The correct answer is: Each bird's survival was strongly influenced by the depth and strength of its beak as the drought persisted.
Grant is a scientist who did his research on finches on the small island of Daphne Major, because the major factor influencing survival of the finches is the weather (there are only few predators or competitors on the island).
The big drought that occurred on the island, affected the survival of finches: the birds with smaller beaks they died of starvation while those with larger beaks could take advantage of alternate food sources because they could eat larger seeds with their larger beak. After two years, Grant found that the beaks of the offspring became larger than their grandparents and that was the evidence that natural selection took place.
