What is a group of organisms that can only reproduce with other members of their group?

A species.

Explanation:

A species is a group of organisms that only reproduces with other members of their group.

A species can be said to be organisms that produce offspring that are fertile, with members of their own group.

Organisms of the same species  tend to also have more characteristics  in common among themselves than with members of a different species.

species

Explanation:

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.