How did seed availability affect traits of individuals in the medium ground finch population of the galapagos islands

A little background may help understand this question better: The Galapagos Islands is home to a lot of species but particular interest here is only the finches and their beak sizes. We have the cactus finch that has a long beak well suited for accessing blossoms; the tree finch with parrot-shaped beaks suited for finding insects underneath the barks of trees; and, the ground finch with beaks suited for eating seeds.

Grant and colleagues studied the medium ground finch in order to observe natural selection at work. Medium ground finch were perfect to study because the only major factor that affected their hypothesized evolution was the climate, which affected food availability. The researchers observed that when food became scarce and only large seeds were present, only the finch with bigger beaks were able to survive. Those with smaller beaks died of starvation because they could not eat the big seeds. Time went on, and when the researchers measure beak size, they found a significant change in size compared to previous generations.