Marine biologists have long thought that variation in the shell color of aquatic snails evolved as a protective camou...

Mehran October 29, 2013

Of course! Let's break down the stimulus first. It is important to note that we are presented with a set of facts explaining different theories as to why there is variation in the shell color of aquatic snails. One theory, i.e. the traditional theory, attributes this to protective camouflage, while the other theory, i.e. the new theory, attributes this to physiological stress associated with heat absorption.

Notice the cause and effect relationship. The observed effect that these theories are trying to explain is the variation in shell color of aquatic snails. The proposed causes are (1) protective camouflage (the traditional theory) and (2) physiological stress associated with heat absorption (the new theory).

We are being asked to find evidence that would strengthen the new theory over the traditional theory. There are two possibilities here:

(1) snails with brown shells on a white floor in an area with a lot of predators and a lot of wave action.

(2) snails with white shells on a dark floor in an area with calm waters and a lot of predators.

Let's take a look at our answer choices:

(A) does not favor the new theory over the traditional one since we are seeing dark-shelled snails in a calm inlet with a dark, rocky bottom and many predators. This actually favors the traditional theory over the new theory because it is entirely consistent with the theory of protective camouflage (i.e. many predators, darks shells and dark-colored seafloor) but inconsistent with the theory of heat absorption (i.e. calmer waters but dark shells).

(B) does not favor the new theory over the traditional theory because it is actually inconsistent with the new theory (i.e. calm waters but dark shells). This also does nothing to the traditional theory because there is no mention of many predators.

(C) does not favor the new theory over the traditional theory because it is actually inconsistent with the new theory (i.e. much wave action but light shells). This also does nothing to the traditional theory because there is no mention of many predators.

(D) is correct because it favors the new theory over the traditional theory. According to the traditional theory, if there are many predators, the shell would match the sea-floor. Here, however, we see many predators but we encounter light shells on a dark floor. Furthermore, (D) is consistent with the new theory because we see a calm inlet and light shells. Notice this is the second possibility set forth above (i.e. snails with white shells on a dark floor in an area with calm waters and a lot of predators).

(E) is incorrect because it is entirely consistent with both theories as opposed to favoring the new theory over the traditional theory.

Hope this helps! Please let us know if you have any other questions.

Mehran October 29, 2013

Also, please note that this is a Strengthen question so we are not inferring anything from the stimulus. Rather we are looking for the answer choice that strengthens the new theory over the traditional theory.

Hope this helps! Please let us know if you have any other questions.

Advaith October 27, 2015

Sorry but Im wondering if something is wrong here with the explanation. I see a big blank space then the second response from Mehran.

Naz October 27, 2015

Yes you should see a long explanation from Mehran. Contact the support team if this is still occurring after you close and reopen your app.

julia September 10, 2018

I don't get how Mehran got the two possibilities in his explanation. It doesn't mention them anywhere in the passage.

midasbo December 4, 2018

I really don't see how Mehran was able to conclude white shells on a dark floor

Jacob-R December 5, 2018

I’m happy to try to help @julia @midasbo

As Mehran noted, the old theory states that shell color is related to sea floor color, whereas the new theory is related to heat absorption.

For the old theory, dark sea floor -> brown shells, and light sea floor -> white shells.

For the new theory, wave action great -> brown shells, and wave action calm -> white shells.

We are looking for evidence that would favor the new theory, and here is how Mehran got to his two options:

If the shells are brown, under the new theory we need great wave action. And under the old theory we would need dark sea floor. So to favor the new theory, we would want great wave action and LIGHT sea floor. (That would indicate that the new theory is a better predictor than the old theory!)

Same idea for white shells: we would need calm wave action, whereas the old theory would say light sea floor. So to favor the new theory we would want calm wave action and DARK sea floor. Make sense?

And that latter option is exactly option D! Which is the right answer. Does that help clarify?

kassidee March 20, 2020

We would want dark sea floors to show the color of the floor has nothing to do with the color of the shells is that correct?

Ravi March 20, 2020

@kassidee, that's correct because if the floor doesn't have anything to do with the color, then it makes it less likely that the shells were colored like the floor for camouflage and more likely for the new theory's reason.

kens February 14, 2022

I'm still a little confused. Isn't the main difference between old and new theories based on the "reasons" for discernable shell colors? I thought the passage already established the fact that shell colors often match the floor color.