Question

Is there evidence of selection for defensive adaptations in mollusc populations exposed to predators?

The fossil record provides evidence that predator-prey interactions may have shaped the course of animal evolution. Predators and prey are important selective agents on each other. Unsuccessful predators cannot obtain food, while prey that are unsuccessful at resisting attack are eaten.
A team of researchers studied a predator, the European green crab (Carcinus maenas), and its prey, a snail called the flat periwinkle (Littorina obtusata), in the Gulf of Maine. European green crabs have preyed on flat periwinkles in the southern part of the Gulf for over 100 generations. Periwinkles from northern sites in the Gulf have been interacting with the invasive green crabs for relatively few generations, as the invasive crabs spread to the northern Gulf comparatively recently.
Previous research shows that (1) flat periwinkle shells recently collected from the Gulf of Maine are thicker than those collected in the late 1800s, and (2) flat periwinkle populations from southern sites of the Gulf have thicker shells than populations from northern sites.
The researchers collected periwinkles and crabs from sites in the northern and southern parts of the Gulf. A single crab was placed in a cage with eight periwinkles of different sizes. (All crabs were of similar size and included equal numbers of males and females.) Four treatments were set up, and each treatment was tested 12 to 14 times:
-Northern crab caged with northern periwinkles
-Northern crab caged with southern periwinkles
-Southern crab caged with northern periwinkles
-Southern crab caged with southern periwinkles
After three days, the researchers assessed the fate of the eight periwinkles. The bar graph shows the data from this experiment.
In a second experiment, the researchers removed the bodies of northern and southern periwinkles from their shells and presented the bodies to northern and southern crabs. The crabs consumed all of the unshelled periwinkles in less than an hour.
Which of the following is a hypothesis the researchers were testing in this study?
a) Through evolution, periwinkle populations with a history of predation by crabs have become more vulnerable to crab predation.
b)Through evolution, all periwinkle populations have become less vulnerable to crab predation.
c)Through evolution, periwinkle populations without a history of predation by crabs have become less vulnerable to crab predation.
d)Through evolution, periwinkle populations with a history of predation by crabs have become less vulnerable to crab predation.

Answer 1

d)Through evolution, periwinkle populations with a history of predation by crabs have become less vulnerable to crab predation.

Step-by-step explanation:

It seems apparent that the periwinkle snails with softer shells were in the are that were not preyed by the crabs. These snails did not have a selective pressure and thus the shells remains soft/missing and on the forced encounter with the crabs they were instantly consumed.

In contrast, the snails that had been in co-existence with the crabs had developed a harder shell because those that did not have a solid shell were unable to survive predation. These individuals did not reproduce and that trait did not pass on. If it did, these were consumed by the crabs.

Meanwhile, the snails with thicker shells were not preyed upon by the crabs and were able to survive to pass along the genes that preserved them.

As time went on the selective pressure put on the snails by the crabs changed the phenotype and resulted in a resistant variety in the are where the crabs were.