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 a clear decrease in shell thickness over time, indicating that the snails with thicker shells were being selectively preyed upon by the introduced crab. This is an example of directional selection, where individuals with a certain trait (thicker shells) are selected against and the population shifts towards individuals with a different trait (thinner shells).

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