The mutation in the species occurred as a result of natural selection. As the larger sized beaks helped certain birds of the species to open clams and other sources of food, that gave these individuals an advantage at survival, meaning better chances of reproducing and passing on their genes and their characteristic large beak. The larger beak also helped the birds to defend themselves from predators, giving them better chances at survival, and passing on their genes. Eventually, generation after generation the weaker members of the species (birds with regular beaks) died off, as they had less chances of survival, therefore less birds to pass on their genes/characteristics.
