What environmental variable caused the beaks in Galapagos finches to evolve?

Changes in the weather led to changes in the food supply available. Sometimes there would be hard nuts, and the finches with tough beaks would survive. However, if, on that same island, the weather changed and longer beaks would be advantageous, the tough-beaked finches would die out and slender beaks would dominate. This is proven by the extensive research carried out by Peter and Rosemary Grant from 1973 to the present.