, The Population Bomb addressed ideas about speciation, but not directly about species diversity. This statement is NOT supported by the passage.
  • Evolutionary changes unfold imperceptibly over time: The passage indicates scientists initially believed evolutionary changes were imperceptible over millions of years. This statement aligns with the passage.
  • Checkerspot butterflies are known to exhibit speciation while living in close proximity: The research on checkerspot butterflies showed distinct groups rarely interacted despite geographical proximity, supporting the idea of speciation. This statement is true.
Based on this analysis, the statement that does not directly align with the passage and thus is not true according to it is:
The Population Bomb questioned dominant ideas about species diversity
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Question: 3

The author discusses Mayr, Ehrlich and Raven to demonstrate that

Updated On: Jul 31, 2025
  • evolution is a sensitive and controversial topic
  • Ehrlich and Raven's ideas about evolutionary divergence are widely accepted by scientists.
  • the causes of speciation are debated by scientists
  • checkerspot butterflies offer the best example of Ehrlich and Raven's ideas about speciation
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The Correct Option is C

Solution and Explanation

The passage discusses the different perspectives held by scientists on the topic of speciation, specifically between Ernst Mayr and the team of Paul Ehrlich and Peter Raven. Mayr proposed that speciation results from populations becoming geographically isolated, which over time leads to evolution into distinct species due to lack of gene flow. On the other hand, Ehrlich and Raven challenged this, suggesting that gene flow isn't as predominant as Mayr believed and that evolutionary divergence can occur even in neighboring populations, primarily driven by natural selection rather than isolation. They pointed out that within a seemingly singular population, distinct groups can form with minimal interaction, indicating there's more to speciation than just geographical separation. Their findings on checkerspot butterflies highlighted the potential for speciation without the factors Mayr emphasized, suggesting ongoing debate among scientists regarding the causes and mechanisms of speciation. Thus, the passage illustrates that the causes of speciation are debated by scientists.
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