Q12 - Populations of shrimp species

[amt33]

Please explain this question to me. And why E isn't the correct answer.

[ManhattanPrepLSAT1]

Happy to help!

We need to explain why these shrimps that were investigated are genetically so different from each other. The genetically dissimilarity is surprising given that the strong ocean currents probably carry the baby shrimps between the reefs - so they should interbred, thereby eliminating the genetic dissimilarity. Answer choice (C) does this nicely by pointing out that even though the shrimps get carried to neighboring reefs, that's not where they breed. The idea of migrating right before breeding doesn't seem all that strange either; salmon migrate to spawn in the creeks and rivers inland from the ocean, and turtles I believe always return to the place where they were born to nest. So, even though the shrimp are being all mixed up on the reefs, they're not interbreeding with each other because they return back to where they were born when it's time.

Let's look at the incorrect answers:

(A) is out of scope. "Other marine animals" really shouldn't have anything to do with explaining why the shrimp are not interbreeding.
(B) misses the issue. The stimulus is about the differences between different populations of shrimp. This answer choice would only explain the differences amongst individuals.
(D) suggests that the shrimp are moving around, so why aren't they interbreeding? This only makes matters worse.
(E) is too weak. "Many" is equivalent to "some" and this answer choice would still leave the possibility that some do go to other reefs. Sure the ones who get swept out to into the open ocean won't be able to interbreed, but that's just some of the shrimp. What about the ones who aren't swept out to the ocean. This answer choice still leaves it unclear why they're not interbreeding.

Notice at this point if you were debating between answer choices (C) and (E) that answer choice (C) encompasses "shrimp of the species examined" whereas answer choice (E) encompasses "many of the baby shrimp". Answer choice (E) is just not inclusive enough.

Hope that helps!

[shirando21]

still not sure about B. I feel B is more like a premise booster which restates they are genetically different, but does not tell us why and does not solve the paradox.

For E, I think carrying baby shrimps to the open ocean does not tell us if they interbreed or not.

[james.h.meyers]

I got this one right but basically because I got lucky and went with (C). But, I still can't figure out why (B) is wrong. Is it because it attacks the evidence? I thought of it like this:

Reef 1: AA
Reef 2: AB
Reef 3: BB
...

So the shrimp at different reefs have different genes. The paradox is that you would think that they would all be the same (maybe all AB or BB) because currents move them around and shrimp from various reefs would interbreed. It seems that (B) eliminates the paradox. If the shrimp at a single reef come in all genetic forms, despite interbreeding, then that means that interbreeding does not make shrimp genetically indistinguishable, and eliminates the paradox.

[stol1989]

Hi james.h.meyers!

If the shrimp at a single reef come in all genetic forms, despite interbreeding, then that means that interbreeding does not make shrimp genetically indistinguishable, and eliminates the paradox.

Stimulus uses word "substantial" in regard to the extent of genetic differences between populations.

"B" just says that individual shrimp within a given population differ from one another genetically. How much do they differ we can not know from "B". Maybe it is just few of billions genes that differ from one individual shrimp to other.

We can't assume that the extent of variations WITHIN given population at any given Indonesian coral reef is the same with extent of variations BETWEEN populations.

There is a possible world where "B" can help to explain discrepancy, but in order to get there we need to make some assumptions that are not supported by AC.

On the other hand look at "C"! Argument says that ocean currents would ALLOW the population to interbreed. "C" says that shrimp don't use this opportunity and effect of currents is negated by shrimp behavior.

[maryadkins]

Good discussion here.

Yes, (B) is about genetic variation between individuals while we're talking about genetic differences between whole populations.

Here's an analogy. We can say that humans vary genetically from monkeys. We can also say that one human varies genetically from other humans. But does saying the latter explain why we vary genetically from monkeys? No. It's a different issue. That's like (B) here.

[jordansandel]

Here's the part that I don't understand:

In C the baby shrimp may be carried out to another reef, but at least they come back to their original reef before breeding. But if a survey were taken of the shrimp at each reef, there's no reason to believe that it would only count breeding-age shrimp. Wouldn't it include all of the baby shrimp that hadn't yet swum back to the reef where hatched to breed?

[rikky.brown]

I'm going to take a shot at this one even though I initially missed it on my timed PT. I just couldn't wrap my head around the stimulus. Jordansandel, I don't think that we need to assume that the results include "only breeding age shrimp." I think what (C) is saying is that the shrimp in this example all migrate back to where they were born before they breed. Therefore, it seems that these shrimp are all breeding with shrimp in their respective areas, not with the shrimp from different reefs, as the argument expects. That is why if we look at all the reef areas as a whole, there exists substantial genetic diversity. Whenever shrimp breed they're are only receiving genes from shrimp with similar genes, so the population as a whole never diversifies.

Let me see if I can use an oversimplified example to make it more clear. Let's assume the population consists of members of 2 coral reefs (instead of 11). Let's say members of the first reef only have genetic material A, and members of the second reef only have genetic material B. The paradox in the stimulus is that the ocean current pushes these groups together, therefore we should expect to find some shrimp with AB genes, as a result of the inbreeding (group 1's A gene + group 2's B gene = AB gene); yet we don't. If as (C) presents, group 1 and group 2 each return back to their respective native areas to breed, then group 1 is only breeding amongst itself, and group 2 is only breeding amongst itself (the A's are only getting A genes and the B's only get B genes). This resolves the paradox because it helps to explain why, although these groups have access to one another, their genetic materials remain distinct from one another, as opposed to looking the same.