Researcher: It is commonly believed

Answer choices

1. A

   Too Weak14% picked this

   The researcher examined the genetic material of only 3 of over 2,000

   This is somewhat compelling, since an examination of more rodent species might help show us that it's not so rare for rodents, despite having a common ancestor, to be very genetically divergent from each other. But we're not really weakening the argument until we get to that last idea ... "hey, we looked at a 4th rodent species, and it's super different from a mouse, too!" This answer isn't giving us this objection; it's only suggesting that maybe if we did more digging we might come across a data point like that. Thus, as is, it has very little logical force.

   14%
2. B

   Correct62% picked this

   Some pairs of species not having a common ancestor are genetically more similar to each other than are some pairs that

   Why this is right

   This is weakly worded ("some"), but because it is perfectly designed to target the author's big assumption, it still has huge impact. Our author was thinking that if mice are as genetically different from guinea pigs as they are from nonrodent species X, then that means that guinea pigs and mice have different common ancestors. This answer is saying, "Whoa, whoa, whoa --- you can't assume just because two species have a lot of genetic differences that they don't have a common ancestor." Suppose species A and B are from the same ancestor, while species X is not. This answer is saying that in some cases, B and X are more genetically similar to each other than A and B are. Yet we know that A and B have the same ancestor, while X does not. So this undermines the author's whole mode of reasoning. Apparently, it is NOT in any way a safe inference to compare in-family vs. out-family genetic differences as a way of measuring whether or not two species have a common ancestor. If the conclusion were less certain of itself, if it were only saying "guinea pigs probably don't have the same ancestor", then this answer would probably be too weak to be correct. But since the author's conclusion is certain of itself, since she is making a definite move from "if mice are as diff from guineas and they are from nonrodents, then no common ancestor", then even showing one exception to that "rule" the author is assuming qualifies as hurting the author's reasoning.

   Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

   62%
3. C

   Unclear Impact10% picked this

   The researcher selected nonrodent species that have the specific cell structures she wanted to analyze genetically, though many nonrodent

   This might seem tempting if we were thinking we could object to the original argument by saying that the study was cherry-picking nonrodent mammals who are atypically likely to have genetic similarities to mice. i.e., "Author, guineas and mice have a common ancestor. The only reason you're seeing that mice have about as much in common with guineas as they do with some nonrodent mammals, is because you're specifically selecting the most mouse-like nonrodent mammals with which to make this comparison." However, this answer doesn't really go as far as indicating that the selected nonrodent species were more "mouse-like" then your average nonrodent mammal. It just says they had a particular cell structure the researcher wanted to study.

   10%
4. D

   No Impact9% picked this

   For some genuine biological orders, the most recent common ancestor dates from later epochs than does the most recent common

   This is just saying, the most recent common ancestor of primates might have occurred 10 million years ago, whereas the most recent common ancestor of rodents might have occurred 20 million years ago. Who cares? Nothing in this argument is concerned with how far back in Earth time a common ancestor would be found. It's only concerned with whether or not there is a common ancestor to mice and guinea pigs.

   9%
5. E

   No Impact5% picked this

   Peculiarities of body structure, such as distinctive teeth and olfactory structures, are shared by all

   On its face, this answer is helping us fight the fight of arguing that guinea pigs and mice do have a common ancestor. After all, they (like all rodents) share certain peculiar body traits. But this doesn't address the author's evidence (why are mice as different from guinea pigs as they are from some nonrodent mammals), and it tries to argue via a counterpoint that the author already acknowledged in her conclusion. Since the author already conceded, "Thus, despite their similar physical form", we can't weaken the argument by saying, "But ... did you consider that guinea pigs and all other rodents have a similar physical form?" She would say, "Yes, I mentioned that. What do you have to say about the fact that guinea pigs are as different from mice as mice are from some nonrodents?"

   5%