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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.

QUESTION-19: Which of the following best sums up Ehrlich and Raven’s argument in their classic 1969 paper?
A. Ernst Mayr was wrong in identifying physical separation as the cause of species diversity.
B. Checkerspot butterflies in the 50-acre Jasper Ridge Preserve formed three groups that rarely interacted with each other.
C. While a factor, isolation was not as important to speciation as natural selection.
D. Gene flow is less common and more erratic than Mayr and his colleagues claimed

Answer: (C)
Explanation for the Question:

In this case, you need to identify the answer option which best sums up Ehrlich and Raven’s argument in their classic 1969 paper: keep your focus on this specific aspect.
Option A talks about Mary and not Ehrlich and Raven’s argument.
Option B highlights a fact and not the main argument.
Option C is the main point of their argument; this is the most essential feature they are talking about.
Option D again focuses on Mayr and that too in an incorrect manner. ‘Less common and more erratic’ point to frequency when gene flow is valid; again this is not mentioned in the passage.

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