LSAT CR

[rico16rad]

Source is LSAT .


The similarity between ichthyosaurs and fish is an example of convergence, a process by which different classes of organisms adapt to the same environment by independently developing one or more similar external body features. Ichthyosaurs were marine reptiles and thus do not belong to the same class of organisms as fish. However, ichthyosaurs adapted to their marine environment by converging on external body features similar to those of fish. Most strikingly, ichthyosaurs, like fish, had fins.
If the statements above are true, which one of the following is an inference that can be properly drawn on the basis of them?
(A) The members of a single class of organisms that inhabit the same environment must be identical in all their external body features.
(B) The members of a single class of organisms must exhibit one or more similar external body features that differentiate that class from all other classes of organisms.
(C) It is only as a result of adaptation to similar environments that one class of organisms develops external body features similar to those of another class of organisms.
(D) An organism does not necessarily belong to a class simply because the organism has one or more external body features similar to those of members of that class.
(E) Whenever two classes of organisms share the same environment, members of one class will differ from members of the other class in several external body features.


OA: D
Initially I got down to E,but I think E specially talks about only two classes and above that it talks about difference of members' external body feature.So this make E a wrong choice.
Experts can you please explain me about the elimination of each answer choices.So I can decide whether I am moving in right way of selecting answer.

[rajkapoor]

will give a crack at the question

the correct answer choice must be 100% provable

convergence : two classes with different traits ,are/put in same eniv, will adapt exclusive of each other and acquire some external features (e.g. i-saurs and fish - both had fins / but belonged to diff class )


(A) The members of a single class of organisms that inhabit the same environment must be identical in all their external body features.

"single class must be identical in all external " - no where mentioned or implied

(B) The members of a single class of organisms must exhibit one or more similar external body features that differentiate that class from all other classes of organisms.

"there is no mention of how the classes are differentiated - only examples of two different classes are given (marine reptiles and fish)"

(C) It is only as a result of adaptation to similar environments that one class of organisms develops external body features similar to those of another class of organisms.

"only as a result" - extreme - there could be other causes too

(D) An organism does not necessarily belong to a class simply because the organism has one or more external body features similar to those of members of that class.

"i-saurs has fins , fish has fins but they belong to different class" - hence proved - CORRECT

(E) Whenever two classes of organisms share the same environment, members of one class will differ from members of the other class in several external body features.

"whenever" - is too extreme +
plus there is no mention about how organisms are classified - the criteria could be 1 external feature , 2 external feature or a totally different set of features than external attributes (e.g. carnivores / herbivores ) - so "several external body features" cannot be proven

[RonPurewal]

Source is LSAT .


The similarity between ichthyosaurs and fish is an example of convergence, a process by which different classes of organisms adapt to the same environment by independently developing one or more similar external body features. Ichthyosaurs were marine reptiles and thus do not belong to the same class of organisms as fish. However, ichthyosaurs adapted to their marine environment by converging on external body features similar to those of fish. Most strikingly, ichthyosaurs, like fish, had fins.
If the statements above are true, which one of the following is an inference that can be properly drawn on the basis of them?
(A) The members of a single class of organisms that inhabit the same environment must be identical in all their external body features.
(B) The members of a single class of organisms must exhibit one or more similar external body features that differentiate that class from all other classes of organisms.
(C) It is only as a result of adaptation to similar environments that one class of organisms develops external body features similar to those of another class of organisms.
(D) An organism does not necessarily belong to a class simply because the organism has one or more external body features similar to those of members of that class.
(E) Whenever two classes of organisms share the same environment, members of one class will differ from members of the other class in several external body features.


OA: D
Initially I got down to E,but I think E specially talks about only two classes and above that it talks about difference of members' external body feature.So this make E a wrong choice.
Experts can you please explain me about the elimination of each answer choices.So I can decide whether I am moving in right way of selecting answer.

well, remember the basic premise of these "draw the conclusion" problems -- you have to pick the answer choice that can be PROVED from the existing statements.

therefore, there are three principles that you can use: (warning! these principles do not apply to other problem types, such as strengthening and weakening)

when you DRAW THE CONCLUSION:
1) you can eliminate any answers that are more general in scope than the passage itself;
2) you can eliminate any answers that mention topics irrelevant to the passage;
3) weaker statements are usually more attractive as conclusions that are stronger statements.

--

you are considering choice (e):
Whenever two classes of organisms share the same environment
--> as soon as you see these words, you can eliminate this choice: the scope is way, way, WAY too broad. since the statement starts with "whenever", the statement actually demands knowledge about ALL such pairs of classes of organisms.

one might complain, then, that choice (d) shares the same problem, since it contains the following words:
An organism does not necessarily belong to a class
--> while this choice seems to be speaking about all organisms and classes, the words "not necessarily" actually indicate that one example is sufficient. i.e., "not necessarily" means that all we actually need is one counterexample to the statement.

--

by the way, you just hit upon one problem with using the lsat as a source of critical reasoning problems. while the lsat is not, of course, a worthless source, you should be cognizant of the fact that the format of lsat problems often differs dramatically from that of gmat problems.
for instance, this problem is basically an exercise in formal logic, a topic that is a mainstay of the lsat, but that appears with extremely scant frequency on the gmat.
i.e., i'm talking about the problems whose answer choices usually look like this:
Some X's in Z are not Y's
Some Y's in Z are not X's
It is only in Z that X's are not Y's
X's that are not Y's are in Z
etc.

the gmat is really not very big on this sort of thing -- the only two problems i recall seeing in this format are #95 and #106 from the OG12. on the other hand, if you study the lsat, you will see a huge proportion of these in every logical reasoning section.

in any case, though, at least you won't learn the WRONG things -- the three principles above, which are the bedrock of "draw the conclusion problems" on the gmat, are every bit as applicable to the lsat.
the real thing you have to watch out for, on the other hand, is the presence of problems on the lsat in formats that completely don't exist on the gmat. post back if you want more information on that.