#26 - Researcher: It is commonly believed that species

[angie23]

I have trouble understanding why certain answer choices are correct in these questions and I hope someone would explain the reasoning behind the correct and incorrect answers:

26) I didn't feel B) (which is the correct answer) is strong enough to undermine the argument, especially when it uses the word "some" and doesn't specify which species it referred to when talking about how some pairs of species without a common ancestor are genetically more similar. I picked C because the selection was somewhat bias and not representative of nonrodent mammals.

Thank you!

[Nikki Siclunov]

Hi angie23,

Thanks for your question!

For answer choice (B) to be correct, it does not need to destroy the argument: it simply needs to undermine it. The author observes that guinea pigs are genetically different from mice, and attributes this difference to a separate ancestor. However, if -- as answer choice (B) claims -- some pairs of species who have separate ancestors are genetically more similar to each other than are some pairs who have a common ancestor, then the genetic difference between guinea pigs and mice means very little: it no longer represents definitive proof that guinea pigs and mice stem from separate ancestors.

Regarding answer choice (C), just because the sample is biased in favor of specific cell structures does not weaken the conclusion. First, it is unclear how this bias affects the genetic similarities (or lack thereof) between the samples in question. Second, a biased sample is only a problem when we try to extrapolate from it and make a more general conclusion. This is not the case here: the author is only trying to explain why guinea pigs and mice are genetically different. She makes no claims regarding all rodents.

Hope this helps! Let me know

[dbpk]

The only problem I had with B was that it says some pairs of species without a common ancestor are genetically MORE SIMILAR to each other than some pairs with a common ancestor, but the stimulus says the difference between guinea pigs and mice are AS GREAT as the difference between mice and some nonrodents, so I thought it wasn't applicable.

Should I understand B to undermine conclusions about ancestry based on genetic differences? Or that because pairs without a common ancestor can be MORE similar than those that share an ancestor, it is entirely possible that guinea pigs share an ancestor with mice despite their genetic differences being as great as those between mice and nonrodents?

Thank you!

[AthenaDalton]

Hi dbpk,

You're correct when you say that answer choice (B) undermines the argument because 'pairs without a common ancestor can be more similar than pairs that share an ancestor.'

Essentially, the argument in the stimulus says that since there are significant genetic differences between mice and guinea pigs, they can't have a common ancestor. Answer choice (B) tells us that species pairs with dissimilar genetic codes can still share a common ancestor, thereby undermining the author's argument that genetic similarity determines common ancestry.

Hope this makes sense. Good luck studying!

[smtq123]

Hi,

I am unable to understand why A is not undermining the argument.

A part of the argument is based on "commonly believed". What if this commonly held belief is wrong; in such a case rodents wouldn't have to be belonging to the same biological order and then we can conclude that physical form is not relevant to arrive at the conclusion that "guinea pigs stem from a separate ancestor".

Commonly held belief could have been proven wrong if we would have analyzed much larger number of species than 3.

Please let me know where I am making the mistake.

Thanks & Regards.

[Adam Tyson]

The small sample size pointed out in answer A doesn't undermine the argument, smtq123, because the author is not arguing that this small sample is representative of a larger group. In this case, the author is arguing that guinea pigs are not descended from the same ancestor as mice and rats based on the genetic differences between those three species. Why would they need to study any other species of rodent when the conclusion is only about those three? Answer A doesn't address anything relevant to the argument, and so it cannot weaken it. It doesn't matter what may be true of any other rodent species, because this argument is not about any other species. It's just about guinea pigs being different from the other two.

Keep in mind that the author defined certain terms for us, and we aren't looking to challenge those definitions. We were told that rodents are a single biological order, and we were told that mice, rats, and guinea pigs are all in that order. We have to accept all that as true. The only issue is whether the commonly held belief that they must all share a common ancestor is true or not, and the author argues that it is not true based on the differences between these three rodent species. If, as you said, the commonly held belief is wrong, then the argument is strengthened, not weakened, because that is exactly what the author is trying to prove!