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Biologist: Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas. Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time.
The biologist's conclusion follows logically from the above if which of the following is assumed?
| Text from Passage | Analysis |
| Species with broad geographic ranges probably tend to endure longer than species with narrow ranges. |
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| The broader a species' range, the more likely that species is to survive the extinction of populations in a few areas. |
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| Therefore, it is likely that the proportion of species with broad ranges tends to gradually increase with time. |
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The biologist starts with a general claim about survival rates, then explains the mechanism behind this pattern (geographic spread provides protection), and finally concludes what this means for species composition over time.
The proportion of species with broad geographic ranges will gradually increase over time.
The argument moves from survival advantage (broad range = longer survival) to future prediction (more broad-range species over time). The logic assumes that if broad-range species survive longer, they'll eventually make up a bigger portion of all species as narrow-range ones die off more frequently.
Assumption - We need to find what must be true for the biologist's conclusion to logically follow from the premises
The conclusion makes a specific claim about proportional change over time - that broad-range species will gradually increase as a percentage of total species
Look for gaps between the premises and conclusion. The premises tell us broad-range species survive longer, but the conclusion jumps to saying their proportion will increase over time. We need to identify what missing links would make this logical jump valid
This tells us about the current state - that there are now more broad-range species than narrow-range species. However, the conclusion is about a trend over time (gradual increase), not about current numbers. Even if narrow-range species currently outnumber broad-range species, the proportion of broad-range species could still be increasing gradually. This choice doesn't provide the logical link we need between survival advantage and proportional change over time.
This restates information already given in the premises about survival capabilities based on geographic range. The argument already tells us that broader ranges help species survive local extinctions. This choice doesn't bridge the gap between survival advantage and the conclusion about changing proportions over time. It's more of a restatement than a necessary assumption.
This addresses what happens to specific geographic areas after local extinctions, but the conclusion is about overall species proportions across time, not about repopulation of specific areas. Whether species repopulate particular areas doesn't affect the logical connection between survival advantage and increasing proportions of broad-range species overall.
This provides the crucial missing link in the argument. The premises establish that broad geographic range is a characteristic that helps species endure longer. The conclusion claims that the proportion of species with broad ranges will increase over time. For this logic to work, we need the general principle that characteristics helping species endure longer will indeed lead to proportional increases over time. This choice states exactly that principle, making it a necessary assumption for the argument to be valid.
This reverses the causal relationship presented in the argument. The passage states that broad ranges help species endure longer because they can survive local extinctions better. This choice suggests that anything helping species endure longer will make them better at surviving local extinctions. This gets the causality backwards and isn't necessary for the conclusion about changing proportions over time.