CAT 2017 Slot 1 — VARC Question 19

Mixed PracticeEasy

Passage / Data

Answer the following question based on the information given below.

Scientists have long recognized 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, while 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 Standford University colleagues 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 interacted despite their very clear 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…

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

Answer & solution

Correct answer: While a factor, isolation was not as important to speciation as natural selection.

A
Ernst Mayr was wrong in identifying physical separation as the cause of species diversity.
B
Checkerspot butterflies in the 50-acer 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.
D
Gene flow is less common and more erratic than Mayr and his colleagues claimed.

Solution

Easy

"Best sums up the argument" asks for the central claim of Ehrlich and Raven's 1969 paper, not a side detail or an observation that merely set it up. The third paragraph states their core thesis: gene flow was less predictable than Mayr held, AND isolation and gene flow were "less important to evolutionary divergence than natural selection." The best summary captures that comparative ranking. Eliminate options that are too strong, merely factual, or vague.

"Ernst Mayr was wrong in identifying physical separation as the cause." This overstates their position. Ehrlich and Raven did not say separation was no cause — they argued it was not the only or most important cause, ranking natural selection above it. A misrepresentation. Eliminate.

"Checkerspot butterflies formed three groups that rarely interacted." This is a true observation from the study, but it is the evidence, not the argument it was used to support. It does not "sum up" the paper's claim. Eliminate.

"While a factor, isolation was not as important to speciation as natural selection." This is exactly their central assertion — isolation and gene flow were "less important to evolutionary divergence than natural selection." It acknowledges isolation as a factor yet subordinates it, capturing the whole thesis. Correct.

"Gene flow is less common and more erratic than Mayr claimed." This captures only one half of their argument (the point about gene flow) and omits the headline claim that natural selection outweighs isolation. It is also vague about which population. A partial summary. Eliminate.

Option C — the paper's central argument is that isolation, though a factor, mattered less to speciation than natural selection did.

CAT 2017 Slot 1 — VARC Question 19

Answer & solution

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