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.

Part A - Understanding the experimental design

Which of the following is a hypothesis the researchers were testing in this study?

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

The researchers' hypothesis was that southern periwinkle populations with a history of predation by crabs have become less vulnerable to crab predation through evolution.