PSAT Writing and Language Practice Question 339
Question: 339
Gause's Law: It's a Competition
By the nineteenth and early twentieth centuries, scientists had observed many ecological 1 principles, including the principle of competition. In this principle, different organisms or species compete for a limited supply of a certain resource. Scientists posited that whenever two species are in competition, they will not coexist with constant population sizes. 2 To prevent going extinct, a species can either evolve or develop a new niche—the species's unique role in a certain environment. While this idea had been proposed earlier, biologist Georgy Gause was 3 born in Moscow, Russia, in 1910.
[1] In order to demonstrate this theory, Gause conducted a laboratory experiment. [2] He used two species of Paramecium (a type of microscopic organism) known as P. aurelia and P. caudatum. [3] To test the theory of competitive exclusion, Gause also combined the two species and observed what happened. [4] For the experiment, he created separate groups of each organism as control groups. [5] He provided the control and combined groups with the same amount of food and water and used a sample to determine the population of each species every day. [6] He wanted to see whether the two species would grow equally 4 well or whether, as the principle of competitive exclusion suggests, one group would become dominant and overtake the other. 5
6 The experiment contradicted scientists' expectations. In the control groups, the two species had similar populations to each other. However, in the group that was combined, 7 Gause saw no clear pattern: as the two species competed for resources, P. aurelia emerged as the dominant organism. Initially, the two species grew at a similar rate, but P. aurelia soon surpassed P. caudatum and approached the population levels seen in the individual groups. P. caudatum decreased in population, 8 but it later increased and caused the P. aurelia to go extinct.
Scientists continue to study competitive exclusion today, among plants, animals, and even humans, and Gause's experiments are credited for demonstrating this ecological idea experimentally. 9 Ecologists know that, at least under the controlled conditions of a laboratory, 10 when two species compete for the same resources, one will overtake the other one.
Which choice, if added here, would most effectively introduce the main topic of the paragraph?
Correct Answer: B
Explanation:
Note the question! The question asks which choice introduces the main topic of the paragraph, so it's testing consistency of ideas. Determine the subject of the paragraph and find the answer that is consistent with that idea. The paragraph discusses the results of Gause's experiment, which proved the validity of the theory of competitive exclusion. Eliminate (A) because Gause's experiment confirmed, not contradicted, the validity of the theory, which had been hypothesized previously by scientists. Keep (B) because it is consistent with the subject of the paragraph. Eliminate (C) because the importance of control groups to scientific experiments is not consistent with the results of Gause's experiment. Eliminate (D) because Gause's publication of his results does not convey the significance of his experiment. The correct answer is (B).