Eighty years ago,a predatory crab was introduced to a region it previously did not inhabit and began preying on a species of marine snail.A scientist was able to locate museum specimens of the snail's shells from prior to the crab's introduction and to compare their thickness-an important factor in how protective the shells are-to the thickness of shells from current snails.Based on the graphs of her results (shown here),what form of evolution/natural selection has taken place?

Question

Quizplus · Accepted Answer

The graphs show that the thickness of the snail shells has decreased over time. This suggests that there has been selection pressure for snails with thinner shells since the introduction of the predatory crab. Therefore, the form of evolution that has taken place is directional selection.

Eighty Years Ago,a Predatory Crab Was Introduced to a Region