Peter and Rosemary Grant studied natural selection in Galápagos finches for more than 40 years; which species was the focus of their research?

Medium ground finch

Explanation:

Peter and Rosemary Grant were biologists who studied natural selection in Galápagos finches for more than 40 years.

The medium ground finch species was the focus of their research. They conducted various experiments and were able to gather information on how very rapid changes in body and beak size according to varied food supply was as a result of natural selection.