PrepTest 135, Section 1, Question 12
Populations of a shrimp species at eleven different(40 more words not shown)
Which one of the following,(14 more words not shown)
- AThe genetic differences(17 more words not shown)
- BThe individual shrimp(25 more words not shown)
- CBefore breeding, shrimp(15 more words not shown)
- DMost shrimp hatched(20 more words not shown)
- EOcean currents probably(24 more words not shown)
Reading the argument
If the currents keep mixing the shrimp, why aren't their populations becoming genetically alike?
Picture a baby shrimp moving from Reef A to Reef B. If it grows up and breeds at Reef B, its genes can enter that population. Repeat that movement often enough, and we can see why the author expects the reefs' populations to become harder to distinguish. Yet substantial differences remain.
Maybe the arriving babies don't survive to breed. They could be eaten before they contribute any offspring. Or perhaps the number arriving is tiny compared with the population already there. Five newcomers among a billion shrimp might have very little effect. Those are possible explanations to look for, not facts we've established.
What we’re looking for
The question asks us to resolve the puzzle. We can accept both the travel between reefs and the genetic differences. We need to understand why travel isn't producing the expected mixing through reproduction.
Working the answers
- A
Comparing shrimp with other marine species doesn't explain the differences between these shrimp populations. Shrimp might be far more different from turtles than from each other, and we'd still wonder why the currents haven't reduced the differences from reef to reef.
- B
Variation among individuals within one reef isn't the same as variation between the reefs' populations. Two groups could each contain genetically varied individuals while still having different overall genetic patterns. B reminds us that interbreeding needn't make individuals identical, but it doesn't explain why exchange between reefs leaves their populations substantially different.
- CCorrect
The shrimp return to their home reef before breeding. So a shrimp carried from Reef A to Reef B doesn't necessarily contribute offspring at B; it goes back to A first. The movement is real, but the reproductive mixing we expected doesn't follow.
This takes a different route from my guesses about survival and small numbers. The shrimp can survive the journey and still fail to mix the populations because they breed back where they started.
- D
Leaving the birth reef by the time they are old enough to breed doesn't explain why the populations remain genetically distinct. We still need to know where they actually breed. D gives us movement without the crucial return-before-breeding fact in C.
- E
Some of the travel goes into the open ocean rather than to another reef. But how much exchange still happens between reefs, and how much reproduction follows? “Many” shrimp going elsewhere doesn't tell us that the remaining exchange is too small to mix the populations. It isn't necessary for every traveling shrimp to die; we just need information that actually explains why the relevant genetic exchange stays limited. C supplies a direct explanation.
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