Bunuel wrote:
Researcher: It is commonly believed that species belonging to the same biological order, such as rodents, descended from a single common ancestor. However, I compared the genetic pattern in 3 rodent species—guinea pigs, rats, and mice—as well as in 13 nonrodent mammals, and found that while rats and mice are genetically quite similar, the genetic differences between guinea pigs and mice are as great as those between mice and some nonrodent species. Thus, despite their similar physical form, guinea pigs stem from a separate ancestor.
Which one of the following, if true, most seriously undermines the researcher’s reasoning?
(A) The researcher examined the genetic material of only 3 of over 2,000 species of rodents.
(B) Some pairs of species not having a common ancestor are genetically more similar to each other than are some pairs that do have a common ancestor.
(C) The researcher selected nonrodent species that have the specific cell structures she wanted to analyze genetically, though many nonrodent mammals lack these cell structures.
(D) For some genuine biological orders, the most recent common ancestor dates from later epochs than does the most recent common ancestor of other biological orders.
(E) Peculiarities of body structure, such as distinctive teeth and olfactory structures, are shared by all rodents, including guinea pigs.
EXPLANATION FROM Fox LSAT
The problem with this argument is that there is no evidence about what having a separate ancestor would look like when you compare the genetic patterns of two different creatures. On the LSAT, you really need to have evidence for every piece of your argument. Here, the researcher
assumes that because guinea pigs and mice have genetic differences “as great as those between mice and some nonrodent species,” that guinea pigs and mice must have a separate ancestor. When I say that she has
assumed this, what I mean is that she doesn’t have a piece of evidence that says, “Any two species with genetic differences as great as those between one of the species and some species from some other order must have a separate ancestor.” This is a critical part of the argument (it must be true in order for the conclusion of the argument to be valid) and it hasn’t been made explicit, therefore it has been assumed.
One of the best ways to weaken an argument that contains an assumption is just to say the opposite of the assumption. For example, consider this argument: “I’m at the ballpark, therefore I’m in San Francisco.” The assumption of the argument is, “The ballpark is AT&T Park, or some other San Francisco ballpark.” To destroy this argument, you could say, “You’re at Yankee Stadium in New York.” Here, we can do much the same thing. To weaken the conclusion “guinea pigs and mice have a separate ancestor,” I would say, “Two species without separate ancestors can have enormous genetic differences.” If this is true, then the given argument makes no sense at all.
A) Why would the researcher be responsible for examining the genetic material of every species of rodent? This doesn’t weaken the argument, because the researcher might respond, “Yes, I examined only three species, and that was enough to prove my case.”
B) This is very similar to our prediction. If it’s possible for species with no common ancestor whatsoever to have more genetic code in common than two species
with a common ancestor, then how has the researcher proven her case? This really deflates the researcher’s argument, so it’s a good answer to a Weaken question.
C) I don’t see how cell structures are relevant to the researcher’s argument.
D) How recently two species shared a common ancestor is irrelevant to the researcher’s argument. There’s nothing at all about dates in the argument, and we are not allowed to assume that more recent ancestors have more genetics in common. This may or may not be true, and requires a big leap in logic without any justifying evidence.
E) Body structure is not at issue, genetics are.
Our answer is B.