Final answer:
The beak size of the Galapagos Medium Ground Finches has changed over generations due to natural selection. During a drought period, large-billed birds with larger bills were able to survive better than small-billed birds, leading to the evolution of beak size. Subsequent studies have further supported the selection and evolution of beak size in response to changing conditions.
Step-by-step explanation:
Within the Galapagos Medium Ground Finches, the beaks have changed over generations due to natural selection. The beak size variation in the finch population is inherited, with some individuals having wide, deep bills and others having thinner bills. During a drought period, the availability of seeds changed, with a decline in small, soft seeds being greater than the decline in large, hard seeds. As a result, large-billed birds were able to survive better than small-billed birds. The surviving large-billed birds with larger bills produced offspring with larger bills, leading to the evolution of beak size in response to changing conditions on the island. Subsequent studies by the Grants have also shown selection on and evolution of beak size in this species in response to changing conditions.