A species of bird ate many types of seeds, each type coming from a different species
of tree. The birds' beaks varied in size, with some individuals having slightly smaller
beaks and others having slightly larger beaks.
slightly smaller beak
slightly larger beak
A few years went by without much rain, and the only species of tree that survived had
large seeds. Many generations later, almost all the birds had the slightly larger beaks.
How is this best explained?