A species of finch living on an isolated island shows variation in beak size. Birds with larger beaks can eat larger seeds.

After a period of drought on the island, large seeds were more plentiful than small seeds and the average size of the finches’ beaks increased.

What explains this increase in size of beak?

A. Artificial selection acting against finches with small beaks

B. Directional selection acting against finches with small beaks

C. Increased rate of mutation resulting in finches with larger beaks

D. Stabilising selection acting against finches with the smallest and largest beaks