The Geospiza fortis, or medium ground finch, live on the Galapagos Islands.

Although the medium ground finch normally eats small, soft seeds, a severe drought in the 1970s reduced the supply of these seeds. As a result, finches with slightly larger beaks, who could eat larger and harder seeds, were naturally selected over finches with smaller beaks. After a few generations, whole populations of these finches developed beaks that are ten percent larger than they were before the drought.

This gradual change in beak size over time is known as _.
A.
evaporation
B.
evolution
C.
extinction
D.
regeneration