Logical Reasoning

PT121 · S4 · Q26 Researcher: It is commonly believed

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Researcher: It is commonly believed that species belonging to the same biological order, such as rodents, descended from a single common ancestor.

Conclusion

Guinea pigs stem from a separate ancestor than that of other rodents.

Evidence

Comparing genetic patterns in guinea pigs, rats, and mice, as well as 13 nonrodent mammals, we see that rats and mice are very similar, but the genetic differences between guinea pigs and mice are as great as those between mice and some nonrodent species.

Evaluate

We can reduce the author's big linking assumption to the idea that, .

We could weaken this argument by challenging this assumption with examples of species that we know did stem from the same ancestor but yet still have huge genetic differences.

We might also object to the limited sample size of rodents here: guinea pigs, rats, and mice.

What if threw in hamsters and squirrels and beavers?

Maybe the difference in genes between a hamster and a beaver are also as different as between a hamster and a nonrodent mammal.

In other words, maybe it's very typical for there to be wide differences in genes, even among animals from a common ancestor, and it's actually the similarity between rats and mice that is the atypical trait.

Goal

We need a way to argue that

Even though guinea pigs are as genetically different from mice as mice are from some nonrodent mammals,

It's still possible that guinea pigs stem from the same ancestor as do other rodents.

26.

Which one of the following, if true, most seriously undermines the researcher's reasoning?

The researcher examined the genetic
Too Weak
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% picked this
Correct
Some pairs of species not
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% picked this
The researcher selected nonrodent species
Unclear Impact
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% picked this
For some genuine biological orders,
No Impact
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.
10% picked this
Peculiarities of body structure, such
No Impact
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% picked this

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