Certain groups of Asian snails include both “left-handed” and “right-h

Hmm... I had no idea that snails could be right- or left- handed, seeing as they have no hands that I've ever noticed. On to your question, though:

As sakshamchhabra pointed out, the main reason to eliminate (D) is that it supports a conclusion opposed to the conclusion of the passage.

To answer the question, we need to strengthen the specific argument that "asymmetrical snake jaws helped drive left-handed snail evolution." Because the information in (D) introduces a competitive advantage of right-handed snails, it does not support the argument that " the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species." (D) is out.

Take another look at (E):

From the passage, we know that snail-eating snakes with asymmetrical jaws have more trouble grasping left-handed snails than right-handed snails. (E) goes a bit further, stating that the snakes fail more often when they try to eat left-handed snails. This strengthens the argument that "asymmetrical snake jaws helped drive left-handed snail evolution," because more left-handed snails survive snake attacks and live to create left-handed snail babies. (E) is the correct answer.

I hope that helps!
_________________

Breaking down our argument:
1. there are left and right snails; some left evolved from right
2. snakes have jaws adapted to right snails
3. (if) snakes ate more right snails (then) left would have had advantage (and) become new species
4. (therefore) snakes' jaws drove evolution of left snails

The logic is extremely straightforward so we can look for a likely answer (a Precise approach). In particular, to strengthen the argument we can show that the logical connections (marked by 'then' and therefore') are correct: the snakes did in fact eat more right-snails; the left-snails did in fact have an advantage (no other predators); there were sufficient snakes for this to be an issue; etc..

Skimming through our options, (E) is the only direct answer -- it states that snakes are bad at eating left-snails so they have an advantage.

I was initially down to A and E , but saw A is wrong for one very strong reason i.e.

Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws

option A is tying to put up the same in a little different fashion.

I was down to c and e. And the reason I didn't choose e because it stated "in this region snake could not eat left snail's then that would mean in some other they can.
So isn't this not a strengthening statement.

Posted from my mobile device

GMATNinja, gmat1393 plz explain why is D wrong and E correct ?

Greetings Alpha14,

While experts pour in their comments here are my 2 cents,

The question is asking us to find the option that would- most strengthen the argument that asymmetrical snake jaws helped drive left-handed snail evolution

In simpler words, we need to find an option that would justify how a small advantage(i.e snail-eating snakes had a hard time catching left-handed snails) helped drive left-handed snail evolution.

Option D says,

D. Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells.


Here, the option is contrary to what we want. The option is saying that right-handed snails already had an advantage and snakes had a hard time catching them.

Furthermore, another reason this option faults is we are not told whether the snakes ate the snails with/without shells and we don't care also.

Here is a small quote from the argument stem, option D is just trying to distort a fact presented in the question.

snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily

Please let me know if you have any questions, I would love to expand more on my explanantion

Understanding the passage

Certain groups of Asian snails include both “left-handed” and “right-handed” species, with shells coiling to the left and right, respectively.

• There are certain groups of Asian snails which include both species, left-handed and right-handed snails.
• Left-handed snails are snails with shells coiling to the left, and Right-handed snails are snails with shells coiling to the right.

Some left-handed species have evolved from right-handed ones.

• Some of the left-handed species of snails have evolved from the right-handed species.

Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily.

• Additionally, researchers found that a particular species of snake, which is the snail-eating snake, had asymmetrical jaws.
• The asymmetrical jaws in snail-eating snakes help them grasp right-handed snails shells more easily.
• Inference: It is less easy or difficult to grasp left-handed snails shells for snail-eating snakes with asymmetrical jaws.

If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species.

• Since snail-eating snakes find it easier to grasp right-handed snails shells, it would mean that catching right-handed snails is easier for these species of snakes.
• Therefore, this could mean that these snakes consume more right-handed snails over time, giving an evolutionary advantage to left-handed snails.
• This advantage for the left-handed snails can lead them to become a new species.

Thus, the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

• Therefore, the snakes’ asymmetrical jaws are likely responsible for the rise of left-handed snail species.

Conclusion: The snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

Pre-thinking

Strengthen Framework

What information will help us believe more in the conclusion?

Conclusion: The snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

Given that:
(i) Left-handed snails are snails with shells coiling to the left, and Right-handed snails are snails with shells coiling to the right.
(ii) Snail-eating snakes have asymmetrical jaws.
(iii) The asymmetrical jaws in snail-eating snakes help them grasp right-coiled snails shells more easily.
(iv) If snail-eating snakes consumed more right-coiled snails, then this could give an advantage to left-coiled snakes and can lead them to become a new species.

Thought Process

There are both Asian snails with shells that coil to the right and Asian snails with shells that coil to the left, and the latter have evolved from the former; furthermore, there are snakes that have asymmetrical jaws that allow the snakes to grasp the snails with right-coiled shells more easily.

Strengthener

Since we are told that if over time the snakes with asymmetrical jaws ate more snails with right-coiled shells than snails with left-coiled shells, then this would give snails with left-coiled shells an evolutionary survival advantage
A statement that indicates that the snakes with asymmetrical jaws, in fact, were more likely to have successfully eaten the snails with the right-coiled shells, then we would have good reason to think the snakes' asymmetrical jaws helped drive the emergence of snails with left-coiled shells.

Answer Choice Analysis

A. In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails. INCORRECT
• The fact that snakes with asymmetrical jaws eat snails and other snakes do not give any indication as to whether snails with left-coiled shells have any evolutionary advantages over snails with right-coiled shells. Thus, this choice is incorrect.

B. Some species of Asian snails contain either all right-handed snails, or all left-handed snails. INCORRECT
• The information in this statement is irrelevant to the conclusion because it does not tell us whether they have any evolutionary advantages over other snails, or whether the snakes' asymmetrical jaws had any effect on any such evolutionary advantages.
• Thus, this is not the correct choice.

C. Anatomical differences prevent left-handed snails from mating easily with right-handed snails. INCORRECT
• Mating of left-handed snails with right-handed snails will not have any impact on the conclusion of the passage. Thus, the information present in the statement is irrelevant, and this is not the correct choice.

D. Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells. INCORRECT
• This statement will weaken the conclusion.
• The fact that snakes cannot extract some snails with right-coiled shells from their shells would suggest that these snails might have an evolutionary advantage, but the author's conclusion is about an evolutionary advantage that snails with left-coiled shells possibly have, not an advantage that snails with right-coiled shells would have.

E. Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails. CORRECT
• The fact that experiments show that the snakes are more successful at eating snails with right-coiled shells than they are at eating snails with left-coiled shells would support the author’s conclusion. Therefore, this is the correct choice.

Cheers!

GMATNinja Could you please help explain why A and C are wrong ? Thanks for collaboration.

Hi

Let me try to address your query. Analyzing the stimulus, we find:

Conclusion: The snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

Premises on which it is based (why):

1) Left-handed species have evolved from right-handed ones.
2) Snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily.
3) If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species.

We are asked to find that which strengthens the argument. Let us consider options (A) and (C).

(A) In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails.

At first sight, this looks to be a tempting option. However, two factors make me cautious:
1) If we consider this to be new information, this does not explain which snails the asymmetrical jawed snakes prefer.
2) Importantly, we already know this from the stimulus, which states: " snail-eating snakes in the same habitat have asymmetrical jaws". Hence this does not further strengthen the argument presented in any way since this is already factored in to draw the conclusion.

We can hold on to this option, however.

(C) Anatomical differences prevent left-handed snails from mating easily with right-handed snails.

It also means that right handed snails can not mate with left handed snails. Therefore, it must me other factors which led to the evolution of left handed snails, since this option does not show any kind of preference towards them.

In the correct option (E): Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails.

While the premises presented in the stimulus are merely hypothetical, this option shows us that the hypothesis actually holds true in the real world, thereby strengthening the argument.

Hope this helps.

I was extremely tempted to choose C but didn't end up doing so. The reason is as follows:

RHS = Right Handed Snails
LHS = Left handed snails

From the passage, we know that snakes are able to to grasp RHS shells more easily. Why? because a unique jaw design enables these snakes to grasp RHS shells much easier.

We have been asked to strengthen the idea that this jaw design of snakes is the major factor that led to LHS evolution.

Let's take a look at the available answer choices:

A. In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails. - In other other words, this option lets us know that some snakes eat snails, while some don't. But how does knowing (A) as a fact help solidify the jaw design as the lead facilitator of LHS evolution? (A) does not provide any explanation for this. Hence, eliminate (A).

B. Some species of Asian snails contain either all right-handed snails, or all left-handed snails. - This option simply reiterates the existence of LHS and RHS species of Asian snails. The simple acknowledgement of the existence of such species does nothing to explain the hypothesized link between jaw design and evolution. Hence, eliminate (B)

C. Anatomical differences prevent left-handed snails from mating easily with right-handed snails. - On one side, we know that snakes are able to grasp RHS snails more easily than the LHS. So, the RHS numbers are expected to decline (assuming that the snake kills these RHS snails after grabbing them). On the other side, if we know that LHS cannot mate with RHS, we can make an inference that LHS snails will mate with its own kind more often and hence this could explain the increase in the # of LHS snails. I was pretty much sold on this answer until I realized an important point - evolution is NOT the same thing as INCREASE in population. Evolution tries to explain why (or how) certain features of animals/plants came into existence. (C) is irrelevant in this context. Hence, eliminate (C).

D. Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells. - "Some" could literally mean 'only a few' or it could also refer to 'a significant majority'. So, we are not entirely sure about the size of RHS population that is being effected. Moreover, How does (D) relate to the evolution of LHS? As you can see, there are many moving parts here that don't offer clear meaning. Hence, eliminate (D).

E. Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails. - (E) suggests that LHS snails had a better chance of survival than RHS snails. A better chance of survival is a natural advantage to evolution. Hence, (E) is the right answer.

Certain groups of Asian snails include both “left-handed” and “right-handed” species, with shells coiling to the left and right, respectively. Some left-handed species have evolved from right-handed ones. Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily. If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species. Thus, the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

Which of the following would, if true, most strengthen the argument that asymmetrical snake jaws helped drive left-handed snail evolution?

A. In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails.
-which snails are we talking about here? Left-handed? Right-handed?
-all this choice tells us is that snakes with symmetrical jaws didn’t evolve to eat snails…but it doesn’t give more credence to the idea that left-handed snails species’ emergence was driven by the snakes with asymmetrical jaws
B. Some species of Asian snails contain either all right-handed snails, or all left-handed snails.
-tempting trap choice, but a shallow answer that fails to capture anything of significance in the logic behind the passage
C. Anatomical differences prevent left-handed snails from mating easily with right-handed snails.
-mating is irrelevant
D. Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells.
-OK, this might even weaken the argument b/c it suggests that right-handed snails were not eaten as frequently…something that was key to the argument
E. Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails.

-Correct…the passage argument flows like so:
-snail-eating snakes have asymmetrical jaws
-such snakes allowed them to eat right-handed snail shells more easily
-left-handed snails got an evolutionary advantage
-since they avoided being eaten by snakes more often…they became a new species

-E gives us experimental/scientific evidence to support the claim that such snakes struggled to eat left-handed species…as a result this would indeed give the left-handed species an evolutionary advantage (natural selection anyone?) …; an advantage that could lead to their becoming a new species.
_________________

Dear DavidTutorexamPAL

GMATNinja, GMATNinjaTwo, VeritasKarishma, AnthonyRitz, CJAnish, MartyTargetTestPrep, AndrewN,
VeritasPrepBrian,fiftyoneverbal
GMATRockstar

I haven't yet gotten the E fulfill the connection between "then" and "therefore", for me E just restates the premise that If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species.. I don't think restatement can strengthen argument.

I thought C is better because if left hand snails cannot mate with right hand snails, then it is safe to infer that left hand snails can mate only with left hand snails, then more and more left hand snails emerge. so do right hand snails, but snakes will ate more right hand snails, (premise: Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily), that means snakes help the drive the left hand snails, so C strengthen the conclusion.

after reading explanation of DavidTutorexamPAL, I am curious whether he points out If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species. is not a fact, it is a hyphothesis, it can not be premise? so need E to turn into fact ?

Bikki says C is irrelevant, I did not get it. I don't think it is irrelevant.

CrackVerbalGMAT says in C other factors which led to the evolution of left handed snails, but I think it can be snake as the other factors.

jaisonsunny77 says evolution is NOT the same thing as INCREASE in population. but conclusion is snakes is the driven, so it doesn't matter the difference between evolution and increase in population.

I am lost

please, help.

thanks in advance.

You seem lost because you missed key points. In strenghten questions, your aim to strengthen conclusion. not strengthen in general.
ExampleC: it says differene prevent LHsnails with RH snails. But try to connect it with conclusion.
Conclusion: the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.
If we add C in the premise, then it has no relation with snakes. Thats why it is irrelevant.
No matter how good is this option. but if it is not related with conclusion, it is useless.
You aim is to strenghthen the conclusion based on information what you have .


On other side, A , B and D doesn't strengthen the conclusion.
You need to find an option that How snakes' asymmetrical jaws could contribute in the emergence of the left-handed snail species.

You think for a while. You may come up with some thought:
Maybe snakes don't eat LH snails over time. and in long terms all right hand snails are gone , thus could contribute in emergence of xx


E is saying the same thing. In experiments it is shown snail-eating snakes fail more often in trying to eat left-handed snails .

E option doesn't mean it would happen for sure . but it is an indication that how snake could play a role in emergency of LH snails.
'
Other options are not even near because they either don't consider snake as factor or they don't bring any affect on conclusion.

Hope it is clear.

Certain groups of Asian snails include both “left-handed” and “right-handed” species, with shells coiling to the left and right, respectively. Some left-handed species have evolved from right-handed ones. Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily. If these snakes ate more right-handed snails over time, this would have given left-handed snails an evolutionary advantage over right-handed snails, with the left-handed snails eventually becoming a new species. Thus, the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species.

P : Some Asian snails include left-handed and right-handed.
P : left handed evolved from right handed
P : Some snail-eating snakes have asymmetrical jaws, which is beneficial in eating right-handed snails.
C : snail-eating snakes have driven left-handed snails to emerge.

Any information supporting linkage between left-handed snails and snail eating snakes will support the argument.

Which of the following would, if true, most strengthen the argument that asymmetrical snake jaws helped drive left-handed snail evolution?

A. In one snake species, the snakes with asymmetrical jaws eat snails, while the snakes with symmetrical jaws do not eat snails.
-> This information only gives idea that snakes with asymmetrical jaws eat snails but does not provide linkage about left-handed snails. Moreover, this information does not give information whether snakes with asymmetrical jaws eat left-handed snails with ease.

B. Some species of Asian snails contain either all right-handed snails, or all left-handed snails.
-> This information is already given in the premise.

C. Anatomical differences prevent left-handed snails from mating easily with right-handed snails.
-> This information is about mating not predation.

D. Some right-handed snails in this habitat have shells with a very narrow opening that helps prevent snakes from extracting the snails from inside their shells.
-> This information gives linkage between right-handed snails and snail-eating snakes but not about left-handed snails.

E. Experiments show that the snail-eating snakes in this habitat fail more often in trying to eat left-handed snails than in trying to eat right-handed snails.
-> Correct. This information claims that same notion with the argument's conclusion implying that left-handed snails would have had benefit in surviving.

I thought C is better because if left hand snails cannot mate with right hand snails, then it is safe to infer that left hand snails can mate only with left hand snails, then more and more left hand snails emerge. so do right hand snails, but snakes will ate more right hand snails, (premise: Also, researchers found that snail-eating snakes in the same habitat have asymmetrical jaws, allowing them to grasp right-handed snail shells more easily), that means snakes help the drive the left hand snails, so C strengthen the conclusion.


zoezhuyan

I think you have made a lot of assumptions in C to support the argument. We can assume things if they are supported by the passage or universally true.
First assumption - We can't assume that mating btw left hand snails will produce only left hand only (I googled to understand their reproduction system and found 2 left hand snails can also produce right hand snails. Weird. Right). Learning - Don't assume reproduction process is similar to humans
Second assumption - We don't know the number of snails produces by right hand and left hand species. May be right hand species are very large in number. In that case, even if snakes will eat more right hand snails, it does not affect their population.
The point is we need to make no assumption to strengthen the argument by E.

Rejecting an option just because it restates the conclusion is not a reliable method. IMO, the impact/indication of options on the conclusion helps to identify the correct option.

i am interested in option E since it adds additional information to the statement than others

I am a bit confused as to how the author can discuss at first just some left-handed species and use it as a premise: "SOME left-handed species have evolved from right-handed ones" ....

to a conclusion that generalizes all left-handed snail species: "thus the snakes' asymmetrical jaws probably helped drive the emergence of the left-handed snail species".

Unless "some left-handed species have evolved from right-handed ones" refers just to snails and not all type of animal species in general?

Thank you in advance!

I wouldn't overthink this. The argument, in essence, is that because it's easier for the snakes with the asymmetrical jaws to grasp the right-handed snails than the left-handed snails, these asymmetrical jaws drove the emergence of the left-handed snails. Well, the fact that some of the left-handed snails evolved from right-handed snails is evidence in support of the theory. After all, if a couple of right-handed parents had one left-handed snail and one-right-handed, the left-handed offspring would be more likely to avoid the grasp of those asymmetrical jaws, right?

(I'm trying really hard not to make a bad escargot joke. Moving on...)

Is that proof that the snakes drove this development? Nope. You're absolutely right about that. But if the argument were air-tight, there'd be no need to strengthen it! Your job isn't to understand why the argument is right. It's to understand how the author is reasoning. There may well be some flaws or illogical leaps in that reasoning, but the fact that these arguments are incomplete is kind of the point.

Put another way, if you can go to the answers with a mindset of "I'm looking for something that makes it MORE likely that it was, in fact, the snakes' asymmetrical jaws that drove the development of left-handed snails," then you've held up your end of the bargain, even if the author is making some questionable leaps in getting to the conclusion.

I hope that clears things up!
_________________

AjiteshArun GMATNinja @MagooshExpert@GMATGuruNY

Will you please help explaining why C is incorrect & E is correct?
IMO C eliminates one additional cause of left handed snail evolution & should be the right answer strengthening the fact that asymmetrical jaws of snakes give rise to the evolution of left handed snails , while E presents an already stated fact in the argument.
Also in other strengthen questions we generally eliminate the premise statement if available in the answer choices.
So what is the exception here?