Science Assessment

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.

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?

1. The birds with larger beaks were better at eating the large seeds than those with smaller beaks, so only the birds with larger beaks got enough food to survive, reproduce, and pass the trait of large beaks to the next generation.
2. The birds with smaller beaks had to work harder than those with larger beaks to crack open the large seeds.  The more they used their beaks, the larger their beaks became, so they were able to get enough food to survive, reproduce, and pass the traits of large beaks to the next generation.
3. The birds with smaller beaks grew their beaks so that they would be better able to eat the large seeds and get enough food to survive, reproduce, and pass the trait of larger beaks to the next generation.
4. It was a chance occurrence that all the individual birds' beaks in the next generation were larger.  They were therefore able to eat the large seeds and get enough food to survive, reproduce, and pass the trait of large beaks to the next generation.