Explanation
Correct answer: B.
Choice B is the best answer because it correctly applies the model presented in Text 1 to explain the different outcomes in the two tanks described in Text 2. Text 1 begins by explaining that fertile female teleost fish will sometimes become fertile males through protogyny. The model, or explanation, of protogyny advanced in Text 1 holds that among female teleost fish, size doesn’t confer any reproductive advantage, but among male teleost fish, “only individuals comparatively large enough to defend territory reproduce”—in other words, there is a direct relationship between relative body size and reproductive success, but only among male fish. Text 1 concludes by saying that, according to this model, individuals can maximize their reproductive success by “staying female when small” (when size has no relevance to reproduction) and only becoming male when they reach a certain size threshold by becoming large enough relative to other male fish to be able to defend territory. In sum, according to the model, whether protogyny is advantageous depends on a female’s body size relative to the males currently present. Text 2 describes an experiment in which fish of “typical size distributions” (meaning, size distributions consistent with Text 1’s note about the tendency of males to be larger than females) were placed in two tanks. Applying Text 1’s model, once the males (that is, the typically larger fish) were removed from tank 1, the remaining females would no longer be at a size disadvantage and could potentially defend territory as males at their current size, making sex change advantageous for some of the fish in tank 1. This would therefore explain the finding that 81 percent of female fish began changing into males. In tank 2, however, the continued presence of larger males meant that on average, females would not be large enough to outcompete the males for territory and would therefore have greater reproductive success by remaining female. In this way, the size-advantage model presented in Text 1 would also account for why no fish changed sex in tank 2.
Why the other choices are wrong
Choice A
Choice A is incorrect because it claims that “even small females” could succeed as males once territories were undefended, contradicting Text 1’s emphasis on there being a “critical size threshold” for male reproductive success, wherein only male fish who were sufficiently large to defend territory would have a reproductive advantage. Under this model, small females would have more reproductive success by remaining female than by becoming male and being unable to compete with larger fish for territory. Additionally, this choice’s claim about tank 2—that “only the largest females” could take over territories—implies either that females defend territory (when the model in Text 1 makes no such claim) or that at least some females were sufficiently large to change sex and compete for territory with fish that began the experiment as males, yet the results show that no females in tank 2 did so.
Choice C
Choice C is incorrect. Not only is it based on a flawed understanding of the model presented in Text 1, it flatly contradicts the experimental results summarized in Text 2. While Text 1 clearly states that body size is irrelevant to female reproductive success, choice C states that the presence of males in tank 2 made differences in body size relevant to the females’ reproductive success. This choice also claims that the removal of males from tank 1 eliminated any advantage to becoming male, yet the researchers reported that 81 percent of females in tank 1 began changing sex.
Choice D
Choice D is incorrect because it doesn’t use the model proposed in Text 1 to explain the findings described in Text 2. It is common sense that a certain number of male fish are required for reproduction to occur in tank 1, but the model presented by Text 1 focuses on the individual reproductive benefits of protogyny, not on its potential to benefit the population by maintaining a favorable sex ratio.