Based on what you know about natural selection, which of the following must be true if evolution is the explanation for what the Grants have observed over time on Daphne? Select all that apply.

Food must be plentiful for all birds.
Beak depth and length must be heritable.
Beak depth and length must show variation among individuals of a single species.
Beak depth and length must affect survival and/or reproduction of birds.
Over time, individuals can evolve to better fit the environment by changing heritable traits.
The beaks of individual birds can change depending on what food is available.