Recently biologists have been interested in a tide-associated periodic behavior displayed by the diatom Hantzschia virgata, a microscopic golden-brown...
GMAT Reading Comprehension : (RC) Questions
Recently biologists have been interested in a tide-associated periodic behavior displayed by the diatom Hantzschia virgata, a microscopic golden-brown alga that inhabits that portion of a shoreline washed by tides (the intertidal zone). Diatoms of this species, sometimes called "commuter" diatoms, remain burrowed in the sand during high tide, and emerge on the sand surface during the daytime low tide. Just before the sand is inundated by the rising tide, the diatoms burrow again. Some scientists hypothesize that commuter diatoms know that it is low tide because they sense an environmental change, such as an alteration in temperature or a change in pressure caused by tidal movement. However, when diatoms are observed under constant conditions in a laboratory, they still display periodic behavior, continuing to burrow on schedule for several weeks. This indicates that commuter diatoms, rather than relying on environmental cues to keep time, possess an internal pacemaker or biological clock that enables them to anticipate periodic changes in the environment. A commuter diatom has an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away.
This is not to suggest that the period of this biological clock is immutably fixed. Biologists have concluded that even though a diatom does not rely on the environment to keep time, environmental factors—including changes in the tide's hydrostatic pressure, salinity, mechanical agitation, and temperature—can alter the period of its biological clock according to changes in the tidal cycle. In short, the relation between an organism's biological clock and its environment is similar to that between a wristwatch and its owner: the owner cannot make the watch run faster or slower, but can reset the hands. However, this relation is complicated in intertidal dwellers such as commuter diatoms by the fact that these organisms are exposed to the solar-day cycle as well as to the tidal cycle, and sometimes display both solar-day and tidal periods in a single behavior. Commuter diatoms, for example, emerge only during those low tides that occur during the day.
The passage suggests which of the following about the accuracy of the commuter diatom's biological clock?
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Recently biologists have been interested in a tide-associated periodic behavior displayed by the diatom Hantzschia virgata, a microscopic golden-brown alga that inhabits that portion of a shoreline washed by tides (the intertidal zone). | What it says: Scientists are studying a tiny sea organism that lives in areas covered and uncovered by tides, and this organism does something in a regular pattern related to tides. What it does: Introduces the main subject and sets up the scientific context Source/Type: Factual statement about scientific research Connection to Previous Sentences: First sentence - establishes the foundation Visualization: Imagine a beach where tides come in every 12 hours. In the sand live millions of microscopic organisms smaller than the period at the end of this sentence. What We Know So Far: There's a tiny organism that lives in tidal zones and has some tide-related behavior What We Don't Know Yet: What exactly does this behavior look like? Why is it interesting? |
Question Analysis:
The question asks what the passage suggests about the accuracy of the commuter diatom's biological clock. This is asking us to make an inference about the precision or reliability of their internal timing mechanism based on information provided in the passage.
Connecting to Our Passage Analysis:
From our passage analysis, several key points relate to the accuracy of the biological clock:
- Evolutionary pressure creates accuracy: The passage states that diatoms have "an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away."
- Survival depends on precision: The analysis showed that natural selection over time would favor diatoms with more accurate timing - those with poor timing get swept away and cannot reproduce.
- Environmental pressure as driving force: The harsh tidal environment creates a life-or-death situation where timing accuracy directly determines survival.
Prethinking:
The passage clearly establishes a cause-and-effect relationship: intense environmental pressures → highly accurate biological clock. The logic is that in harsh environments where timing mistakes are fatal, only organisms with very accurate internal clocks survive to reproduce. This suggests that species facing more intense environmental pressures would develop more accurate biological clocks than species in less demanding environments.
Why It's Wrong:
- The passage states environmental factors can alter the clock's period, but this refers to resetting/adjusting the clock, not changing its fundamental accuracy
- The watch analogy clarifies this: you can reset a watch but can't make it run faster or slower - the accuracy of the mechanism itself doesn't change
- Confuses the clock's adjustability with its precision
Common Student Mistakes:
- Does "environmental factors can alter the period" mean the clock becomes less accurate?
→ No, this means the clock can be reset to match environmental changes, like adjusting a watch for a new time zone - the watch's accuracy doesn't change - Are period adjustments the same as accuracy changes?
→ No, period = timing cycle length; accuracy = precision of the timing mechanism itself
Why It's Wrong:
- The passage doesn't compare commuter diatoms to other intertidal species
- Only discusses commuter diatoms specifically, not other organisms in tidal zones
- Makes an unsupported claim about rarity among intertidal species
Common Student Mistakes:
- Since the passage mentions "unusual accuracy," doesn't this mean it's rare among similar species?
→ "Unusual" here refers to the high degree of accuracy, not its rarity compared to other intertidal species - Can I infer information about other intertidal species?
→ No, the passage only provides information about commuter diatoms specifically
Why It's Right:
- Directly supported by the passage's explanation of evolutionary pressure
- Follows the logical chain: intense environmental pressure → need for precision → highly accurate biological clock
- Aligns with the passage's core argument about survival-driven accuracy
Key Evidence: "A commuter diatom has an unusually accurate biological clock, a consequence of the unrelenting environmental pressures to which it is subjected; any diatoms that do not burrow before the tide arrives are washed away."
Why It's Wrong:
- Misattributes the accuracy to tidal cycle consistency rather than environmental pressure intensity
- The passage emphasizes "unrelenting environmental pressures" and survival consequences, not tidal consistency
- Focuses on the wrong causal factor for the clock's accuracy
Common Student Mistakes:
- Don't consistent tides create more accurate biological clocks?
→ The passage emphasizes survival pressure (wash away if timing is wrong) rather than tidal consistency as the driving factor - Isn't regularity of tides important for accuracy?
→ While regular tides may help, the passage specifically credits "unrelenting environmental pressures" and survival consequences
Why It's Wrong:
- Contradicts the passage evidence - lab experiments showed diatoms maintain consistent timing under constant conditions
- The passage states diatoms "continuing to burrow on schedule for several weeks" in stable lab conditions
- Confuses what happens in variable vs. constant laboratory conditions
Common Student Mistakes:
- Don't laboratory conditions affect biological clock accuracy?
→ The passage describes constant lab conditions where timing remained consistent, not variable conditions - Would variable conditions cause fluctuations?
→ While logical, the passage doesn't provide evidence about variable lab conditions - only constant ones