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: Sep 26, 2024
  • 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

Option (1) is intricate as it fails to directly address the core issue – if Mayr was incorrect, what was the correct perspective? Ehrlich and Raven's argument is not clear. Similarly, (2) provides evidence but lacks a clear thesis statement. The ideal approach is to express your viewpoint on the topic directly and concisely in one sentence. Although (4) comes close by referencing "gene flow," the answer is (3) – as indicated in the third paragraph - where it is stated that "isolation and gene flow were less important to evolutionary divergence than natural selection." Therefore, the correct answer is (3).

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Question: 2

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

Updated On: Sep 26, 2024
  • 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

Ehrlich and Raven acknowledge in the third paragraph that gene flow contributes to evolutionary divergence, stating, "isolation and gene flow were less important to evolutionary divergence than natural selection," indicating that isolation and gene flow have some importance in evolutionary divergence. This point is also reiterated in the last sentence of the passage. Therefore, (2) is the correct choice. Additionally, (3) is supported by information in the first paragraph, mentioning the separation of populations over geologic scales of time. Furthermore, (4) finds support in the second paragraph, which describes three groups that rarely interacted despite their close proximity.

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Question: 3

The author discusses Mayr, Ehrlich and Raven to demonstrate that

Updated On: Sep 26, 2024
  • 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 does not suggest that evolution is a sensitive or controversial topic, so (1) is eliminated. The determination of whether Ehrlich and Raven's thesis superseded Mayr's is not made in the passage, ruling out (2). The merits of checkerspot butterflies are not discussed in the passage, so (4) is also eliminated. The passage focuses on Mayr, Ehrlich, and Raven in the context of theories of speciation, making (3) the correct answer.

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